(* Title: HOL/Algebra/Polynomial_Divisibility.thy
Author: Paulo EmÃlio de Vilhena
*)
theory Polynomial_Divisibility
imports Polynomials Embedded_Algebras "HOL-Library.Multiset"
begin
section \<open>Divisibility of Polynomials\<close>
subsection \<open>Definitions\<close>
abbreviation poly_ring :: "_ \<Rightarrow> ('a list) ring"
where "poly_ring R \<equiv> univ_poly R (carrier R)"
abbreviation pirreducible :: "_ \<Rightarrow> 'a set \<Rightarrow> 'a list \<Rightarrow> bool" ("pirreducible\<index>")
where "pirreducible\<^bsub>R\<^esub> K p \<equiv> ring_irreducible\<^bsub>(univ_poly R K)\<^esub> p"
abbreviation pprime :: "_ \<Rightarrow> 'a set \<Rightarrow> 'a list \<Rightarrow> bool" ("pprime\<index>")
where "pprime\<^bsub>R\<^esub> K p \<equiv> ring_prime\<^bsub>(univ_poly R K)\<^esub> p"
definition pdivides :: "_ \<Rightarrow> 'a list \<Rightarrow> 'a list \<Rightarrow> bool" (infix "pdivides\<index>" 65)
where "p pdivides\<^bsub>R\<^esub> q = p divides\<^bsub>(univ_poly R (carrier R))\<^esub> q"
definition rupture :: "_ \<Rightarrow> 'a set \<Rightarrow> 'a list \<Rightarrow> (('a list) set) ring" ("Rupt\<index>")
where "Rupt\<^bsub>R\<^esub> K p = (K[X]\<^bsub>R\<^esub>) Quot (PIdl\<^bsub>K[X]\<^bsub>R\<^esub>\<^esub> p)"
abbreviation (in ring) rupture_surj :: "'a set \<Rightarrow> 'a list \<Rightarrow> 'a list \<Rightarrow> ('a list) set"
where "rupture_surj K p \<equiv> (\<lambda>q. (PIdl\<^bsub>K[X]\<^esub> p) +>\<^bsub>K[X]\<^esub> q)"
subsection \<open>Basic Properties\<close>
lemma (in ring) carrier_polynomial_shell [intro]:
assumes "subring K R" and "p \<in> carrier (K[X])" shows "p \<in> carrier (poly_ring R)"
using carrier_polynomial[OF assms(1), of p] assms(2) unfolding sym[OF univ_poly_carrier] by simp
lemma (in domain) pdivides_zero:
assumes "subring K R" and "p \<in> carrier (K[X])" shows "p pdivides []"
using ring.divides_zero[OF univ_poly_is_ring[OF carrier_is_subring]
carrier_polynomial_shell[OF assms]]
unfolding univ_poly_zero pdivides_def .
lemma (in domain) zero_pdivides_zero: "[] pdivides []"
using pdivides_zero[OF carrier_is_subring] univ_poly_carrier by blast
lemma (in domain) zero_pdivides:
shows "[] pdivides p \<longleftrightarrow> p = []"
using ring.zero_divides[OF univ_poly_is_ring[OF carrier_is_subring]]
unfolding univ_poly_zero pdivides_def .
lemma (in domain) pprime_iff_pirreducible:
assumes "subfield K R" and "p \<in> carrier (K[X])"
shows "pprime K p \<longleftrightarrow> pirreducible K p"
using principal_domain.primeness_condition[OF univ_poly_is_principal] assms by simp
lemma (in domain) pirreducibleE:
assumes "subring K R" "p \<in> carrier (K[X])" "pirreducible K p"
shows "p \<noteq> []" "p \<notin> Units (K[X])"
and "\<And>q r. \<lbrakk> q \<in> carrier (K[X]); r \<in> carrier (K[X])\<rbrakk> \<Longrightarrow>
p = q \<otimes>\<^bsub>K[X]\<^esub> r \<Longrightarrow> q \<in> Units (K[X]) \<or> r \<in> Units (K[X])"
using domain.ring_irreducibleE[OF univ_poly_is_domain[OF assms(1)] _ assms(3)] assms(2)
by (auto simp add: univ_poly_zero)
lemma (in domain) pirreducibleI:
assumes "subring K R" "p \<in> carrier (K[X])" "p \<noteq> []" "p \<notin> Units (K[X])"
and "\<And>q r. \<lbrakk> q \<in> carrier (K[X]); r \<in> carrier (K[X])\<rbrakk> \<Longrightarrow>
p = q \<otimes>\<^bsub>K[X]\<^esub> r \<Longrightarrow> q \<in> Units (K[X]) \<or> r \<in> Units (K[X])"
shows "pirreducible K p"
using domain.ring_irreducibleI[OF univ_poly_is_domain[OF assms(1)] _ assms(4)] assms(2-3,5)
by (auto simp add: univ_poly_zero)
lemma (in domain) univ_poly_carrier_units_incl:
shows "Units ((carrier R) [X]) \<subseteq> { [ k ] | k. k \<in> carrier R - { \<zero> } }"
proof
fix p assume "p \<in> Units ((carrier R) [X])"
then obtain q
where p: "polynomial (carrier R) p" and q: "polynomial (carrier R) q" and pq: "poly_mult p q = [ \<one> ]"
unfolding Units_def univ_poly_def by auto
hence not_nil: "p \<noteq> []" and "q \<noteq> []"
using poly_mult_integral[OF carrier_is_subring p q] poly_mult_zero[OF polynomial_incl[OF p]] by auto
hence "degree p = 0"
using poly_mult_degree_eq[OF carrier_is_subring p q] unfolding pq by simp
hence "length p = 1"
using not_nil by (metis One_nat_def Suc_pred length_greater_0_conv)
then obtain k where k: "p = [ k ]"
by (metis One_nat_def length_0_conv length_Suc_conv)
hence "k \<in> carrier R - { \<zero> }"
using p unfolding polynomial_def by auto
thus "p \<in> { [ k ] | k. k \<in> carrier R - { \<zero> } }"
unfolding k by blast
qed
lemma (in field) univ_poly_carrier_units:
"Units ((carrier R) [X]) = { [ k ] | k. k \<in> carrier R - { \<zero> } }"
proof
show "Units ((carrier R) [X]) \<subseteq> { [ k ] | k. k \<in> carrier R - { \<zero> } }"
using univ_poly_carrier_units_incl by simp
next
show "{ [ k ] | k. k \<in> carrier R - { \<zero> } } \<subseteq> Units ((carrier R) [X])"
proof (auto)
fix k assume k: "k \<in> carrier R" "k \<noteq> \<zero>"
hence inv_k: "inv k \<in> carrier R" "inv k \<noteq> \<zero>" and "k \<otimes> inv k = \<one>" "inv k \<otimes> k = \<one>"
using subfield_m_inv[OF carrier_is_subfield, of k] by auto
hence "poly_mult [ k ] [ inv k ] = [ \<one> ]" and "poly_mult [ inv k ] [ k ] = [ \<one> ]"
by (auto simp add: k)
moreover have "polynomial (carrier R) [ k ]" and "polynomial (carrier R) [ inv k ]"
using const_is_polynomial k inv_k by auto
ultimately show "[ k ] \<in> Units ((carrier R) [X])"
unfolding Units_def univ_poly_def by (auto simp del: poly_mult.simps)
qed
qed
lemma (in domain) univ_poly_units_incl:
assumes "subring K R" shows "Units (K[X]) \<subseteq> { [ k ] | k. k \<in> K - { \<zero> } }"
using domain.univ_poly_carrier_units_incl[OF subring_is_domain[OF assms]]
univ_poly_consistent[OF assms] by auto
lemma (in ring) univ_poly_units:
assumes "subfield K R" shows "Units (K[X]) = { [ k ] | k. k \<in> K - { \<zero> } }"
using field.univ_poly_carrier_units[OF subfield_iff(2)[OF assms]]
univ_poly_consistent[OF subfieldE(1)[OF assms]] by auto
lemma (in domain) univ_poly_units':
assumes "subfield K R" shows "p \<in> Units (K[X]) \<longleftrightarrow> p \<in> carrier (K[X]) \<and> p \<noteq> [] \<and> degree p = 0"
unfolding univ_poly_units[OF assms] sym[OF univ_poly_carrier] polynomial_def
by (auto, metis hd_in_set le_0_eq le_Suc_eq length_0_conv length_Suc_conv list.sel(1) subsetD)
corollary (in domain) rupture_one_not_zero:
assumes "subfield K R" and "p \<in> carrier (K[X])" and "degree p > 0"
shows "\<one>\<^bsub>Rupt K p\<^esub> \<noteq> \<zero>\<^bsub>Rupt K p\<^esub>"
proof (rule ccontr)
interpret UP: principal_domain "K[X]"
using univ_poly_is_principal[OF assms(1)] .
assume "\<not> \<one>\<^bsub>Rupt K p\<^esub> \<noteq> \<zero>\<^bsub>Rupt K p\<^esub>"
then have "PIdl\<^bsub>K[X]\<^esub> p +>\<^bsub>K[X]\<^esub> \<one>\<^bsub>K[X]\<^esub> = PIdl\<^bsub>K[X]\<^esub> p"
unfolding rupture_def FactRing_def by simp
hence "\<one>\<^bsub>K[X]\<^esub> \<in> PIdl\<^bsub>K[X]\<^esub> p"
using ideal.rcos_const_imp_mem[OF UP.cgenideal_ideal[OF assms(2)]] by auto
then obtain q where "q \<in> carrier (K[X])" and "\<one>\<^bsub>K[X]\<^esub> = q \<otimes>\<^bsub>K[X]\<^esub> p"
using assms(2) unfolding cgenideal_def by auto
hence "p \<in> Units (K[X])"
unfolding Units_def using assms(2) UP.m_comm by auto
hence "degree p = 0"
unfolding univ_poly_units[OF assms(1)] by auto
with \<open>degree p > 0\<close> show False
by simp
qed
corollary (in ring) pirreducible_degree:
assumes "subfield K R" "p \<in> carrier (K[X])" "pirreducible K p"
shows "degree p \<ge> 1"
proof (rule ccontr)
assume "\<not> degree p \<ge> 1" then have "length p \<le> 1"
by simp
moreover have "p \<noteq> []" and "p \<notin> Units (K[X])"
using assms(3) by (auto simp add: ring_irreducible_def irreducible_def univ_poly_zero)
ultimately obtain k where k: "p = [ k ]"
by (metis append_butlast_last_id butlast_take diff_is_0_eq le_refl self_append_conv2 take0 take_all)
hence "k \<in> K" and "k \<noteq> \<zero>"
using assms(2) by (auto simp add: polynomial_def univ_poly_def)
hence "p \<in> Units (K[X])"
using univ_poly_units[OF assms(1)] unfolding k by auto
from \<open>p \<in> Units (K[X])\<close> and \<open>p \<notin> Units (K[X])\<close> show False by simp
qed
corollary (in domain) univ_poly_not_field:
assumes "subring K R" shows "\<not> field (K[X])"
proof -
have "X \<in> carrier (K[X]) - { \<zero>\<^bsub>(K[X])\<^esub> }" and "X \<notin> { [ k ] | k. k \<in> K - { \<zero> } }"
using var_closed(1)[OF assms] unfolding univ_poly_zero var_def by auto
thus ?thesis
using field.field_Units[of "K[X]"] univ_poly_units_incl[OF assms] by blast
qed
lemma (in domain) rupture_is_field_iff_pirreducible:
assumes "subfield K R" and "p \<in> carrier (K[X])"
shows "field (Rupt K p) \<longleftrightarrow> pirreducible K p"
proof
assume "pirreducible K p" thus "field (Rupt K p)"
using principal_domain.field_iff_prime[OF univ_poly_is_principal[OF assms(1)]] assms(2)
pprime_iff_pirreducible[OF assms] pirreducibleE(1)[OF subfieldE(1)[OF assms(1)]]
by (simp add: univ_poly_zero rupture_def)
next
interpret UP: principal_domain "K[X]"
using univ_poly_is_principal[OF assms(1)] .
assume field: "field (Rupt K p)"
have "p \<noteq> []"
proof (rule ccontr)
assume "\<not> p \<noteq> []" then have p: "p = []"
by simp
hence "Rupt K p \<simeq> (K[X])"
using UP.FactRing_zeroideal(1) UP.genideal_zero
UP.cgenideal_eq_genideal[OF UP.zero_closed]
by (simp add: rupture_def univ_poly_zero)
then obtain h where h: "h \<in> ring_iso (Rupt K p) (K[X])"
unfolding is_ring_iso_def by blast
moreover have "ring (Rupt K p)"
using field by (simp add: cring_def domain_def field_def)
ultimately interpret R: ring_hom_ring "Rupt K p" "K[X]" h
unfolding ring_hom_ring_def ring_hom_ring_axioms_def ring_iso_def
using UP.ring_axioms by simp
have "field (K[X])"
using field.ring_iso_imp_img_field[OF field h] by simp
thus False
using univ_poly_not_field[OF subfieldE(1)[OF assms(1)]] by simp
qed
thus "pirreducible K p"
using UP.field_iff_prime pprime_iff_pirreducible[OF assms] assms(2) field
by (simp add: univ_poly_zero rupture_def)
qed
lemma (in domain) rupture_surj_hom:
assumes "subring K R" and "p \<in> carrier (K[X])"
shows "(rupture_surj K p) \<in> ring_hom (K[X]) (Rupt K p)"
and "ring_hom_ring (K[X]) (Rupt K p) (rupture_surj K p)"
proof -
interpret UP: domain "K[X]"
using univ_poly_is_domain[OF assms(1)] .
interpret I: ideal "PIdl\<^bsub>K[X]\<^esub> p" "K[X]"
using UP.cgenideal_ideal[OF assms(2)] .
show "(rupture_surj K p) \<in> ring_hom (K[X]) (Rupt K p)"
and "ring_hom_ring (K[X]) (Rupt K p) (rupture_surj K p)"
using ring_hom_ring.intro[OF UP.ring_axioms I.quotient_is_ring] I.rcos_ring_hom
unfolding symmetric[OF ring_hom_ring_axioms_def] rupture_def by auto
qed
corollary (in domain) rupture_surj_norm_is_hom:
assumes "subring K R" and "p \<in> carrier (K[X])"
shows "((rupture_surj K p) \<circ> poly_of_const) \<in> ring_hom (R \<lparr> carrier := K \<rparr>) (Rupt K p)"
using ring_hom_trans[OF canonical_embedding_is_hom[OF assms(1)] rupture_surj_hom(1)[OF assms]] .
lemma (in domain) norm_map_in_poly_ring_carrier:
assumes "p \<in> carrier (poly_ring R)" and "\<And>a. a \<in> carrier R \<Longrightarrow> f a \<in> carrier (poly_ring R)"
shows "ring.normalize (poly_ring R) (map f p) \<in> carrier (poly_ring (poly_ring R))"
proof -
have "set p \<subseteq> carrier R"
using assms(1) unfolding sym[OF univ_poly_carrier] polynomial_def by auto
hence "set (map f p) \<subseteq> carrier (poly_ring R)"
using assms(2) by auto
thus ?thesis
using ring.normalize_gives_polynomial[OF univ_poly_is_ring[OF carrier_is_subring]]
unfolding univ_poly_carrier by simp
qed
lemma (in domain) map_in_poly_ring_carrier:
assumes "p \<in> carrier (poly_ring R)" and "\<And>a. a \<in> carrier R \<Longrightarrow> f a \<in> carrier (poly_ring R)"
and "\<And>a. a \<noteq> \<zero> \<Longrightarrow> f a \<noteq> []"
shows "map f p \<in> carrier (poly_ring (poly_ring R))"
proof -
interpret UP: ring "poly_ring R"
using univ_poly_is_ring[OF carrier_is_subring] .
have "lead_coeff p \<noteq> \<zero>" if "p \<noteq> []"
using that assms(1) unfolding sym[OF univ_poly_carrier] polynomial_def by auto
hence "ring.normalize (poly_ring R) (map f p) = map f p"
by (cases p) (simp_all add: assms(3) univ_poly_zero)
thus ?thesis
using norm_map_in_poly_ring_carrier[of p f] assms(1-2) by simp
qed
lemma (in domain) map_norm_in_poly_ring_carrier:
assumes "subring K R" and "p \<in> carrier (K[X])"
shows "map poly_of_const p \<in> carrier (poly_ring (K[X]))"
using domain.map_in_poly_ring_carrier[OF subring_is_domain[OF assms(1)]]
proof -
have "\<And>a. a \<in> K \<Longrightarrow> poly_of_const a \<in> carrier (K[X])"
and "\<And>a. a \<noteq> \<zero> \<Longrightarrow> poly_of_const a \<noteq> []"
using ring_hom_memE(1)[OF canonical_embedding_is_hom[OF assms(1)]]
by (auto simp: poly_of_const_def)
thus ?thesis
using domain.map_in_poly_ring_carrier[OF subring_is_domain[OF assms(1)]] assms(2)
unfolding univ_poly_consistent[OF assms(1)] by simp
qed
lemma (in domain) polynomial_rupture:
assumes "subring K R" and "p \<in> carrier (K[X])"
shows "(ring.eval (Rupt K p)) (map ((rupture_surj K p) \<circ> poly_of_const) p) (rupture_surj K p X) = \<zero>\<^bsub>Rupt K p\<^esub>"
proof -
let ?surj = "rupture_surj K p"
interpret UP: domain "K[X]"
using univ_poly_is_domain[OF assms(1)] .
interpret Hom: ring_hom_ring "K[X]" "Rupt K p" ?surj
using rupture_surj_hom(2)[OF assms] .
have "(Hom.S.eval) (map (?surj \<circ> poly_of_const) p) (?surj X) = ?surj ((UP.eval) (map poly_of_const p) X)"
using Hom.eval_hom[OF UP.carrier_is_subring var_closed(1)[OF assms(1)]
map_norm_in_poly_ring_carrier[OF assms]] by simp
also have " ... = ?surj p"
unfolding sym[OF eval_rewrite[OF assms]] ..
also have " ... = \<zero>\<^bsub>Rupt K p\<^esub>"
using UP.a_rcos_zero[OF UP.cgenideal_ideal[OF assms(2)] UP.cgenideal_self[OF assms(2)]]
unfolding rupture_def FactRing_def by simp
finally show ?thesis .
qed
subsection \<open>Division\<close>
definition (in ring) long_divides :: "'a list \<Rightarrow> 'a list \<Rightarrow> ('a list \<times> 'a list) \<Rightarrow> bool"
where "long_divides p q t \<longleftrightarrow>
\<comment> \<open>i\<close> (t \<in> carrier (poly_ring R) \<times> carrier (poly_ring R)) \<and>
\<comment> \<open>ii\<close> (p = (q \<otimes>\<^bsub>poly_ring R\<^esub> (fst t)) \<oplus>\<^bsub>poly_ring R\<^esub> (snd t)) \<and>
\<comment> \<open>iii\<close> (snd t = [] \<or> degree (snd t) < degree q)"
definition (in ring) long_division :: "'a list \<Rightarrow> 'a list \<Rightarrow> ('a list \<times> 'a list)"
where "long_division p q = (THE t. long_divides p q t)"
definition (in ring) pdiv :: "'a list \<Rightarrow> 'a list \<Rightarrow> 'a list" (infixl "pdiv" 65)
where "p pdiv q = (if q = [] then [] else fst (long_division p q))"
definition (in ring) pmod :: "'a list \<Rightarrow> 'a list \<Rightarrow> 'a list" (infixl "pmod" 65)
where "p pmod q = (if q = [] then p else snd (long_division p q))"
lemma (in ring) long_dividesI:
assumes "b \<in> carrier (poly_ring R)" and "r \<in> carrier (poly_ring R)"
and "p = (q \<otimes>\<^bsub>poly_ring R\<^esub> b) \<oplus>\<^bsub>poly_ring R\<^esub> r" and "r = [] \<or> degree r < degree q"
shows "long_divides p q (b, r)"
using assms unfolding long_divides_def by auto
lemma (in domain) exists_long_division:
assumes "subfield K R" and "p \<in> carrier (K[X])" and "q \<in> carrier (K[X])" "q \<noteq> []"
obtains b r where "b \<in> carrier (K[X])" and "r \<in> carrier (K[X])" and "long_divides p q (b, r)"
using subfield_long_division_theorem_shell[OF assms(1-3)] assms(4)
carrier_polynomial_shell[OF subfieldE(1)[OF assms(1)]]
unfolding long_divides_def univ_poly_zero univ_poly_add univ_poly_mult by auto
lemma (in domain) exists_unique_long_division:
assumes "subfield K R" and "p \<in> carrier (K[X])" and "q \<in> carrier (K[X])" "q \<noteq> []"
shows "\<exists>!t. long_divides p q t"
proof -
let ?padd = "\<lambda>a b. a \<oplus>\<^bsub>poly_ring R\<^esub> b"
let ?pmult = "\<lambda>a b. a \<otimes>\<^bsub>poly_ring R\<^esub> b"
let ?pminus = "\<lambda>a b. a \<ominus>\<^bsub>poly_ring R\<^esub> b"
interpret UP: domain "poly_ring R"
using univ_poly_is_domain[OF carrier_is_subring] .
obtain b r where ldiv: "long_divides p q (b, r)"
using exists_long_division[OF assms] by metis
moreover have "(b, r) = (b', r')" if "long_divides p q (b', r')" for b' r'
proof -
have q: "q \<in> carrier (poly_ring R)" "q \<noteq> []"
using assms(3-4) carrier_polynomial[OF subfieldE(1)[OF assms(1)]]
unfolding univ_poly_carrier by auto
hence in_carrier: "q \<in> carrier (poly_ring R)"
"b \<in> carrier (poly_ring R)" "r \<in> carrier (poly_ring R)"
"b' \<in> carrier (poly_ring R)" "r' \<in> carrier (poly_ring R)"
using assms(3) that ldiv unfolding long_divides_def by auto
have "?pminus (?padd (?pmult q b) r) r' = ?pminus (?padd (?pmult q b') r') r'"
using ldiv and that unfolding long_divides_def by auto
hence eq: "?padd (?pmult q (?pminus b b')) (?pminus r r') = \<zero>\<^bsub>poly_ring R\<^esub>"
using in_carrier by algebra
have "b = b'"
proof (rule ccontr)
assume "b \<noteq> b'"
hence pminus: "?pminus b b' \<noteq> \<zero>\<^bsub>poly_ring R\<^esub>" "?pminus b b' \<in> carrier (poly_ring R)"
using in_carrier(2,4) by (metis UP.add.inv_closed UP.l_neg UP.minus_eq UP.minus_unique, algebra)
hence degree_ge: "degree (?pmult q (?pminus b b')) \<ge> degree q"
using poly_mult_degree_eq[OF carrier_is_subring, of q "?pminus b b'"] q
unfolding univ_poly_zero univ_poly_carrier univ_poly_mult by simp
have "?pminus b b' = \<zero>\<^bsub>poly_ring R\<^esub>" if "?pminus r r' = \<zero>\<^bsub>poly_ring R\<^esub>"
using eq pminus(2) q UP.integral univ_poly_zero unfolding that by auto
hence "?pminus r r' \<noteq> []"
using pminus(1) unfolding univ_poly_zero by blast
moreover have "?pminus r r' = []" if "r = []" and "r' = []"
using univ_poly_a_inv_def'[OF carrier_is_subring UP.zero_closed] that
unfolding a_minus_def univ_poly_add univ_poly_zero by auto
ultimately have "r \<noteq> [] \<or> r' \<noteq> []"
by blast
hence "max (degree r) (degree r') < degree q"
using ldiv and that unfolding long_divides_def by auto
moreover have "degree (?pminus r r') \<le> max (degree r) (degree r')"
using poly_add_degree[of r "map (a_inv R) r'"]
unfolding a_minus_def univ_poly_add univ_poly_a_inv_def'[OF carrier_is_subring in_carrier(5)]
by auto
ultimately have degree_lt: "degree (?pminus r r') < degree q"
by linarith
have is_poly: "polynomial (carrier R) (?pmult q (?pminus b b'))" "polynomial (carrier R) (?pminus r r')"
using in_carrier pminus(2) unfolding univ_poly_carrier by algebra+
have "degree (?padd (?pmult q (?pminus b b')) (?pminus r r')) = degree (?pmult q (?pminus b b'))"
using poly_add_degree_eq[OF carrier_is_subring is_poly] degree_ge degree_lt
unfolding univ_poly_carrier sym[OF univ_poly_add[of R "carrier R"]] max_def by simp
hence "degree (?padd (?pmult q (?pminus b b')) (?pminus r r')) > 0"
using degree_ge degree_lt by simp
moreover have "degree (?padd (?pmult q (?pminus b b')) (?pminus r r')) = 0"
using eq unfolding univ_poly_zero by simp
ultimately show False by simp
qed
hence "?pminus r r' = \<zero>\<^bsub>poly_ring R\<^esub>"
using in_carrier eq by algebra
hence "r = r'"
using in_carrier by (metis UP.add.inv_closed UP.add.right_cancel UP.minus_eq UP.r_neg)
with \<open>b = b'\<close> show ?thesis
by simp
qed
ultimately show ?thesis
by auto
qed
lemma (in domain) long_divisionE:
assumes "subfield K R" and "p \<in> carrier (K[X])" and "q \<in> carrier (K[X])" "q \<noteq> []"
shows "long_divides p q (p pdiv q, p pmod q)"
using theI'[OF exists_unique_long_division[OF assms]] assms(4)
unfolding pmod_def pdiv_def long_division_def by auto
lemma (in domain) long_divisionI:
assumes "subfield K R" and "p \<in> carrier (K[X])" and "q \<in> carrier (K[X])" "q \<noteq> []"
shows "long_divides p q (b, r) \<Longrightarrow> (b, r) = (p pdiv q, p pmod q)"
using exists_unique_long_division[OF assms] long_divisionE[OF assms] by metis
lemma (in domain) long_division_closed:
assumes "subfield K R" and "p \<in> carrier (K[X])" "q \<in> carrier (K[X])"
shows "p pdiv q \<in> carrier (K[X])" and "p pmod q \<in> carrier (K[X])"
proof -
have "p pdiv q \<in> carrier (K[X]) \<and> p pmod q \<in> carrier (K[X])"
using assms univ_poly_zero_closed[of R] long_divisionI[of K] exists_long_division[OF assms]
by (cases "q = []") (simp add: pdiv_def pmod_def, metis Pair_inject)+
thus "p pdiv q \<in> carrier (K[X])" and "p pmod q \<in> carrier (K[X])"
by auto
qed
lemma (in domain) pdiv_pmod:
assumes "subfield K R" and "p \<in> carrier (K[X])" "q \<in> carrier (K[X])"
shows "p = (q \<otimes>\<^bsub>K[X]\<^esub> (p pdiv q)) \<oplus>\<^bsub>K[X]\<^esub> (p pmod q)"
proof (cases)
interpret UP: ring "K[X]"
using univ_poly_is_ring[OF subfieldE(1)[OF assms(1)]] .
assume "q = []" thus ?thesis
using assms(2) unfolding pdiv_def pmod_def sym[OF univ_poly_zero[of R K]] by simp
next
assume "q \<noteq> []" thus ?thesis
using long_divisionE[OF assms] unfolding long_divides_def univ_poly_mult univ_poly_add by simp
qed
lemma (in domain) pmod_degree:
assumes "subfield K R" and "p \<in> carrier (K[X])" and "q \<in> carrier (K[X])" "q \<noteq> []"
shows "p pmod q = [] \<or> degree (p pmod q) < degree q"
using long_divisionE[OF assms] unfolding long_divides_def by auto
lemma (in domain) pmod_const:
assumes "subfield K R" and "p \<in> carrier (K[X])" "q \<in> carrier (K[X])" and "degree q > degree p"
shows "p pdiv q = []" and "p pmod q = p"
proof -
have "p pdiv q = [] \<and> p pmod q = p"
proof (cases)
interpret UP: ring "K[X]"
using univ_poly_is_ring[OF subfieldE(1)[OF assms(1)]] .
assume "q \<noteq> []"
have "p = (q \<otimes>\<^bsub>K[X]\<^esub> []) \<oplus>\<^bsub>K[X]\<^esub> p"
using assms(2-3) unfolding sym[OF univ_poly_zero[of R K]] by simp
moreover have "([], p) \<in> carrier (poly_ring R) \<times> carrier (poly_ring R)"
using carrier_polynomial_shell[OF subfieldE(1)[OF assms(1)] assms(2)] by auto
ultimately have "long_divides p q ([], p)"
using assms(4) unfolding long_divides_def univ_poly_mult univ_poly_add by auto
with \<open>q \<noteq> []\<close> show ?thesis
using long_divisionI[OF assms(1-3)] by auto
qed (simp add: pmod_def pdiv_def)
thus "p pdiv q = []" and "p pmod q = p"
by auto
qed
lemma (in domain) long_division_zero:
assumes "subfield K R" and "q \<in> carrier (K[X])" shows "[] pdiv q = []" and "[] pmod q = []"
proof -
interpret UP: ring "poly_ring R"
using univ_poly_is_ring[OF carrier_is_subring] .
have "[] pdiv q = [] \<and> [] pmod q = []"
proof (cases)
assume "q \<noteq> []"
have "q \<in> carrier (poly_ring R)"
using carrier_polynomial_shell[OF subfieldE(1)[OF assms(1)] assms(2)] .
hence "long_divides [] q ([], [])"
unfolding long_divides_def sym[OF univ_poly_zero[of R "carrier R"]] by auto
with \<open>q \<noteq> []\<close> show ?thesis
using long_divisionI[OF assms(1) univ_poly_zero_closed assms(2)] by simp
qed (simp add: pmod_def pdiv_def)
thus "[] pdiv q = []" and "[] pmod q = []"
by auto
qed
lemma (in domain) long_division_a_inv:
assumes "subfield K R" and "p \<in> carrier (K[X])" "q \<in> carrier (K[X])"
shows "((\<ominus>\<^bsub>K[X]\<^esub> p) pdiv q) = \<ominus>\<^bsub>K[X]\<^esub> (p pdiv q)" (is "?pdiv")
and "((\<ominus>\<^bsub>K[X]\<^esub> p) pmod q) = \<ominus>\<^bsub>K[X]\<^esub> (p pmod q)" (is "?pmod")
proof -
interpret UP: ring "K[X]"
using univ_poly_is_ring[OF subfieldE(1)[OF assms(1)]] .
have "?pdiv \<and> ?pmod"
proof (cases)
assume "q = []" thus ?thesis
unfolding pmod_def pdiv_def sym[OF univ_poly_zero[of R K]] by simp
next
assume not_nil: "q \<noteq> []"
have "\<ominus>\<^bsub>K[X]\<^esub> p = \<ominus>\<^bsub>K[X]\<^esub> ((q \<otimes>\<^bsub>K[X]\<^esub> (p pdiv q)) \<oplus>\<^bsub>K[X]\<^esub> (p pmod q))"
using pdiv_pmod[OF assms] by simp
hence "\<ominus>\<^bsub>K[X]\<^esub> p = (q \<otimes>\<^bsub>K[X]\<^esub> (\<ominus>\<^bsub>K[X]\<^esub> (p pdiv q))) \<oplus>\<^bsub>K[X]\<^esub> (\<ominus>\<^bsub>K[X]\<^esub> (p pmod q))"
using assms(2-3) long_division_closed[OF assms] by algebra
moreover have "\<ominus>\<^bsub>K[X]\<^esub> (p pdiv q) \<in> carrier (K[X])" "\<ominus>\<^bsub>K[X]\<^esub> (p pmod q) \<in> carrier (K[X])"
using long_division_closed[OF assms] by algebra+
hence "(\<ominus>\<^bsub>K[X]\<^esub> (p pdiv q), \<ominus>\<^bsub>K[X]\<^esub> (p pmod q)) \<in> carrier (poly_ring R) \<times> carrier (poly_ring R)"
using carrier_polynomial_shell[OF subfieldE(1)[OF assms(1)]] by auto
moreover have "\<ominus>\<^bsub>K[X]\<^esub> (p pmod q) = [] \<or> degree (\<ominus>\<^bsub>K[X]\<^esub> (p pmod q)) < degree q"
using univ_poly_a_inv_length[OF subfieldE(1)[OF assms(1)]
long_division_closed(2)[OF assms]] pmod_degree[OF assms not_nil]
by auto
ultimately have "long_divides (\<ominus>\<^bsub>K[X]\<^esub> p) q (\<ominus>\<^bsub>K[X]\<^esub> (p pdiv q), \<ominus>\<^bsub>K[X]\<^esub> (p pmod q))"
unfolding long_divides_def univ_poly_mult univ_poly_add by simp
thus ?thesis
using long_divisionI[OF assms(1) UP.a_inv_closed[OF assms(2)] assms(3) not_nil] by simp
qed
thus ?pdiv and ?pmod
by auto
qed
lemma (in domain) long_division_add:
assumes "subfield K R" and "a \<in> carrier (K[X])" "b \<in> carrier (K[X])" "q \<in> carrier (K[X])"
shows "(a \<oplus>\<^bsub>K[X]\<^esub> b) pdiv q = (a pdiv q) \<oplus>\<^bsub>K[X]\<^esub> (b pdiv q)" (is "?pdiv")
and "(a \<oplus>\<^bsub>K[X]\<^esub> b) pmod q = (a pmod q) \<oplus>\<^bsub>K[X]\<^esub> (b pmod q)" (is "?pmod")
proof -
let ?pdiv_add = "(a pdiv q) \<oplus>\<^bsub>K[X]\<^esub> (b pdiv q)"
let ?pmod_add = "(a pmod q) \<oplus>\<^bsub>K[X]\<^esub> (b pmod q)"
interpret UP: ring "K[X]"
using univ_poly_is_ring[OF subfieldE(1)[OF assms(1)]] .
have "?pdiv \<and> ?pmod"
proof (cases)
assume "q = []" thus ?thesis
using assms(2-3) unfolding pmod_def pdiv_def sym[OF univ_poly_zero[of R K]] by simp
next
note in_carrier = long_division_closed[OF assms(1,2,4)]
long_division_closed[OF assms(1,3,4)]
assume "q \<noteq> []"
have "a \<oplus>\<^bsub>K[X]\<^esub> b = ((q \<otimes>\<^bsub>K[X]\<^esub> (a pdiv q)) \<oplus>\<^bsub>K[X]\<^esub> (a pmod q)) \<oplus>\<^bsub>K[X]\<^esub>
((q \<otimes>\<^bsub>K[X]\<^esub> (b pdiv q)) \<oplus>\<^bsub>K[X]\<^esub> (b pmod q))"
using assms(2-3)[THEN pdiv_pmod[OF assms(1) _ assms(4)]] by simp
hence "a \<oplus>\<^bsub>K[X]\<^esub> b = (q \<otimes>\<^bsub>K[X]\<^esub> ?pdiv_add) \<oplus>\<^bsub>K[X]\<^esub> ?pmod_add"
using assms(4) in_carrier by algebra
moreover have "(?pdiv_add, ?pmod_add) \<in> carrier (poly_ring R) \<times> carrier (poly_ring R)"
using in_carrier carrier_polynomial_shell[OF subfieldE(1)[OF assms(1)]] by auto
moreover have "?pmod_add = [] \<or> degree ?pmod_add < degree q"
proof (cases)
assume "?pmod_add \<noteq> []"
hence "a pmod q \<noteq> [] \<or> b pmod q \<noteq> []"
using in_carrier(2,4) unfolding sym[OF univ_poly_zero[of R K]] by auto
moreover from \<open>q \<noteq> []\<close>
have "a pmod q = [] \<or> degree (a pmod q) < degree q" and "b pmod q = [] \<or> degree (b pmod q) < degree q"
using assms(2-3)[THEN pmod_degree[OF assms(1) _ assms(4)]] by auto
ultimately have "max (degree (a pmod q)) (degree (b pmod q)) < degree q"
by auto
thus ?thesis
using poly_add_degree le_less_trans unfolding univ_poly_add by blast
qed simp
ultimately have "long_divides (a \<oplus>\<^bsub>K[X]\<^esub> b) q (?pdiv_add, ?pmod_add)"
unfolding long_divides_def univ_poly_mult univ_poly_add by simp
with \<open>q \<noteq> []\<close> show ?thesis
using long_divisionI[OF assms(1) UP.a_closed[OF assms(2-3)] assms(4)] by simp
qed
thus ?pdiv and ?pmod
by auto
qed
lemma (in domain) long_division_add_iff:
assumes "subfield K R"
and "a \<in> carrier (K[X])" "b \<in> carrier (K[X])" "c \<in> carrier (K[X])" "q \<in> carrier (K[X])"
shows "a pmod q = b pmod q \<longleftrightarrow> (a \<oplus>\<^bsub>K[X]\<^esub> c) pmod q = (b \<oplus>\<^bsub>K[X]\<^esub> c) pmod q"
proof -
interpret UP: ring "K[X]"
using univ_poly_is_ring[OF subfieldE(1)[OF assms(1)]] .
show ?thesis
using assms(2-4)[THEN long_division_closed(2)[OF assms(1) _ assms(5)]]
unfolding assms(2-3)[THEN long_division_add(2)[OF assms(1) _ assms(4-5)]] by auto
qed
lemma (in domain) pdivides_iff:
assumes "subfield K R" and "polynomial K p" "polynomial K q"
shows "p pdivides q \<longleftrightarrow> p divides\<^bsub>K[X]\<^esub> q"
proof
show "p divides\<^bsub>K [X]\<^esub> q \<Longrightarrow> p pdivides q"
using carrier_polynomial[OF subfieldE(1)[OF assms(1)]]
unfolding pdivides_def factor_def univ_poly_mult univ_poly_carrier by auto
next
interpret UP: ring "poly_ring R"
using univ_poly_is_ring[OF carrier_is_subring] .
have in_carrier: "p \<in> carrier (poly_ring R)" "q \<in> carrier (poly_ring R)"
using carrier_polynomial[OF subfieldE(1)[OF assms(1)]] assms
unfolding univ_poly_carrier by auto
assume "p pdivides q"
then obtain b where "b \<in> carrier (poly_ring R)" and "q = p \<otimes>\<^bsub>poly_ring R\<^esub> b"
unfolding pdivides_def factor_def by blast
show "p divides\<^bsub>K[X]\<^esub> q"
proof (cases)
assume "p = []"
with \<open>b \<in> carrier (poly_ring R)\<close> and \<open>q = p \<otimes>\<^bsub>poly_ring R\<^esub> b\<close> have "q = []"
unfolding univ_poly_mult sym[OF univ_poly_carrier]
using poly_mult_zero(1)[OF polynomial_incl] by simp
with \<open>p = []\<close> show ?thesis
using poly_mult_zero(2)[of "[]"]
unfolding factor_def univ_poly_mult by auto
next
interpret UP: ring "poly_ring R"
using univ_poly_is_ring[OF carrier_is_subring] .
assume "p \<noteq> []"
from \<open>p pdivides q\<close> obtain b where "b \<in> carrier (poly_ring R)" and "q = p \<otimes>\<^bsub>poly_ring R\<^esub> b"
unfolding pdivides_def factor_def by blast
moreover have "p \<in> carrier (poly_ring R)" and "q \<in> carrier (poly_ring R)"
using assms carrier_polynomial[OF subfieldE(1)[OF assms(1)]] unfolding univ_poly_carrier by auto
ultimately have "q = (p \<otimes>\<^bsub>poly_ring R\<^esub> b) \<oplus>\<^bsub>poly_ring R\<^esub> \<zero>\<^bsub>poly_ring R\<^esub>"
by algebra
with \<open>b \<in> carrier (poly_ring R)\<close> have "long_divides q p (b, [])"
unfolding long_divides_def univ_poly_zero by auto
with \<open>p \<noteq> []\<close> have "b \<in> carrier (K[X])"
using long_divisionI[of K q p b] long_division_closed[of K q p] assms
unfolding univ_poly_carrier by auto
with \<open>q = p \<otimes>\<^bsub>poly_ring R\<^esub> b\<close> show ?thesis
unfolding factor_def univ_poly_mult by blast
qed
qed
lemma (in domain) pdivides_iff_shell:
assumes "subfield K R" and "p \<in> carrier (K[X])" "q \<in> carrier (K[X])"
shows "p pdivides q \<longleftrightarrow> p divides\<^bsub>K[X]\<^esub> q"
using pdivides_iff assms by (simp add: univ_poly_carrier)
lemma (in domain) pmod_zero_iff_pdivides:
assumes "subfield K R" and "p \<in> carrier (K[X])" "q \<in> carrier (K[X])"
shows "p pmod q = [] \<longleftrightarrow> q pdivides p"
proof -
interpret UP: domain "K[X]"
using univ_poly_is_domain[OF subfieldE(1)[OF assms(1)]] .
show ?thesis
proof
assume pmod: "p pmod q = []"
have "p pdiv q \<in> carrier (K[X])" and "p pmod q \<in> carrier (K[X])"
using long_division_closed[OF assms] by auto
hence "p = q \<otimes>\<^bsub>K[X]\<^esub> (p pdiv q)"
using pdiv_pmod[OF assms] assms(3) unfolding pmod sym[OF univ_poly_zero[of R K]] by algebra
with \<open>p pdiv q \<in> carrier (K[X])\<close> show "q pdivides p"
unfolding pdivides_iff_shell[OF assms(1,3,2)] factor_def by blast
next
assume "q pdivides p" show "p pmod q = []"
proof (cases)
assume "q = []" with \<open>q pdivides p\<close> show ?thesis
using zero_pdivides unfolding pmod_def by simp
next
assume "q \<noteq> []"
from \<open>q pdivides p\<close> obtain r where "r \<in> carrier (K[X])" and "p = q \<otimes>\<^bsub>K[X]\<^esub> r"
unfolding pdivides_iff_shell[OF assms(1,3,2)] factor_def by blast
hence "p = (q \<otimes>\<^bsub>K[X]\<^esub> r) \<oplus>\<^bsub>K[X]\<^esub> []"
using assms(2) unfolding sym[OF univ_poly_zero[of R K]] by simp
moreover from \<open>r \<in> carrier (K[X])\<close> have "r \<in> carrier (poly_ring R)"
using carrier_polynomial_shell[OF subfieldE(1)[OF assms(1)]] by auto
ultimately have "long_divides p q (r, [])"
unfolding long_divides_def univ_poly_mult univ_poly_add by auto
with \<open>q \<noteq> []\<close> show ?thesis
using long_divisionI[OF assms] by simp
qed
qed
qed
lemma (in domain) same_pmod_iff_pdivides:
assumes "subfield K R" and "a \<in> carrier (K[X])" "b \<in> carrier (K[X])" "q \<in> carrier (K[X])"
shows "a pmod q = b pmod q \<longleftrightarrow> q pdivides (a \<ominus>\<^bsub>K[X]\<^esub> b)"
proof -
interpret UP: domain "K[X]"
using univ_poly_is_domain[OF subfieldE(1)[OF assms(1)]] .
have "a pmod q = b pmod q \<longleftrightarrow> (a \<oplus>\<^bsub>K[X]\<^esub> (\<ominus>\<^bsub>K[X]\<^esub> b)) pmod q = (b \<oplus>\<^bsub>K[X]\<^esub> (\<ominus>\<^bsub>K[X]\<^esub> b)) pmod q"
using long_division_add_iff[OF assms(1-3) UP.a_inv_closed[OF assms(3)] assms(4)] .
also have " ... \<longleftrightarrow> (a \<ominus>\<^bsub>K[X]\<^esub> b) pmod q = \<zero>\<^bsub>K[X]\<^esub> pmod q"
using assms(2-3) by algebra
also have " ... \<longleftrightarrow> q pdivides (a \<ominus>\<^bsub>K[X]\<^esub> b)"
using pmod_zero_iff_pdivides[OF assms(1) UP.minus_closed[OF assms(2-3)] assms(4)]
unfolding univ_poly_zero long_division_zero(2)[OF assms(1,4)] .
finally show ?thesis .
qed
lemma (in domain) pdivides_imp_degree_le:
assumes "subring K R" and "p \<in> carrier (K[X])" "q \<in> carrier (K[X])" "q \<noteq> []"
shows "p pdivides q \<Longrightarrow> degree p \<le> degree q"
proof -
assume "p pdivides q"
then obtain r where r: "polynomial (carrier R) r" "q = poly_mult p r"
unfolding pdivides_def factor_def univ_poly_mult univ_poly_carrier by blast
moreover have p: "polynomial (carrier R) p"
using assms(2) carrier_polynomial[OF assms(1)] unfolding univ_poly_carrier by auto
moreover have "p \<noteq> []" and "r \<noteq> []"
using poly_mult_zero(2)[OF polynomial_incl[OF p]] r(2) assms(4) by auto
ultimately show "degree p \<le> degree q"
using poly_mult_degree_eq[OF carrier_is_subring, of p r] by auto
qed
lemma (in domain) pprimeE:
assumes "subfield K R" "p \<in> carrier (K[X])" "pprime K p"
shows "p \<noteq> []" "p \<notin> Units (K[X])"
and "\<And>q r. \<lbrakk> q \<in> carrier (K[X]); r \<in> carrier (K[X])\<rbrakk> \<Longrightarrow>
p pdivides (q \<otimes>\<^bsub>K[X]\<^esub> r) \<Longrightarrow> p pdivides q \<or> p pdivides r"
using assms(2-3) poly_mult_closed[OF subfieldE(1)[OF assms(1)]] pdivides_iff[OF assms(1)]
unfolding ring_prime_def prime_def
by (auto simp add: univ_poly_mult univ_poly_carrier univ_poly_zero)
lemma (in domain) pprimeI:
assumes "subfield K R" "p \<in> carrier (K[X])" "p \<noteq> []" "p \<notin> Units (K[X])"
and "\<And>q r. \<lbrakk> q \<in> carrier (K[X]); r \<in> carrier (K[X])\<rbrakk> \<Longrightarrow>
p pdivides (q \<otimes>\<^bsub>K[X]\<^esub> r) \<Longrightarrow> p pdivides q \<or> p pdivides r"
shows "pprime K p"
using assms(2-5) poly_mult_closed[OF subfieldE(1)[OF assms(1)]] pdivides_iff[OF assms(1)]
unfolding ring_prime_def prime_def
by (auto simp add: univ_poly_mult univ_poly_carrier univ_poly_zero)
lemma (in domain) associated_polynomials_iff:
assumes "subfield K R" and "p \<in> carrier (K[X])" "q \<in> carrier (K[X])"
shows "p \<sim>\<^bsub>K[X]\<^esub> q \<longleftrightarrow> (\<exists>k \<in> K - { \<zero> }. p = [ k ] \<otimes>\<^bsub>K[X]\<^esub> q)"
using domain.ring_associated_iff[OF univ_poly_is_domain[OF subfieldE(1)[OF assms(1)]] assms(2-3)]
unfolding univ_poly_units[OF assms(1)] by auto
corollary (in domain) associated_polynomials_imp_same_length: (* stronger than "imp_same_degree" *)
assumes "subring K R" and "p \<in> carrier (K[X])" and "q \<in> carrier (K[X])"
shows "p \<sim>\<^bsub>K[X]\<^esub> q \<Longrightarrow> length p = length q"
proof -
{ fix p q
assume p: "p \<in> carrier (K[X])" and q: "q \<in> carrier (K[X])" and "p \<sim>\<^bsub>K[X]\<^esub> q"
have "length p \<le> length q"
proof (cases "q = []")
case True with \<open>p \<sim>\<^bsub>K[X]\<^esub> q\<close> have "p = []"
unfolding associated_def True factor_def univ_poly_def by auto
thus ?thesis
using True by simp
next
case False
from \<open>p \<sim>\<^bsub>K[X]\<^esub> q\<close> have "p divides\<^bsub>K [X]\<^esub> q"
unfolding associated_def by simp
hence "p divides\<^bsub>poly_ring R\<^esub> q"
using carrier_polynomial[OF assms(1)]
unfolding factor_def univ_poly_carrier univ_poly_mult by auto
with \<open>q \<noteq> []\<close> have "degree p \<le> degree q"
using pdivides_imp_degree_le[OF assms(1) p q] unfolding pdivides_def by simp
with \<open>q \<noteq> []\<close> show ?thesis
by (cases "p = []", auto simp add: Suc_leI le_diff_iff)
qed
} note aux_lemma = this
interpret UP: domain "K[X]"
using univ_poly_is_domain[OF assms(1)] .
assume "p \<sim>\<^bsub>K[X]\<^esub> q" thus ?thesis
using aux_lemma[OF assms(2-3)] aux_lemma[OF assms(3,2) UP.associated_sym] by simp
qed
lemma (in ring) divides_pirreducible_condition:
assumes "pirreducible K q" and "p \<in> carrier (K[X])"
shows "p divides\<^bsub>K[X]\<^esub> q \<Longrightarrow> p \<in> Units (K[X]) \<or> p \<sim>\<^bsub>K[X]\<^esub> q"
using divides_irreducible_condition[of "K[X]" q p] assms
unfolding ring_irreducible_def by auto
subsection \<open>Polynomial Power\<close>
lemma (in domain) polynomial_pow_not_zero:
assumes "p \<in> carrier (poly_ring R)" and "p \<noteq> []"
shows "p [^]\<^bsub>poly_ring R\<^esub> (n::nat) \<noteq> []"
proof -
interpret UP: domain "poly_ring R"
using univ_poly_is_domain[OF carrier_is_subring] .
from assms UP.integral show ?thesis
unfolding sym[OF univ_poly_zero[of R "carrier R"]]
by (induction n, auto)
qed
lemma (in domain) subring_polynomial_pow_not_zero:
assumes "subring K R" and "p \<in> carrier (K[X])" and "p \<noteq> []"
shows "p [^]\<^bsub>K[X]\<^esub> (n::nat) \<noteq> []"
using domain.polynomial_pow_not_zero[OF subring_is_domain, of K p n] assms
unfolding univ_poly_consistent[OF assms(1)] by simp
lemma (in domain) polynomial_pow_degree:
assumes "p \<in> carrier (poly_ring R)"
shows "degree (p [^]\<^bsub>poly_ring R\<^esub> n) = n * degree p"
proof -
interpret UP: domain "poly_ring R"
using univ_poly_is_domain[OF carrier_is_subring] .
show ?thesis
proof (induction n)
case 0 thus ?case
using UP.nat_pow_0 unfolding univ_poly_one by auto
next
let ?ppow = "\<lambda>n. p [^]\<^bsub>poly_ring R\<^esub> n"
case (Suc n) thus ?case
proof (cases "p = []")
case True thus ?thesis
using univ_poly_zero[of R "carrier R"] UP.r_null assms by auto
next
case False
hence "?ppow n \<in> carrier (poly_ring R)" and "?ppow n \<noteq> []" and "p \<noteq> []"
using polynomial_pow_not_zero[of p n] assms by (auto simp add: univ_poly_one)
thus ?thesis
using poly_mult_degree_eq[OF carrier_is_subring, of "?ppow n" p] Suc assms
unfolding univ_poly_carrier univ_poly_zero
by (auto simp add: add.commute univ_poly_mult)
qed
qed
qed
lemma (in domain) subring_polynomial_pow_degree:
assumes "subring K R" and "p \<in> carrier (K[X])"
shows "degree (p [^]\<^bsub>K[X]\<^esub> n) = n * degree p"
using domain.polynomial_pow_degree[OF subring_is_domain, of K p n] assms
unfolding univ_poly_consistent[OF assms(1)] by simp
lemma (in domain) polynomial_pow_division:
assumes "p \<in> carrier (poly_ring R)" and "(n::nat) \<le> m"
shows "(p [^]\<^bsub>poly_ring R\<^esub> n) pdivides (p [^]\<^bsub>poly_ring R\<^esub> m)"
proof -
interpret UP: domain "poly_ring R"
using univ_poly_is_domain[OF carrier_is_subring] .
let ?ppow = "\<lambda>n. p [^]\<^bsub>poly_ring R\<^esub> n"
have "?ppow n \<otimes>\<^bsub>poly_ring R\<^esub> ?ppow k = ?ppow (n + k)" for k
using assms(1) by (simp add: UP.nat_pow_mult)
thus ?thesis
using dividesI[of "?ppow (m - n)" "poly_ring R" "?ppow m" "?ppow n"] assms
unfolding pdivides_def by auto
qed
lemma (in domain) subring_polynomial_pow_division:
assumes "subring K R" and "p \<in> carrier (K[X])" and "(n::nat) \<le> m"
shows "(p [^]\<^bsub>K[X]\<^esub> n) divides\<^bsub>K[X]\<^esub> (p [^]\<^bsub>K[X]\<^esub> m)"
using domain.polynomial_pow_division[OF subring_is_domain, of K p n m] assms
unfolding univ_poly_consistent[OF assms(1)] pdivides_def by simp
lemma (in domain) pirreducible_pow_pdivides_iff:
assumes "subfield K R" "p \<in> carrier (K[X])" "q \<in> carrier (K[X])" "r \<in> carrier (K[X])"
and "pirreducible K p" and "\<not> (p pdivides q)"
shows "(p [^]\<^bsub>K[X]\<^esub> (n :: nat)) pdivides (q \<otimes>\<^bsub>K[X]\<^esub> r) \<longleftrightarrow> (p [^]\<^bsub>K[X]\<^esub> n) pdivides r"
proof -
interpret UP: principal_domain "K[X]"
using univ_poly_is_principal[OF assms(1)] .
show ?thesis
proof (cases "r = []")
case True with \<open>q \<in> carrier (K[X])\<close> have "q \<otimes>\<^bsub>K[X]\<^esub> r = []" and "r = []"
unfolding sym[OF univ_poly_zero[of R K]] by auto
thus ?thesis
using pdivides_zero[OF subfieldE(1),of K] assms by auto
next
case False then have not_zero: "p \<noteq> []" "q \<noteq> []" "r \<noteq> []" "q \<otimes>\<^bsub>K[X]\<^esub> r \<noteq> []"
using subfieldE(1) pdivides_zero[OF _ assms(2)] assms(1-2,5-6) pirreducibleE(1)
UP.integral_iff[OF assms(3-4)] univ_poly_zero[of R K] by auto
from \<open>p \<noteq> []\<close>
have ppow: "p [^]\<^bsub>K[X]\<^esub> (n :: nat) \<noteq> []" "p [^]\<^bsub>K[X]\<^esub> (n :: nat) \<in> carrier (K[X])"
using subring_polynomial_pow_not_zero[OF subfieldE(1)] assms(1-2) by auto
have not_pdiv: "\<not> (p divides\<^bsub>mult_of (K[X])\<^esub> q)"
using assms(6) pdivides_iff_shell[OF assms(1-3)] unfolding pdivides_def by auto
have prime: "prime (mult_of (K[X])) p"
using assms(5) pprime_iff_pirreducible[OF assms(1-2)]
unfolding sym[OF UP.prime_eq_prime_mult[OF assms(2)]] ring_prime_def by simp
have "a pdivides b \<longleftrightarrow> a divides\<^bsub>mult_of (K[X])\<^esub> b"
if "a \<in> carrier (K[X])" "a \<noteq> \<zero>\<^bsub>K[X]\<^esub>" "b \<in> carrier (K[X])" "b \<noteq> \<zero>\<^bsub>K[X]\<^esub>" for a b
using that UP.divides_imp_divides_mult[of a b] divides_mult_imp_divides[of "K[X]" a b]
unfolding pdivides_iff_shell[OF assms(1) that(1,3)] by blast
thus ?thesis
using UP.mult_of.prime_pow_divides_iff[OF _ _ _ prime not_pdiv, of r] ppow not_zero assms(2-4)
unfolding nat_pow_mult_of carrier_mult_of mult_mult_of sym[OF univ_poly_zero[of R K]]
by (metis DiffI UP.m_closed singletonD)
qed
qed
lemma (in domain) subring_degree_one_imp_pirreducible:
assumes "subring K R" and "a \<in> Units (R \<lparr> carrier := K \<rparr>)" and "b \<in> K"
shows "pirreducible K [ a, b ]"
proof (rule pirreducibleI[OF assms(1)])
have "a \<in> K" and "a \<noteq> \<zero>"
using assms(2) subringE(1)[OF assms(1)] unfolding Units_def by auto
thus "[ a, b ] \<in> carrier (K[X])" and "[ a, b ] \<noteq> []" and "[ a, b ] \<notin> Units (K [X])"
using univ_poly_units_incl[OF assms(1)] assms(2-3)
unfolding sym[OF univ_poly_carrier] polynomial_def by auto
next
interpret UP: domain "K[X]"
using univ_poly_is_domain[OF assms(1)] .
{ fix q r
assume q: "q \<in> carrier (K[X])" and r: "r \<in> carrier (K[X])" and "[ a, b ] = q \<otimes>\<^bsub>K[X]\<^esub> r"
hence not_zero: "q \<noteq> []" "r \<noteq> []"
by (metis UP.integral_iff list.distinct(1) univ_poly_zero)+
have "degree (q \<otimes>\<^bsub>K[X]\<^esub> r) = degree q + degree r"
using not_zero poly_mult_degree_eq[OF assms(1)] q r
by (simp add: univ_poly_carrier univ_poly_mult)
with sym[OF \<open>[ a, b ] = q \<otimes>\<^bsub>K[X]\<^esub> r\<close>] have "degree q + degree r = 1" and "q \<noteq> []" "r \<noteq> []"
using not_zero by auto
} note aux_lemma1 = this
{ fix q r
assume q: "q \<in> carrier (K[X])" "q \<noteq> []" and r: "r \<in> carrier (K[X])" "r \<noteq> []"
and "[ a, b ] = q \<otimes>\<^bsub>K[X]\<^esub> r" and "degree q = 1" and "degree r = 0"
hence "length q = Suc (Suc 0)" and "length r = Suc 0"
by (linarith, metis add.right_neutral add_eq_if length_0_conv)
from \<open>length q = Suc (Suc 0)\<close> obtain c d where q_def: "q = [ c, d ]"
by (metis length_0_conv length_Cons list.exhaust nat.inject)
from \<open>length r = Suc 0\<close> obtain e where r_def: "r = [ e ]"
by (metis length_0_conv length_Suc_conv)
from \<open>r = [ e ]\<close> and \<open>q = [ c, d ]\<close>
have c: "c \<in> K" "c \<noteq> \<zero>" and d: "d \<in> K" and e: "e \<in> K" "e \<noteq> \<zero>"
using r q subringE(1)[OF assms(1)] unfolding sym[OF univ_poly_carrier] polynomial_def by auto
with sym[OF \<open>[ a, b ] = q \<otimes>\<^bsub>K[X]\<^esub> r\<close>] have "a = c \<otimes> e"
using poly_mult_lead_coeff[OF assms(1), of q r]
unfolding polynomial_def sym[OF univ_poly_mult[of R K]] r_def q_def by auto
obtain inv_a where a: "a \<in> K" and inv_a: "inv_a \<in> K" "a \<otimes> inv_a = \<one>" "inv_a \<otimes> a = \<one>"
using assms(2) unfolding Units_def by auto
hence "a \<noteq> \<zero>" and "inv_a \<noteq> \<zero>"
using subringE(1)[OF assms(1)] integral_iff by auto
with \<open>c \<in> K\<close> and \<open>c \<noteq> \<zero>\<close> have in_carrier: "[ c \<otimes> inv_a ] \<in> carrier (K[X])"
using subringE(1,6)[OF assms(1)] inv_a integral
unfolding sym[OF univ_poly_carrier] polynomial_def
by (auto, meson subsetD)
moreover have "[ c \<otimes> inv_a ] \<otimes>\<^bsub>K[X]\<^esub> r = [ \<one> ]"
using \<open>a = c \<otimes> e\<close> a inv_a c e subsetD[OF subringE(1)[OF assms(1)]]
unfolding r_def univ_poly_mult by (auto) (simp add: m_assoc m_lcomm integral_iff)+
ultimately have "r \<in> Units (K[X])"
using r(1) UP.m_comm[OF in_carrier r(1)] unfolding sym[OF univ_poly_one[of R K]] Units_def by auto
} note aux_lemma2 = this
fix q r
assume q: "q \<in> carrier (K[X])" and r: "r \<in> carrier (K[X])" and qr: "[ a, b ] = q \<otimes>\<^bsub>K[X]\<^esub> r"
thus "q \<in> Units (K[X]) \<or> r \<in> Units (K[X])"
using aux_lemma1[OF q r qr] aux_lemma2[of q r] aux_lemma2[of r q] UP.m_comm add_is_1 by auto
qed
lemma (in domain) degree_one_imp_pirreducible:
assumes "subfield K R" and "p \<in> carrier (K[X])" and "degree p = 1"
shows "pirreducible K p"
proof -
from \<open>degree p = 1\<close> have "length p = Suc (Suc 0)"
by simp
then obtain a b where p: "p = [ a, b ]"
by (metis length_0_conv length_Cons nat.inject neq_Nil_conv)
with \<open>p \<in> carrier (K[X])\<close> show ?thesis
using subring_degree_one_imp_pirreducible[OF subfieldE(1)[OF assms(1)], of a b]
subfield.subfield_Units[OF assms(1)]
unfolding sym[OF univ_poly_carrier] polynomial_def by auto
qed
lemma (in ring) degree_oneE[elim]:
assumes "p \<in> carrier (K[X])" and "degree p = 1"
and "\<And>a b. \<lbrakk> a \<in> K; a \<noteq> \<zero>; b \<in> K; p = [ a, b ] \<rbrakk> \<Longrightarrow> P"
shows P
proof -
from \<open>degree p = 1\<close> have "length p = Suc (Suc 0)"
by simp
then obtain a b where "p = [ a, b ]"
by (metis length_0_conv length_Cons nat.inject neq_Nil_conv)
with \<open>p \<in> carrier (K[X])\<close> have "a \<in> K" and "a \<noteq> \<zero>" and "b \<in> K"
unfolding sym[OF univ_poly_carrier] polynomial_def by auto
with \<open>p = [ a, b ]\<close> show ?thesis
using assms(3) by simp
qed
lemma (in domain) subring_degree_one_associatedI:
assumes "subring K R" and "a \<in> K" "a' \<in> K" and "b \<in> K" and "a \<otimes> a' = \<one>"
shows "[ a , b ] \<sim>\<^bsub>K[X]\<^esub> [ \<one>, a' \<otimes> b ]"
proof -
from \<open>a \<otimes> a' = \<one>\<close> have not_zero: "a \<noteq> \<zero>" "a' \<noteq> \<zero>"
using subringE(1)[OF assms(1)] assms(2-3) by auto
hence "[ a, b ] = [ a ] \<otimes>\<^bsub>K[X]\<^esub> [ \<one>, a' \<otimes> b ]"
using assms(2-4)[THEN subsetD[OF subringE(1)[OF assms(1)]]] assms(5) m_assoc
unfolding univ_poly_mult by fastforce
moreover have "[ a, b ] \<in> carrier (K[X])" and "[ \<one>, a' \<otimes> b ] \<in> carrier (K[X])"
using subringE(1,3,6)[OF assms(1)] not_zero one_not_zero assms
unfolding sym[OF univ_poly_carrier] polynomial_def by auto
moreover have "[ a ] \<in> Units (K[X])"
proof -
from \<open>a \<noteq> \<zero>\<close> and \<open>a' \<noteq> \<zero>\<close> have "[ a ] \<in> carrier (K[X])" and "[ a' ] \<in> carrier (K[X])"
using assms(2-3) unfolding sym[OF univ_poly_carrier] polynomial_def by auto
moreover have "a' \<otimes> a = \<one>"
using subsetD[OF subringE(1)[OF assms(1)]] assms m_comm by simp
hence "[ a ] \<otimes>\<^bsub>K[X]\<^esub> [ a' ] = [ \<one> ]" and "[ a' ] \<otimes>\<^bsub>K[X]\<^esub> [ a ] = [ \<one> ]"
using assms unfolding univ_poly_mult by auto
ultimately show ?thesis
unfolding sym[OF univ_poly_one[of R K]] Units_def by blast
qed
ultimately show ?thesis
using domain.ring_associated_iff[OF univ_poly_is_domain[OF assms(1)]] by blast
qed
lemma (in domain) degree_one_associatedI:
assumes "subfield K R" and "p \<in> carrier (K[X])" and "degree p = 1"
shows "p \<sim>\<^bsub>K[X]\<^esub> [ \<one>, inv (lead_coeff p) \<otimes> (const_term p) ]"
proof -
from \<open>p \<in> carrier (K[X])\<close> and \<open>degree p = 1\<close>
obtain a b where "p = [ a, b ]" and "a \<in> K" "a \<noteq> \<zero>" and "b \<in> K"
by auto
thus ?thesis
using subring_degree_one_associatedI[OF subfieldE(1)[OF assms(1)]]
subfield_m_inv[OF assms(1)] subsetD[OF subfieldE(3)[OF assms(1)]]
unfolding const_term_def
by auto
qed
subsection \<open>Ideals\<close>
lemma (in domain) exists_unique_gen:
assumes "subfield K R" "ideal I (K[X])" "I \<noteq> { [] }"
shows "\<exists>!p \<in> carrier (K[X]). lead_coeff p = \<one> \<and> I = PIdl\<^bsub>K[X]\<^esub> p"
(is "\<exists>!p. ?generator p")
proof -
interpret UP: principal_domain "K[X]"
using univ_poly_is_principal[OF assms(1)] .
obtain q where q: "q \<in> carrier (K[X])" "I = PIdl\<^bsub>K[X]\<^esub> q"
using UP.exists_gen[OF assms(2)] by blast
hence not_nil: "q \<noteq> []"
using UP.genideal_zero UP.cgenideal_eq_genideal[OF UP.zero_closed] assms(3)
by (auto simp add: univ_poly_zero)
hence "lead_coeff q \<in> K - { \<zero> }"
using q(1) unfolding univ_poly_def polynomial_def by auto
hence inv_lc_q: "inv (lead_coeff q) \<in> K - { \<zero> }" "inv (lead_coeff q) \<otimes> lead_coeff q = \<one>"
using subfield_m_inv[OF assms(1)] by auto
define p where "p = [ inv (lead_coeff q) ] \<otimes>\<^bsub>K[X]\<^esub> q"
have is_poly: "polynomial K [ inv (lead_coeff q) ]" "polynomial K q"
using inv_lc_q(1) q(1) unfolding univ_poly_def polynomial_def by auto
hence in_carrier: "p \<in> carrier (K[X])"
using UP.m_closed unfolding univ_poly_carrier p_def by simp
have lc_p: "lead_coeff p = \<one>"
using poly_mult_lead_coeff[OF subfieldE(1)[OF assms(1)] is_poly _ not_nil] inv_lc_q(2)
unfolding p_def univ_poly_mult[of R K] by simp
moreover have PIdl_p: "I = PIdl\<^bsub>K[X]\<^esub> p"
using UP.associated_iff_same_ideal[OF in_carrier q(1)] q(2) inv_lc_q(1) p_def
associated_polynomials_iff[OF assms(1) in_carrier q(1)]
by auto
ultimately have "?generator p"
using in_carrier by simp
moreover
have "\<And>r. \<lbrakk> r \<in> carrier (K[X]); lead_coeff r = \<one>; I = PIdl\<^bsub>K[X]\<^esub> r \<rbrakk> \<Longrightarrow> r = p"
proof -
fix r assume r: "r \<in> carrier (K[X])" "lead_coeff r = \<one>" "I = PIdl\<^bsub>K[X]\<^esub> r"
have "subring K R"
by (simp add: \<open>subfield K R\<close> subfieldE(1))
obtain k where k: "k \<in> K - { \<zero> }" "r = [ k ] \<otimes>\<^bsub>K[X]\<^esub> p"
using UP.associated_iff_same_ideal[OF r(1) in_carrier] PIdl_p r(3)
associated_polynomials_iff[OF assms(1) r(1) in_carrier]
by auto
hence "polynomial K [ k ]"
unfolding polynomial_def by simp
moreover have "p \<noteq> []"
using not_nil UP.associated_iff_same_ideal[OF in_carrier q(1)] q(2) PIdl_p
associated_polynomials_imp_same_length[OF \<open>subring K R\<close> in_carrier q(1)] by auto
ultimately have "lead_coeff r = k \<otimes> (lead_coeff p)"
using poly_mult_lead_coeff[OF subfieldE(1)[OF assms(1)]] in_carrier k(2)
unfolding univ_poly_def by (auto simp del: poly_mult.simps)
hence "k = \<one>"
using lc_p r(2) k(1) subfieldE(3)[OF assms(1)] by auto
hence "r = map ((\<otimes>) \<one>) p"
using poly_mult_const(1)[OF subfieldE(1)[OF assms(1)] _ k(1), of p] in_carrier
unfolding k(2) univ_poly_carrier[of R K] univ_poly_mult[of R K] by auto
moreover have "set p \<subseteq> carrier R"
using polynomial_in_carrier[OF subfieldE(1)[OF assms(1)]]
in_carrier univ_poly_carrier[of R K] by auto
hence "map ((\<otimes>) \<one>) p = p"
by (induct p) (auto)
ultimately show "r = p" by simp
qed
ultimately show ?thesis by blast
qed
proposition (in domain) exists_unique_pirreducible_gen:
assumes "subfield K R" "ring_hom_ring (K[X]) R h"
and "a_kernel (K[X]) R h \<noteq> { [] }" "a_kernel (K[X]) R h \<noteq> carrier (K[X])"
shows "\<exists>!p \<in> carrier (K[X]). pirreducible K p \<and> lead_coeff p = \<one> \<and> a_kernel (K[X]) R h = PIdl\<^bsub>K[X]\<^esub> p"
(is "\<exists>!p. ?generator p")
proof -
interpret UP: principal_domain "K[X]"
using univ_poly_is_principal[OF assms(1)] .
have "ideal (a_kernel (K[X]) R h) (K[X])"
using ring_hom_ring.kernel_is_ideal[OF assms(2)] .
then obtain p
where p: "p \<in> carrier (K[X])" "lead_coeff p = \<one>" "a_kernel (K[X]) R h = PIdl\<^bsub>K[X]\<^esub> p"
and unique:
"\<And>q. \<lbrakk> q \<in> carrier (K[X]); lead_coeff q = \<one>; a_kernel (K[X]) R h = PIdl\<^bsub>K[X]\<^esub> q \<rbrakk> \<Longrightarrow> q = p"
using exists_unique_gen[OF assms(1) _ assms(3)] by metis
have "p \<in> carrier (K[X]) - { [] }"
using UP.genideal_zero UP.cgenideal_eq_genideal[OF UP.zero_closed] assms(3) p(1,3)
by (auto simp add: univ_poly_zero)
hence "pprime K p"
using ring_hom_ring.primeideal_vimage[OF assms(2) UP.is_cring zeroprimeideal]
UP.primeideal_iff_prime[of p]
unfolding univ_poly_zero sym[OF p(3)] a_kernel_def' by simp
hence "pirreducible K p"
using pprime_iff_pirreducible[OF assms(1) p(1)] by simp
thus ?thesis
using p unique by metis
qed
lemma (in domain) cgenideal_pirreducible:
assumes "subfield K R" and "p \<in> carrier (K[X])" "pirreducible K p"
shows "\<lbrakk> pirreducible K q; q \<in> PIdl\<^bsub>K[X]\<^esub> p \<rbrakk> \<Longrightarrow> p \<sim>\<^bsub>K[X]\<^esub> q"
proof -
interpret UP: principal_domain "K[X]"
using univ_poly_is_principal[OF assms(1)] .
assume q: "pirreducible K q" "q \<in> PIdl\<^bsub>K[X]\<^esub> p"
hence in_carrier: "q \<in> carrier (K[X])"
using additive_subgroup.a_subset[OF ideal.axioms(1)[OF UP.cgenideal_ideal[OF assms(2)]]] by auto
hence "p divides\<^bsub>K[X]\<^esub> q"
by (meson q assms(2) UP.cgenideal_ideal UP.cgenideal_minimal UP.to_contain_is_to_divide)
then obtain r where r: "r \<in> carrier (K[X])" "q = p \<otimes>\<^bsub>K[X]\<^esub> r"
by auto
hence "r \<in> Units (K[X])"
using pirreducibleE(3)[OF _ in_carrier q(1) assms(2) r(1)] subfieldE(1)[OF assms(1)]
pirreducibleE(2)[OF _ assms(2-3)] by auto
thus "p \<sim>\<^bsub>K[X]\<^esub> q"
using UP.ring_associated_iff[OF in_carrier assms(2)] r(2) UP.associated_sym
unfolding UP.m_comm[OF assms(2) r(1)] by auto
qed
subsection \<open>Roots and Multiplicity\<close>
definition (in ring) is_root :: "'a list \<Rightarrow> 'a \<Rightarrow> bool"
where "is_root p x \<longleftrightarrow> (x \<in> carrier R \<and> eval p x = \<zero> \<and> p \<noteq> [])"
definition (in ring) alg_mult :: "'a list \<Rightarrow> 'a \<Rightarrow> nat"
where "alg_mult p x =
(if p = [] then 0 else
(if x \<in> carrier R then Greatest (\<lambda> n. ([ \<one>, \<ominus> x ] [^]\<^bsub>poly_ring R\<^esub> n) pdivides p) else 0))"
definition (in ring) roots :: "'a list \<Rightarrow> 'a multiset"
where "roots p = Abs_multiset (alg_mult p)"
definition (in ring) roots_on :: "'a set \<Rightarrow> 'a list \<Rightarrow> 'a multiset"
where "roots_on K p = roots p \<inter># mset_set K"
definition (in ring) splitted :: "'a list \<Rightarrow> bool"
where "splitted p \<longleftrightarrow> size (roots p) = degree p"
definition (in ring) splitted_on :: "'a set \<Rightarrow> 'a list \<Rightarrow> bool"
where "splitted_on K p \<longleftrightarrow> size (roots_on K p) = degree p"
lemma (in domain) pdivides_imp_root_sharing:
assumes "p \<in> carrier (poly_ring R)" "p pdivides q" and "a \<in> carrier R"
shows "eval p a = \<zero> \<Longrightarrow> eval q a = \<zero>"
proof -
from \<open>p pdivides q\<close> obtain r where r: "q = p \<otimes>\<^bsub>poly_ring R\<^esub> r" "r \<in> carrier (poly_ring R)"
unfolding pdivides_def factor_def by auto
hence "eval q a = (eval p a) \<otimes> (eval r a)"
using ring_hom_memE(2)[OF eval_is_hom[OF carrier_is_subring assms(3)] assms(1) r(2)] by simp
thus "eval p a = \<zero> \<Longrightarrow> eval q a = \<zero>"
using ring_hom_memE(1)[OF eval_is_hom[OF carrier_is_subring assms(3)] r(2)] by auto
qed
lemma (in domain) degree_one_root:
assumes "subfield K R" and "p \<in> carrier (K[X])" and "degree p = 1"
shows "eval p (\<ominus> (inv (lead_coeff p) \<otimes> (const_term p))) = \<zero>"
and "inv (lead_coeff p) \<otimes> (const_term p) \<in> K"
proof -
from \<open>degree p = 1\<close> have "length p = Suc (Suc 0)"
by simp
then obtain a b where p: "p = [ a, b ]"
by (metis (no_types, hide_lams) Suc_length_conv length_0_conv)
hence "a \<in> K - { \<zero> }" "b \<in> K" and in_carrier: "a \<in> carrier R" "b \<in> carrier R"
using assms(2) subfieldE(3)[OF assms(1)] unfolding sym[OF univ_poly_carrier] polynomial_def by auto
hence inv_a: "inv a \<in> carrier R" "a \<otimes> inv a = \<one>" and "inv a \<in> K"
using subfield_m_inv(1-2)[OF assms(1), of a] subfieldE(3)[OF assms(1)] by auto
hence "eval p (\<ominus> (inv a \<otimes> b)) = a \<otimes> (\<ominus> (inv a \<otimes> b)) \<oplus> b"
using in_carrier unfolding p by simp
also have " ... = \<ominus> (a \<otimes> (inv a \<otimes> b)) \<oplus> b"
using inv_a in_carrier by (simp add: r_minus)
also have " ... = \<zero>"
using in_carrier(2) unfolding sym[OF m_assoc[OF in_carrier(1) inv_a(1) in_carrier(2)]] inv_a(2) by algebra
finally have "eval p (\<ominus> (inv a \<otimes> b)) = \<zero>" .
moreover have ct: "const_term p = b"
using in_carrier unfolding p const_term_def by auto
ultimately show "eval p (\<ominus> (inv (lead_coeff p) \<otimes> (const_term p))) = \<zero>"
unfolding p by simp
from \<open>inv a \<in> K\<close> and \<open>b \<in> K\<close>
show "inv (lead_coeff p) \<otimes> (const_term p) \<in> K"
using p subringE(6)[OF subfieldE(1)[OF assms(1)]] unfolding ct by auto
qed
lemma (in domain) is_root_imp_pdivides:
assumes "p \<in> carrier (poly_ring R)"
shows "is_root p x \<Longrightarrow> [ \<one>, \<ominus> x ] pdivides p"
proof -
let ?b = "[ \<one> , \<ominus> x ]"
interpret UP: domain "poly_ring R"
using univ_poly_is_domain[OF carrier_is_subring] .
assume "is_root p x" hence x: "x \<in> carrier R" and is_root: "eval p x = \<zero>"
unfolding is_root_def by auto
hence b: "?b \<in> carrier (poly_ring R)"
unfolding sym[OF univ_poly_carrier] polynomial_def by auto
then obtain q r where q: "q \<in> carrier (poly_ring R)" and r: "r \<in> carrier (poly_ring R)"
and long_divides: "p = (?b \<otimes>\<^bsub>poly_ring R\<^esub> q) \<oplus>\<^bsub>poly_ring R\<^esub> r" "r = [] \<or> degree r < degree ?b"
using long_division_theorem[OF carrier_is_subring, of p ?b] assms by (auto simp add: univ_poly_carrier)
show ?thesis
proof (cases "r = []")
case True then have "r = \<zero>\<^bsub>poly_ring R\<^esub>"
unfolding univ_poly_zero[of R "carrier R"] .
thus ?thesis
using long_divides(1) q r b dividesI[OF q, of p ?b] by (simp add: pdivides_def)
next
case False then have "length r = Suc 0"
using long_divides(2) le_SucE by fastforce
then obtain a where "r = [ a ]" and a: "a \<in> carrier R" and "a \<noteq> \<zero>"
using r unfolding sym[OF univ_poly_carrier] polynomial_def
by (metis False length_0_conv length_Suc_conv list.sel(1) list.set_sel(1) subset_code(1))
have "eval p x = ((eval ?b x) \<otimes> (eval q x)) \<oplus> (eval r x)"
using long_divides(1) ring_hom_memE[OF eval_is_hom[OF carrier_is_subring x]] by (simp add: b q r)
also have " ... = eval r x"
using ring_hom_memE[OF eval_is_hom[OF carrier_is_subring x]] x b q r by (auto, algebra)
finally have "a = \<zero>"
using a unfolding \<open>r = [ a ]\<close> is_root by simp
with \<open>a \<noteq> \<zero>\<close> have False .. thus ?thesis ..
qed
qed
lemma (in domain) pdivides_imp_is_root:
assumes "p \<noteq> []" and "x \<in> carrier R"
shows "[ \<one>, \<ominus> x ] pdivides p \<Longrightarrow> is_root p x"
proof -
assume "[ \<one>, \<ominus> x ] pdivides p"
then obtain q where q: "q \<in> carrier (poly_ring R)" and pdiv: "p = [ \<one>, \<ominus> x ] \<otimes>\<^bsub>poly_ring R\<^esub> q"
unfolding pdivides_def by auto
moreover have "[ \<one>, \<ominus> x ] \<in> carrier (poly_ring R)"
using assms(2) unfolding sym[OF univ_poly_carrier] polynomial_def by simp
ultimately have "eval p x = \<zero>"
using ring_hom_memE[OF eval_is_hom[OF carrier_is_subring, of x]] assms(2) by (auto, algebra)
with \<open>p \<noteq> []\<close> and \<open>x \<in> carrier R\<close> show "is_root p x"
unfolding is_root_def by simp
qed
lemma (in domain) associated_polynomials_imp_same_is_root:
assumes "p \<in> carrier (poly_ring R)" and "q \<in> carrier (poly_ring R)" and "p \<sim>\<^bsub>poly_ring R\<^esub> q"
shows "is_root p x \<longleftrightarrow> is_root q x"
proof (cases "p = []")
case True with \<open>p \<sim>\<^bsub>poly_ring R\<^esub> q\<close> have "q = []"
unfolding associated_def True factor_def univ_poly_def by auto
thus ?thesis
using True unfolding is_root_def by simp
next
case False
interpret UP: domain "poly_ring R"
using univ_poly_is_domain[OF carrier_is_subring] .
{ fix p q
assume p: "p \<in> carrier (poly_ring R)" and q: "q \<in> carrier (poly_ring R)" and pq: "p \<sim>\<^bsub>poly_ring R\<^esub> q"
have "is_root p x \<Longrightarrow> is_root q x"
proof -
assume is_root: "is_root p x"
then have "[ \<one>, \<ominus> x ] pdivides p" and "p \<noteq> []" and "x \<in> carrier R"
using is_root_imp_pdivides[OF p] unfolding is_root_def by auto
moreover have "[ \<one>, \<ominus> x ] \<in> carrier (poly_ring R)"
using is_root unfolding is_root_def sym[OF univ_poly_carrier] polynomial_def by simp
ultimately have "[ \<one>, \<ominus> x ] pdivides q"
using UP.divides_cong_r[OF _ pq ] unfolding pdivides_def by simp
with \<open>p \<noteq> []\<close> and \<open>x \<in> carrier R\<close> show ?thesis
using associated_polynomials_imp_same_length[OF carrier_is_subring p q pq]
pdivides_imp_is_root[of q x]
by fastforce
qed
}
then show ?thesis
using assms UP.associated_sym[OF assms(3)] by blast
qed
lemma (in ring) monic_degree_one_root_condition:
assumes "a \<in> carrier R" shows "is_root [ \<one>, \<ominus> a ] b \<longleftrightarrow> a = b"
using assms minus_equality r_neg[OF assms] unfolding is_root_def by (auto, fastforce)
lemma (in field) degree_one_root_condition:
assumes "p \<in> carrier (poly_ring R)" and "degree p = 1"
shows "is_root p x \<longleftrightarrow> x = \<ominus> (inv (lead_coeff p) \<otimes> (const_term p))"
proof -
interpret UP: domain "poly_ring R"
using univ_poly_is_domain[OF carrier_is_subring] .
from \<open>degree p = 1\<close> have "length p = Suc (Suc 0)"
by simp
then obtain a b where p: "p = [ a, b ]"
by (metis length_0_conv length_Cons list.exhaust nat.inject)
hence a: "a \<in> carrier R" "a \<noteq> \<zero>" and b: "b \<in> carrier R"
using assms(1) unfolding sym[OF univ_poly_carrier] polynomial_def by auto
hence inv_a: "inv a \<in> carrier R" "(inv a) \<otimes> a = \<one>"
using subfield_m_inv[OF carrier_is_subfield, of a] by auto
hence in_carrier: "[ \<one>, (inv a) \<otimes> b ] \<in> carrier (poly_ring R)"
using b unfolding sym[OF univ_poly_carrier] polynomial_def by auto
have "p \<sim>\<^bsub>poly_ring R\<^esub> [ \<one>, (inv a) \<otimes> b ]"
proof (rule UP.associatedI2'[OF _ _ in_carrier, of _ "[ a ]"])
have "p = [ a ] \<otimes>\<^bsub>poly_ring R\<^esub> [ \<one>, inv a \<otimes> b ]"
using a inv_a b m_assoc[of a "inv a" b] unfolding p univ_poly_mult by (auto, algebra)
also have " ... = [ \<one>, inv a \<otimes> b ] \<otimes>\<^bsub>poly_ring R\<^esub> [ a ]"
using UP.m_comm[OF in_carrier, of "[ a ]"] a
by (auto simp add: sym[OF univ_poly_carrier] polynomial_def)
finally show "p = [ \<one>, inv a \<otimes> b ] \<otimes>\<^bsub>poly_ring R\<^esub> [ a ]" .
next
from \<open>a \<in> carrier R\<close> and \<open>a \<noteq> \<zero>\<close> show "[ a ] \<in> Units (poly_ring R)"
unfolding univ_poly_units[OF carrier_is_subfield] by simp
qed
moreover have "(inv a) \<otimes> b = \<ominus> (\<ominus> (inv (lead_coeff p) \<otimes> (const_term p)))"
and "inv (lead_coeff p) \<otimes> (const_term p) \<in> carrier R"
using inv_a a b unfolding p const_term_def by auto
ultimately show ?thesis
using associated_polynomials_imp_same_is_root[OF assms(1) in_carrier]
monic_degree_one_root_condition
by (metis add.inv_closed)
qed
lemma (in domain) is_root_poly_mult_imp_is_root:
assumes "p \<in> carrier (poly_ring R)" and "q \<in> carrier (poly_ring R)"
shows "is_root (p \<otimes>\<^bsub>poly_ring R\<^esub> q) x \<Longrightarrow> (is_root p x) \<or> (is_root q x)"
proof -
interpret UP: domain "poly_ring R"
using univ_poly_is_domain[OF carrier_is_subring] .
assume is_root: "is_root (p \<otimes>\<^bsub>poly_ring R\<^esub> q) x"
hence "p \<noteq> []" and "q \<noteq> []"
unfolding is_root_def sym[OF univ_poly_zero[of R "carrier R"]]
using UP.l_null[OF assms(2)] UP.r_null[OF assms(1)] by blast+
moreover have x: "x \<in> carrier R" and "eval (p \<otimes>\<^bsub>poly_ring R\<^esub> q) x = \<zero>"
using is_root unfolding is_root_def by simp+
hence "eval p x = \<zero> \<or> eval q x = \<zero>"
using ring_hom_memE[OF eval_is_hom[OF carrier_is_subring], of x] assms integral by auto
ultimately show "(is_root p x) \<or> (is_root q x)"
using x unfolding is_root_def by auto
qed
lemma (in domain) degree_zero_imp_not_is_root:
assumes "p \<in> carrier (poly_ring R)" and "degree p = 0" shows "\<not> is_root p x"
proof (cases "p = []", simp add: is_root_def)
case False with \<open>degree p = 0\<close> have "length p = Suc 0"
using le_SucE by fastforce
then obtain a where "p = [ a ]" and "a \<in> carrier R" and "a \<noteq> \<zero>"
using assms unfolding sym[OF univ_poly_carrier] polynomial_def
by (metis False length_0_conv length_Suc_conv list.sel(1) list.set_sel(1) subset_code(1))
thus ?thesis
unfolding is_root_def by auto
qed
lemma (in domain) finite_number_of_roots:
assumes "p \<in> carrier (poly_ring R)" shows "finite { x. is_root p x }"
using assms
proof (induction "degree p" arbitrary: p)
case 0 thus ?case
by (simp add: degree_zero_imp_not_is_root)
next
case (Suc n) show ?case
proof (cases "{ x. is_root p x } = {}")
case True thus ?thesis
by (simp add: True)
next
interpret UP: domain "poly_ring R"
using univ_poly_is_domain[OF carrier_is_subring] .
case False
then obtain a where is_root: "is_root p a"
by blast
hence a: "a \<in> carrier R" and eval: "eval p a = \<zero>" and p_not_zero: "p \<noteq> []"
unfolding is_root_def by auto
hence in_carrier: "[ \<one>, \<ominus> a ] \<in> carrier (poly_ring R)"
unfolding sym[OF univ_poly_carrier] polynomial_def by auto
obtain q where q: "q \<in> carrier (poly_ring R)" and p: "p = [ \<one>, \<ominus> a ] \<otimes>\<^bsub>poly_ring R\<^esub> q"
using is_root_imp_pdivides[OF Suc(3) is_root] unfolding pdivides_def by auto
with \<open>p \<noteq> []\<close> have q_not_zero: "q \<noteq> []"
using UP.r_null UP.integral in_carrier unfolding sym[OF univ_poly_zero[of R "carrier R"]]
by metis
hence "degree q = n"
using poly_mult_degree_eq[OF carrier_is_subring, of "[ \<one>, \<ominus> a ]" q]
in_carrier q p_not_zero p Suc(2)
unfolding univ_poly_carrier
by (metis One_nat_def Suc_eq_plus1 diff_Suc_1 list.distinct(1)
list.size(3-4) plus_1_eq_Suc univ_poly_mult)
hence "finite { x. is_root q x }"
using Suc(1)[OF _ q] by simp
moreover have "{ x. is_root p x } \<subseteq> insert a { x. is_root q x }"
using is_root_poly_mult_imp_is_root[OF in_carrier q]
monic_degree_one_root_condition[OF a]
unfolding p by auto
ultimately show ?thesis
using finite_subset by auto
qed
qed
lemma (in domain) alg_multE:
assumes "x \<in> carrier R" and "p \<in> carrier (poly_ring R)" and "p \<noteq> []"
shows "([ \<one>, \<ominus> x ] [^]\<^bsub>poly_ring R\<^esub> (alg_mult p x)) pdivides p"
and "\<And>n. ([ \<one>, \<ominus> x ] [^]\<^bsub>poly_ring R\<^esub> n) pdivides p \<Longrightarrow> n \<le> alg_mult p x"
proof -
interpret UP: domain "poly_ring R"
using univ_poly_is_domain[OF carrier_is_subring] .
let ?ppow = "\<lambda>n :: nat. ([ \<one>, \<ominus> x ] [^]\<^bsub>poly_ring R\<^esub> n)"
define S :: "nat set" where "S = { n. ?ppow n pdivides p }"
have "?ppow 0 = \<one>\<^bsub>poly_ring R\<^esub>"
using UP.nat_pow_0 by simp
hence "0 \<in> S"
using UP.one_divides[OF assms(2)] unfolding S_def pdivides_def by simp
hence "S \<noteq> {}"
by auto
moreover have "n \<le> degree p" if "n \<in> S" for n :: nat
proof -
have "[ \<one>, \<ominus> x ] \<in> carrier (poly_ring R)"
using assms unfolding sym[OF univ_poly_carrier] polynomial_def by auto
hence "?ppow n \<in> carrier (poly_ring R)"
using assms unfolding univ_poly_zero by auto
with \<open>n \<in> S\<close> have "degree (?ppow n) \<le> degree p"
using pdivides_imp_degree_le[OF carrier_is_subring _ assms(2-3), of "?ppow n"] by (simp add: S_def)
with \<open>[ \<one>, \<ominus> x ] \<in> carrier (poly_ring R)\<close> show ?thesis
using polynomial_pow_degree by simp
qed
hence "finite S"
using finite_nat_set_iff_bounded_le by blast
ultimately have MaxS: "\<And>n. n \<in> S \<Longrightarrow> n \<le> Max S" "Max S \<in> S"
using Max_ge[of S] Max_in[of S] by auto
with \<open>x \<in> carrier R\<close> have "alg_mult p x = Max S"
using Greatest_equality[of "\<lambda>n. ?ppow n pdivides p" "Max S"] assms(3)
unfolding S_def alg_mult_def by auto
thus "([ \<one>, \<ominus> x ] [^]\<^bsub>poly_ring R\<^esub> (alg_mult p x)) pdivides p"
and "\<And>n. ([ \<one>, \<ominus> x ] [^]\<^bsub>poly_ring R\<^esub> n) pdivides p \<Longrightarrow> n \<le> alg_mult p x"
using MaxS unfolding S_def by auto
qed
lemma (in domain) le_alg_mult_imp_pdivides:
assumes "x \<in> carrier R" and "p \<in> carrier (poly_ring R)"
shows "n \<le> alg_mult p x \<Longrightarrow> ([ \<one>, \<ominus> x ] [^]\<^bsub>poly_ring R\<^esub> n) pdivides p"
proof -
interpret UP: domain "poly_ring R"
using univ_poly_is_domain[OF carrier_is_subring] .
assume le_alg_mult: "n \<le> alg_mult p x"
have in_carrier: "[ \<one>, \<ominus> x ] \<in> carrier (poly_ring R)"
using assms(1) unfolding sym[OF univ_poly_carrier] polynomial_def by auto
hence ppow_pdivides:
"([ \<one>, \<ominus> x ] [^]\<^bsub>poly_ring R\<^esub> n) pdivides
([ \<one>, \<ominus> x ] [^]\<^bsub>poly_ring R\<^esub> (alg_mult p x))"
using polynomial_pow_division[OF _ le_alg_mult] by simp
show ?thesis
proof (cases "p = []")
case True thus ?thesis
using in_carrier pdivides_zero[OF carrier_is_subring] by auto
next
case False thus ?thesis
using ppow_pdivides UP.divides_trans UP.nat_pow_closed alg_multE(1)[OF assms] in_carrier
unfolding pdivides_def by meson
qed
qed
lemma (in domain) alg_mult_gt_zero_iff_is_root:
assumes "p \<in> carrier (poly_ring R)" shows "alg_mult p x > 0 \<longleftrightarrow> is_root p x"
proof -
interpret UP: domain "poly_ring R"
using univ_poly_is_domain[OF carrier_is_subring] .
show ?thesis
proof
assume is_root: "is_root p x" hence x: "x \<in> carrier R" and not_zero: "p \<noteq> []"
unfolding is_root_def by auto
have "[\<one>, \<ominus> x] [^]\<^bsub>poly_ring R\<^esub> (Suc 0) = [\<one>, \<ominus> x]"
using x unfolding univ_poly_def by auto
thus "alg_mult p x > 0"
using is_root_imp_pdivides[OF _ is_root] alg_multE(2)[OF x, of p "Suc 0"] not_zero assms by auto
next
assume gt_zero: "alg_mult p x > 0"
hence x: "x \<in> carrier R" and not_zero: "p \<noteq> []"
unfolding alg_mult_def by (cases "p = []", auto, cases "x \<in> carrier R", auto)
hence in_carrier: "[ \<one>, \<ominus> x ] \<in> carrier (poly_ring R)"
unfolding sym[OF univ_poly_carrier] polynomial_def by auto
with \<open>x \<in> carrier R\<close> have "[ \<one>, \<ominus> x ] pdivides p" and "eval [ \<one>, \<ominus> x ] x = \<zero>"
using le_alg_mult_imp_pdivides[of x p "1::nat"] gt_zero assms by (auto, algebra)
thus "is_root p x"
using pdivides_imp_root_sharing[OF in_carrier] not_zero x by (simp add: is_root_def)
qed
qed
lemma (in domain) alg_mult_eq_count_roots:
assumes "p \<in> carrier (poly_ring R)" shows "alg_mult p = count (roots p)"
using finite_number_of_roots[OF assms]
unfolding sym[OF alg_mult_gt_zero_iff_is_root[OF assms]]
by (simp add: multiset_def roots_def)
lemma (in domain) roots_mem_iff_is_root:
assumes "p \<in> carrier (poly_ring R)" shows "x \<in># roots p \<longleftrightarrow> is_root p x"
using alg_mult_eq_count_roots[OF assms] count_greater_zero_iff
unfolding roots_def sym[OF alg_mult_gt_zero_iff_is_root[OF assms]] by metis
lemma (in domain) degree_zero_imp_empty_roots:
assumes "p \<in> carrier (poly_ring R)" and "degree p = 0" shows "roots p = {#}"
using degree_zero_imp_not_is_root[of p] roots_mem_iff_is_root[of p] assms by auto
lemma (in domain) degree_zero_imp_splitted:
assumes "p \<in> carrier (poly_ring R)" and "degree p = 0" shows "splitted p"
unfolding splitted_def degree_zero_imp_empty_roots[OF assms] assms(2) by simp
lemma (in domain) roots_inclI':
assumes "p \<in> carrier (poly_ring R)" and "\<And>a. \<lbrakk> a \<in> carrier R; p \<noteq> [] \<rbrakk> \<Longrightarrow> alg_mult p a \<le> count m a"
shows "roots p \<subseteq># m"
proof (intro mset_subset_eqI)
fix a show "count (roots p) a \<le> count m a"
using assms unfolding sym[OF alg_mult_eq_count_roots[OF assms(1)]] alg_mult_def
by (cases "p = []", simp, cases "a \<in> carrier R", auto)
qed
lemma (in domain) roots_inclI:
assumes "p \<in> carrier (poly_ring R)" and "q \<in> carrier (poly_ring R)" "q \<noteq> []"
and "\<And>a. \<lbrakk> a \<in> carrier R; p \<noteq> [] \<rbrakk> \<Longrightarrow> ([ \<one>, \<ominus> a ] [^]\<^bsub>poly_ring R\<^esub> (alg_mult p a)) pdivides q"
shows "roots p \<subseteq># roots q"
using roots_inclI'[OF assms(1), of "roots q"] assms alg_multE(2)[OF _ assms(2-3)]
unfolding sym[OF alg_mult_eq_count_roots[OF assms(2)]] by auto
lemma (in domain) pdivides_imp_roots_incl:
assumes "p \<in> carrier (poly_ring R)" and "q \<in> carrier (poly_ring R)" "q \<noteq> []"
shows "p pdivides q \<Longrightarrow> roots p \<subseteq># roots q"
proof (rule roots_inclI[OF assms])
interpret UP: domain "poly_ring R"
using univ_poly_is_domain[OF carrier_is_subring] .
fix a assume "p pdivides q" and a: "a \<in> carrier R"
hence "[ \<one> , \<ominus> a ] \<in> carrier (poly_ring R)"
unfolding sym[OF univ_poly_carrier] polynomial_def by simp
with \<open>p pdivides q\<close> show "([\<one>, \<ominus> a] [^]\<^bsub>poly_ring R\<^esub> (alg_mult p a)) pdivides q"
using UP.divides_trans[of _p q] le_alg_mult_imp_pdivides[OF a assms(1)]
by (auto simp add: pdivides_def)
qed
lemma (in domain) associated_polynomials_imp_same_roots:
assumes "p \<in> carrier (poly_ring R)" and "q \<in> carrier (poly_ring R)" and "p \<sim>\<^bsub>poly_ring R\<^esub> q"
shows "roots p = roots q"
using assms pdivides_imp_roots_incl zero_pdivides
unfolding pdivides_def associated_def
by (metis subset_mset.eq_iff)
lemma (in domain) monic_degree_one_roots:
assumes "a \<in> carrier R" shows "roots [ \<one> , \<ominus> a ] = {# a #}"
proof -
let ?p = "[ \<one> , \<ominus> a ]"
interpret UP: domain "poly_ring R"
using univ_poly_is_domain[OF carrier_is_subring] .
from \<open>a \<in> carrier R\<close> have in_carrier: "?p \<in> carrier (poly_ring R)"
unfolding sym[OF univ_poly_carrier] polynomial_def by simp
show ?thesis
proof (rule subset_mset.antisym)
show "{# a #} \<subseteq># roots ?p"
using roots_mem_iff_is_root[OF in_carrier]
monic_degree_one_root_condition[OF assms]
by simp
next
show "roots ?p \<subseteq># {# a #}"
proof (rule mset_subset_eqI, auto)
fix b assume "a \<noteq> b" thus "count (roots ?p) b = 0"
using alg_mult_gt_zero_iff_is_root[OF in_carrier]
monic_degree_one_root_condition[OF assms]
unfolding sym[OF alg_mult_eq_count_roots[OF in_carrier]]
by fastforce
next
have "(?p [^]\<^bsub>poly_ring R\<^esub> (alg_mult ?p a)) pdivides ?p"
using le_alg_mult_imp_pdivides[OF assms in_carrier] by simp
hence "degree (?p [^]\<^bsub>poly_ring R\<^esub> (alg_mult ?p a)) \<le> degree ?p"
using pdivides_imp_degree_le[OF carrier_is_subring, of _ ?p] in_carrier by auto
thus "count (roots ?p) a \<le> Suc 0"
using polynomial_pow_degree[OF in_carrier]
unfolding sym[OF alg_mult_eq_count_roots[OF in_carrier]]
by auto
qed
qed
qed
lemma (in domain) degree_one_roots:
assumes "a \<in> carrier R" "a' \<in> carrier R" and "b \<in> carrier R" and "a \<otimes> a' = \<one>"
shows "roots [ a , b ] = {# \<ominus> (a' \<otimes> b) #}"
proof -
have "[ a, b ] \<in> carrier (poly_ring R)" and "[ \<one>, a' \<otimes> b ] \<in> carrier (poly_ring R)"
using assms unfolding sym[OF univ_poly_carrier] polynomial_def by auto
thus ?thesis
using subring_degree_one_associatedI[OF carrier_is_subring assms] assms
monic_degree_one_roots associated_polynomials_imp_same_roots
by (metis add.inv_closed local.minus_minus m_closed)
qed
lemma (in field) degree_one_imp_singleton_roots:
assumes "p \<in> carrier (poly_ring R)" and "degree p = 1"
shows "roots p = {# \<ominus> (inv (lead_coeff p) \<otimes> (const_term p)) #}"
proof -
from \<open>p \<in> carrier (poly_ring R)\<close> and \<open>degree p = 1\<close>
obtain a b where "p = [ a, b ]" and "a \<in> carrier R" "a \<noteq> \<zero>" and "b \<in> carrier R"
by auto
thus ?thesis
using degree_one_roots[of a "inv a" b]
by (auto simp add: const_term_def field_Units)
qed
lemma (in field) degree_one_imp_splitted:
assumes "p \<in> carrier (poly_ring R)" and "degree p = 1" shows "splitted p"
using degree_one_imp_singleton_roots[OF assms] assms(2) unfolding splitted_def by simp
lemma (in field) no_roots_imp_same_roots:
assumes "p \<in> carrier (poly_ring R)" "p \<noteq> []" and "q \<in> carrier (poly_ring R)"
shows "roots p = {#} \<Longrightarrow> roots (p \<otimes>\<^bsub>poly_ring R\<^esub> q) = roots q"
proof -
interpret UP: domain "poly_ring R"
using univ_poly_is_domain[OF carrier_is_subring] .
assume no_roots: "roots p = {#}" show "roots (p \<otimes>\<^bsub>poly_ring R\<^esub> q) = roots q"
proof (intro subset_mset.antisym)
have pdiv: "q pdivides (p \<otimes>\<^bsub>poly_ring R\<^esub> q)"
using UP.divides_prod_l assms unfolding pdivides_def by blast
show "roots q \<subseteq># roots (p \<otimes>\<^bsub>poly_ring R\<^esub> q)"
using pdivides_imp_roots_incl[OF _ _ _ pdiv] assms
degree_zero_imp_empty_roots[OF assms(3)]
by (cases "q = []", auto, metis UP.l_null UP.m_rcancel UP.zero_closed univ_poly_zero)
next
show "roots (p \<otimes>\<^bsub>poly_ring R\<^esub> q) \<subseteq># roots q"
proof (cases "p \<otimes>\<^bsub>poly_ring R\<^esub> q = []")
case True thus ?thesis
using degree_zero_imp_empty_roots[OF UP.m_closed[OF assms(1,3)]] by simp
next
case False with \<open>p \<noteq> []\<close> have q_not_zero: "q \<noteq> []"
by (metis UP.r_null assms(1) univ_poly_zero)
show ?thesis
proof (rule roots_inclI[OF UP.m_closed[OF assms(1,3)] assms(3) q_not_zero])
fix a assume a: "a \<in> carrier R"
hence "\<not> ([ \<one>, \<ominus> a ] pdivides p)"
using assms(1-2) no_roots pdivides_imp_is_root roots_mem_iff_is_root[of p] by auto
moreover have in_carrier: "[ \<one>, \<ominus> a ] \<in> carrier (poly_ring R)"
using a unfolding sym[OF univ_poly_carrier] polynomial_def by auto
hence "pirreducible (carrier R) [ \<one>, \<ominus> a ]"
using degree_one_imp_pirreducible[OF carrier_is_subfield] by simp
moreover
have "([ \<one>, \<ominus> a ] [^]\<^bsub>poly_ring R\<^esub> (alg_mult (p \<otimes>\<^bsub>poly_ring R\<^esub> q) a)) pdivides (p \<otimes>\<^bsub>poly_ring R\<^esub> q)"
using le_alg_mult_imp_pdivides[OF a UP.m_closed, of p q] assms by simp
ultimately show "([ \<one>, \<ominus> a ] [^]\<^bsub>poly_ring R\<^esub> (alg_mult (p \<otimes>\<^bsub>poly_ring R\<^esub> q) a)) pdivides q"
using pirreducible_pow_pdivides_iff[OF carrier_is_subfield in_carrier] assms by auto
qed
qed
qed
qed
lemma (in field) poly_mult_degree_one_monic_imp_same_roots:
assumes "a \<in> carrier R" and "p \<in> carrier (poly_ring R)" "p \<noteq> []"
shows "roots ([ \<one>, \<ominus> a ] \<otimes>\<^bsub>poly_ring R\<^esub> p) = add_mset a (roots p)"
proof -
interpret UP: domain "poly_ring R"
using univ_poly_is_domain[OF carrier_is_subring] .
from \<open>a \<in> carrier R\<close> have in_carrier: "[ \<one>, \<ominus> a ] \<in> carrier (poly_ring R)"
unfolding sym[OF univ_poly_carrier] polynomial_def by simp
show ?thesis
proof (intro subset_mset.antisym[OF roots_inclI' mset_subset_eqI])
show "([ \<one>, \<ominus> a ] \<otimes>\<^bsub>poly_ring R\<^esub> p) \<in> carrier (poly_ring R)"
using in_carrier assms(2) by simp
next
fix b assume b: "b \<in> carrier R" and "[ \<one>, \<ominus> a ] \<otimes>\<^bsub>poly_ring R\<^esub> p \<noteq> []"
hence not_zero: "p \<noteq> []"
unfolding univ_poly_def by auto
from \<open>b \<in> carrier R\<close> have in_carrier': "[ \<one>, \<ominus> b ] \<in> carrier (poly_ring R)"
unfolding sym[OF univ_poly_carrier] polynomial_def by simp
show "alg_mult ([ \<one>, \<ominus> a ] \<otimes>\<^bsub>poly_ring R\<^esub> p) b \<le> count (add_mset a (roots p)) b"
proof (cases "a = b")
case False
hence "\<not> [ \<one>, \<ominus> b ] pdivides [ \<one>, \<ominus> a ]"
using assms(1) b monic_degree_one_root_condition pdivides_imp_is_root by blast
moreover have "pirreducible (carrier R) [ \<one>, \<ominus> b ]"
using degree_one_imp_pirreducible[OF carrier_is_subfield in_carrier'] by simp
ultimately
have "[ \<one>, \<ominus> b ] [^]\<^bsub>poly_ring R\<^esub> (alg_mult ([ \<one>, \<ominus> a ] \<otimes>\<^bsub>poly_ring R\<^esub> p) b) pdivides p"
using le_alg_mult_imp_pdivides[OF b UP.m_closed, of _ p] assms(2) in_carrier
pirreducible_pow_pdivides_iff[OF carrier_is_subfield in_carrier' in_carrier, of p]
by auto
with \<open>a \<noteq> b\<close> show ?thesis
using alg_mult_eq_count_roots[OF assms(2)] alg_multE(2)[OF b assms(2) not_zero] by auto
next
case True
have "[ \<one>, \<ominus> a ] pdivides ([ \<one>, \<ominus> a ] \<otimes>\<^bsub>poly_ring R\<^esub> p)"
using dividesI[OF assms(2)] unfolding pdivides_def by auto
with \<open>[ \<one>, \<ominus> a ] \<otimes>\<^bsub>poly_ring R\<^esub> p \<noteq> []\<close>
have "alg_mult ([ \<one>, \<ominus> a ] \<otimes>\<^bsub>poly_ring R\<^esub> p) a \<ge> Suc 0"
using alg_multE(2)[of a _ "Suc 0"] in_carrier assms by auto
then obtain m where m: "alg_mult ([ \<one>, \<ominus> a ] \<otimes>\<^bsub>poly_ring R\<^esub> p) a = Suc m"
using Suc_le_D by blast
hence "([ \<one>, \<ominus> a ] \<otimes>\<^bsub>poly_ring R\<^esub> ([ \<one>, \<ominus> a ] [^]\<^bsub>poly_ring R\<^esub> m)) pdivides
([ \<one>, \<ominus> a ] \<otimes>\<^bsub>poly_ring R\<^esub> p)"
using le_alg_mult_imp_pdivides[OF _ UP.m_closed, of a _ p]
in_carrier assms UP.nat_pow_Suc2 by force
hence "([ \<one>, \<ominus> a ] [^]\<^bsub>poly_ring R\<^esub> m) pdivides p"
using UP.mult_divides in_carrier assms(2)
unfolding univ_poly_zero pdivides_def factor_def
by (simp add: UP.m_assoc UP.m_lcancel univ_poly_zero)
with \<open>a = b\<close> show ?thesis
using alg_mult_eq_count_roots assms in_carrier UP.nat_pow_Suc2
alg_multE(2)[OF assms(1) _ not_zero] m
by auto
qed
next
fix b
have not_zero: "[ \<one>, \<ominus> a ] \<otimes>\<^bsub>poly_ring R\<^esub> p \<noteq> []"
using assms in_carrier univ_poly_zero[of R] UP.integral by auto
show "count (add_mset a (roots p)) b \<le> count (roots ([\<one>, \<ominus> a] \<otimes>\<^bsub>poly_ring R\<^esub> p)) b"
proof (cases "a = b")
case True
have "([ \<one>, \<ominus> a ] \<otimes>\<^bsub>poly_ring R\<^esub> ([ \<one>, \<ominus> a ] [^]\<^bsub>poly_ring R\<^esub> (alg_mult p a))) pdivides
([ \<one>, \<ominus> a ] \<otimes>\<^bsub>poly_ring R\<^esub> p)"
using UP.divides_mult[OF _ in_carrier] le_alg_mult_imp_pdivides[OF assms(1,2)] in_carrier assms
by (auto simp add: pdivides_def)
with \<open>a = b\<close> show ?thesis
using alg_mult_eq_count_roots assms in_carrier UP.nat_pow_Suc2
alg_multE(2)[OF assms(1) _ not_zero]
by auto
next
case False
have "p pdivides ([ \<one>, \<ominus> a ] \<otimes>\<^bsub>poly_ring R\<^esub> p)"
using dividesI[OF in_carrier] UP.m_comm in_carrier assms unfolding pdivides_def by auto
thus ?thesis
using False pdivides_imp_roots_incl assms in_carrier not_zero
by (simp add: subseteq_mset_def)
qed
qed
qed
lemma (in domain) not_empty_rootsE[elim]:
assumes "p \<in> carrier (poly_ring R)" and "roots p \<noteq> {#}"
and "\<And>a. \<lbrakk> a \<in> carrier R; a \<in># roots p;
[ \<one>, \<ominus> a ] \<in> carrier (poly_ring R); [ \<one>, \<ominus> a ] pdivides p \<rbrakk> \<Longrightarrow> P"
shows P
proof -
from \<open>roots p \<noteq> {#}\<close> obtain a where "a \<in># roots p"
by blast
with \<open>p \<in> carrier (poly_ring R)\<close> have "[ \<one>, \<ominus> a ] pdivides p"
and "[ \<one>, \<ominus> a ] \<in> carrier (poly_ring R)" and "a \<in> carrier R"
using is_root_imp_pdivides[of p] roots_mem_iff_is_root[of p] is_root_def[of p a]
unfolding sym[OF univ_poly_carrier] polynomial_def by auto
with \<open>a \<in># roots p\<close> show ?thesis
using assms(3)[of a] by auto
qed
lemma (in field) associated_polynomials_imp_same_roots:
assumes "p \<in> carrier (poly_ring R)" "p \<noteq> []" and "q \<in> carrier (poly_ring R)" "q \<noteq> []"
shows "roots (p \<otimes>\<^bsub>poly_ring R\<^esub> q) = roots p + roots q"
proof -
interpret UP: domain "poly_ring R"
using univ_poly_is_domain[OF carrier_is_subring] .
from assms show ?thesis
proof (induction "degree p" arbitrary: p rule: less_induct)
case less show ?case
proof (cases "roots p = {#}")
case True thus ?thesis
using no_roots_imp_same_roots[of p q] less by simp
next
case False with \<open>p \<in> carrier (poly_ring R)\<close>
obtain a where a: "a \<in> carrier R" and "a \<in># roots p" and pdiv: "[ \<one>, \<ominus> a ] pdivides p"
and in_carrier: "[ \<one>, \<ominus> a ] \<in> carrier (poly_ring R)"
by blast
show ?thesis
proof (cases "degree p = 1")
case True with \<open>p \<in> carrier (poly_ring R)\<close>
obtain b c where p: "p = [ b, c ]" and b: "b \<in> carrier R" "b \<noteq> \<zero>" and c: "c \<in> carrier R"
by auto
with \<open>a \<in># roots p\<close> have roots: "roots p = {# a #}" and a: "\<ominus> a = inv b \<otimes> c" "a \<in> carrier R"
and lead: "lead_coeff p = b" and const: "const_term p = c"
using degree_one_imp_singleton_roots[of p] less(2) field_Units
unfolding const_term_def by auto
hence "(p \<otimes>\<^bsub>poly_ring R\<^esub> q) \<sim>\<^bsub>poly_ring R\<^esub> ([ \<one>, \<ominus> a ] \<otimes>\<^bsub>poly_ring R\<^esub> q)"
using UP.mult_cong_l[OF degree_one_associatedI[OF carrier_is_subfield _ True]] less(2,4)
by (auto simp add: a lead const)
hence "roots (p \<otimes>\<^bsub>poly_ring R\<^esub> q) = roots ([ \<one>, \<ominus> a ] \<otimes>\<^bsub>poly_ring R\<^esub> q)"
using associated_polynomials_imp_same_roots in_carrier less(2,4) unfolding a by simp
thus ?thesis
unfolding poly_mult_degree_one_monic_imp_same_roots[OF a(2) less(4,5)] roots by simp
next
case False
from \<open>[ \<one>, \<ominus> a ] pdivides p\<close>
obtain r where p: "p = [ \<one>, \<ominus> a ] \<otimes>\<^bsub>poly_ring R\<^esub> r" and r: "r \<in> carrier (poly_ring R)"
unfolding pdivides_def by auto
with \<open>p \<noteq> []\<close> have not_zero: "r \<noteq> []"
using in_carrier univ_poly_zero[of R "carrier R"] UP.integral_iff by auto
with \<open>p = [ \<one>, \<ominus> a ] \<otimes>\<^bsub>poly_ring R\<^esub> r\<close> have deg: "degree p = Suc (degree r)"
using poly_mult_degree_eq[OF carrier_is_subring, of _ r] in_carrier r
unfolding univ_poly_carrier sym[OF univ_poly_mult[of R "carrier R"]] by auto
with \<open>r \<noteq> []\<close> and \<open>q \<noteq> []\<close> have "r \<otimes>\<^bsub>poly_ring R\<^esub> q \<noteq> []"
using in_carrier univ_poly_zero[of R "carrier R"] UP.integral less(4) r by auto
hence "roots (p \<otimes>\<^bsub>poly_ring R\<^esub> q) = add_mset a (roots (r \<otimes>\<^bsub>poly_ring R\<^esub> q))"
using poly_mult_degree_one_monic_imp_same_roots[OF a UP.m_closed[OF r less(4)]]
UP.m_assoc[OF in_carrier r less(4)] p by auto
also have " ... = add_mset a (roots r + roots q)"
using less(1)[OF _ r not_zero less(4-5)] deg by simp
also have " ... = (add_mset a (roots r)) + roots q"
by simp
also have " ... = roots p + roots q"
using poly_mult_degree_one_monic_imp_same_roots[OF a r not_zero] p by simp
finally show ?thesis .
qed
qed
qed
qed
lemma (in field) size_roots_le_degree:
assumes "p \<in> carrier (poly_ring R)" shows "size (roots p) \<le> degree p"
using assms
proof (induction "degree p" arbitrary: p rule: less_induct)
case less show ?case
proof (cases "roots p = {#}", simp)
interpret UP: domain "poly_ring R"
using univ_poly_is_domain[OF carrier_is_subring] .
case False with \<open>p \<in> carrier (poly_ring R)\<close>
obtain a where a: "a \<in> carrier R" and "a \<in># roots p" and "[ \<one>, \<ominus> a ] pdivides p"
and in_carrier: "[ \<one>, \<ominus> a ] \<in> carrier (poly_ring R)"
by blast
then obtain q where p: "p = [ \<one>, \<ominus> a ] \<otimes>\<^bsub>poly_ring R\<^esub> q" and q: "q \<in> carrier (poly_ring R)"
unfolding pdivides_def by auto
with \<open>a \<in># roots p\<close> have "p \<noteq> []"
using degree_zero_imp_empty_roots[OF less(2)] by auto
hence not_zero: "q \<noteq> []"
using in_carrier univ_poly_zero[of R "carrier R"] UP.integral_iff p by auto
hence "degree p = Suc (degree q)"
using poly_mult_degree_eq[OF carrier_is_subring, of _ q] in_carrier p q
unfolding univ_poly_carrier sym[OF univ_poly_mult[of R "carrier R"]] by auto
with \<open>q \<noteq> []\<close> show ?thesis
using poly_mult_degree_one_monic_imp_same_roots[OF a q] p less(1)[OF _ q]
by (metis Suc_le_mono lessI size_add_mset)
qed
qed
lemma (in domain) pirreducible_roots:
assumes "p \<in> carrier (poly_ring R)" and "pirreducible (carrier R) p" and "degree p \<noteq> 1"
shows "roots p = {#}"
proof (rule ccontr)
assume "roots p \<noteq> {#}" with \<open>p \<in> carrier (poly_ring R)\<close>
obtain a where a: "a \<in> carrier R" and "a \<in># roots p" and "[ \<one>, \<ominus> a ] pdivides p"
and in_carrier: "[ \<one>, \<ominus> a ] \<in> carrier (poly_ring R)"
by blast
hence "[ \<one>, \<ominus> a ] \<sim>\<^bsub>poly_ring R\<^esub> p"
using divides_pirreducible_condition[OF assms(2) in_carrier]
univ_poly_units_incl[OF carrier_is_subring]
unfolding pdivides_def by auto
hence "degree p = 1"
using associated_polynomials_imp_same_length[OF carrier_is_subring in_carrier assms(1)] by auto
with \<open>degree p \<noteq> 1\<close> show False ..
qed
lemma (in field) pirreducible_imp_not_splitted:
assumes "p \<in> carrier (poly_ring R)" and "pirreducible (carrier R) p" and "degree p \<noteq> 1"
shows "\<not> splitted p"
using pirreducible_roots[of p] pirreducible_degree[OF carrier_is_subfield, of p] assms
by (simp add: splitted_def)
lemma (in field)
assumes "p \<in> carrier (poly_ring R)" and "q \<in> carrier (poly_ring R)"
and "pirreducible (carrier R) p" and "degree p \<noteq> 1"
shows "roots (p \<otimes>\<^bsub>poly_ring R\<^esub> q) = roots q"
using no_roots_imp_same_roots[of p q] pirreducible_roots[of p] assms
unfolding ring_irreducible_def univ_poly_zero by auto
lemma (in field) trivial_factors_imp_splitted:
assumes "p \<in> carrier (poly_ring R)"
and "\<And>q. \<lbrakk> q \<in> carrier (poly_ring R); pirreducible (carrier R) q; q pdivides p \<rbrakk> \<Longrightarrow> degree q \<le> 1"
shows "splitted p"
using assms
proof (induction "degree p" arbitrary: p rule: less_induct)
interpret UP: principal_domain "poly_ring R"
using univ_poly_is_principal[OF carrier_is_subfield] .
case less show ?case
proof (cases "degree p = 0", simp add: degree_zero_imp_splitted[OF less(2)])
case False show ?thesis
proof (cases "roots p = {#}")
case True
from \<open>degree p \<noteq> 0\<close> have "p \<notin> Units (poly_ring R)" and "p \<in> carrier (poly_ring R) - { [] }"
using univ_poly_units'[OF carrier_is_subfield, of p] less(2) by auto
then obtain q where "q \<in> carrier (poly_ring R)" "pirreducible (carrier R) q" and "q pdivides p"
using UP.exists_irreducible_divisor[of p] unfolding univ_poly_zero pdivides_def by auto
with \<open>degree p \<noteq> 0\<close> have "roots p \<noteq> {#}"
using degree_one_imp_singleton_roots[OF _ , of q] less(3)[of q]
pdivides_imp_roots_incl[OF _ less(2), of q]
pirreducible_degree[OF carrier_is_subfield, of q]
by force
from \<open>roots p = {#}\<close> and \<open>roots p \<noteq> {#}\<close> have False
by simp
thus ?thesis ..
next
case False with \<open>p \<in> carrier (poly_ring R)\<close>
obtain a where a: "a \<in> carrier R" and "a \<in># roots p" and "[ \<one>, \<ominus> a ] pdivides p"
and in_carrier: "[ \<one>, \<ominus> a ] \<in> carrier (poly_ring R)"
by blast
then obtain q where p: "p = [ \<one>, \<ominus> a ] \<otimes>\<^bsub>poly_ring R\<^esub> q" and q: "q \<in> carrier (poly_ring R)"
unfolding pdivides_def by blast
with \<open>degree p \<noteq> 0\<close> have "p \<noteq> []"
by auto
with \<open>p = [ \<one>, \<ominus> a ] \<otimes>\<^bsub>poly_ring R\<^esub> q\<close> have "q \<noteq> []"
using in_carrier q unfolding sym[OF univ_poly_zero[of R "carrier R"]] by auto
with \<open>p = [ \<one>, \<ominus> a ] \<otimes>\<^bsub>poly_ring R\<^esub> q\<close> and \<open>p \<noteq> []\<close> have "degree p = Suc (degree q)"
using poly_mult_degree_eq[OF carrier_is_subring] in_carrier q
unfolding univ_poly_carrier sym[OF univ_poly_mult[of R "carrier R"]] by auto
moreover have "q pdivides p"
using p dividesI[OF in_carrier] UP.m_comm[OF in_carrier q] by (auto simp add: pdivides_def)
hence "degree r = 1" if "r \<in> carrier (poly_ring R)" and "pirreducible (carrier R) r"
and "r pdivides q" for r
using less(3)[OF that(1-2)] UP.divides_trans[OF _ _ that(1), of q p] that(3)
pirreducible_degree[OF carrier_is_subfield that(1-2)]
by (auto simp add: pdivides_def)
ultimately have "splitted q"
using less(1)[OF _ q] by auto
with \<open>degree p = Suc (degree q)\<close> and \<open>q \<noteq> []\<close> show ?thesis
using poly_mult_degree_one_monic_imp_same_roots[OF a q]
unfolding sym[OF p] splitted_def
by simp
qed
qed
qed
lemma (in field) pdivides_imp_splitted:
assumes "p \<in> carrier (poly_ring R)" and "q \<in> carrier (poly_ring R)" "q \<noteq> []" and "splitted q"
shows "p pdivides q \<Longrightarrow> splitted p"
proof (cases "p = []")
case True thus ?thesis
using degree_zero_imp_splitted[OF assms(1)] by simp
next
interpret UP: principal_domain "poly_ring R"
using univ_poly_is_principal[OF carrier_is_subfield] .
case False
assume "p pdivides q"
then obtain b where b: "b \<in> carrier (poly_ring R)" and q: "q = p \<otimes>\<^bsub>poly_ring R\<^esub> b"
unfolding pdivides_def by auto
with \<open>q \<noteq> []\<close> have "p \<noteq> []" and "b \<noteq> []"
using assms UP.integral_iff[of p b] unfolding sym[OF univ_poly_zero[of R "carrier R"]] by auto
hence "degree p + degree b = size (roots p) + size (roots b)"
using associated_polynomials_imp_same_roots[of p b] assms b q splitted_def
poly_mult_degree_eq[OF carrier_is_subring,of p b]
unfolding univ_poly_carrier sym[OF univ_poly_mult[of R "carrier R"]]
by auto
moreover have "size (roots p) \<le> degree p" and "size (roots b) \<le> degree b"
using size_roots_le_degree assms(1) b by auto
ultimately show ?thesis
unfolding splitted_def by linarith
qed
lemma (in field) splitted_imp_trivial_factors:
assumes "p \<in> carrier (poly_ring R)" "p \<noteq> []" and "splitted p"
shows "\<And>q. \<lbrakk> q \<in> carrier (poly_ring R); pirreducible (carrier R) q; q pdivides p \<rbrakk> \<Longrightarrow> degree q = 1"
using pdivides_imp_splitted[OF _ assms] pirreducible_imp_not_splitted
by auto
subsection \<open>Link between @{term \<open>(pmod)\<close>} and @{term rupture_surj}\<close>
lemma (in domain) rupture_surj_composed_with_pmod:
assumes "subfield K R" and "p \<in> carrier (K[X])" and "q \<in> carrier (K[X])"
shows "rupture_surj K p q = rupture_surj K p (q pmod p)"
proof -
interpret UP: principal_domain "K[X]"
using univ_poly_is_principal[OF assms(1)] .
interpret Rupt: ring "Rupt K p"
using assms by (simp add: UP.cgenideal_ideal ideal.quotient_is_ring rupture_def)
let ?h = "rupture_surj K p"
have "?h q = (?h p \<otimes>\<^bsub>Rupt K p\<^esub> ?h (q pdiv p)) \<oplus>\<^bsub>Rupt K p\<^esub> ?h (q pmod p)"
and "?h (q pdiv p) \<in> carrier (Rupt K p)" "?h (q pmod p) \<in> carrier (Rupt K p)"
using pdiv_pmod[OF assms(1,3,2)] long_division_closed[OF assms(1,3,2)] assms UP.m_closed
ring_hom_memE[OF rupture_surj_hom(1)[OF subfieldE(1)[OF assms(1)] assms(2)]]
by metis+
moreover have "?h p = PIdl\<^bsub>K[X]\<^esub> p"
using assms by (simp add: UP.a_rcos_zero UP.cgenideal_ideal UP.cgenideal_self)
hence "?h p = \<zero>\<^bsub>Rupt K p\<^esub>"
unfolding rupture_def FactRing_def by simp
ultimately show ?thesis
by simp
qed
corollary (in domain) rupture_carrier_as_pmod_image:
assumes "subfield K R" and "p \<in> carrier (K[X])"
shows "(rupture_surj K p) ` ((\<lambda>q. q pmod p) ` (carrier (K[X]))) = carrier (Rupt K p)"
(is "?lhs = ?rhs")
proof
have "(\<lambda>q. q pmod p) ` carrier (K[X]) \<subseteq> carrier (K[X])"
using long_division_closed(2)[OF assms(1) _ assms(2)] by auto
thus "?lhs \<subseteq> ?rhs"
using ring_hom_memE(1)[OF rupture_surj_hom(1)[OF subfieldE(1)[OF assms(1)] assms(2)]] by auto
next
show "?rhs \<subseteq> ?lhs"
proof
fix a assume "a \<in> carrier (Rupt K p)"
then obtain q where "q \<in> carrier (K[X])" and "a = rupture_surj K p q"
unfolding rupture_def FactRing_def A_RCOSETS_def' by auto
thus "a \<in> ?lhs"
using rupture_surj_composed_with_pmod[OF assms] by auto
qed
qed
lemma (in domain) rupture_surj_inj_on:
assumes "subfield K R" and "p \<in> carrier (K[X])"
shows "inj_on (rupture_surj K p) ((\<lambda>q. q pmod p) ` (carrier (K[X])))"
proof (intro inj_onI)
interpret UP: principal_domain "K[X]"
using univ_poly_is_principal[OF assms(1)] .
fix a b
assume "a \<in> (\<lambda>q. q pmod p) ` carrier (K[X])"
and "b \<in> (\<lambda>q. q pmod p) ` carrier (K[X])"
then obtain q s
where q: "q \<in> carrier (K[X])" "a = q pmod p"
and s: "s \<in> carrier (K[X])" "b = s pmod p"
by auto
moreover assume "rupture_surj K p a = rupture_surj K p b"
ultimately have "q \<ominus>\<^bsub>K[X]\<^esub> s \<in> (PIdl\<^bsub>K[X]\<^esub> p)"
using UP.quotient_eq_iff_same_a_r_cos[OF UP.cgenideal_ideal[OF assms(2)], of q s]
rupture_surj_composed_with_pmod[OF assms] by auto
hence "p pdivides (q \<ominus>\<^bsub>K[X]\<^esub> s)"
using assms q(1) s(1) UP.to_contain_is_to_divide pdivides_iff_shell
by (meson UP.cgenideal_ideal UP.cgenideal_minimal UP.minus_closed)
thus "a = b"
unfolding q s same_pmod_iff_pdivides[OF assms(1) q(1) s(1) assms(2)] .
qed
subsection \<open>Dimension\<close>
definition (in ring) exp_base :: "'a \<Rightarrow> nat \<Rightarrow> 'a list"
where "exp_base x n = map (\<lambda>i. x [^] i) (rev [0..< n])"
lemma (in ring) exp_base_closed:
assumes "x \<in> carrier R" shows "set (exp_base x n) \<subseteq> carrier R"
using assms by (induct n) (auto simp add: exp_base_def)
lemma (in ring) exp_base_append:
shows "exp_base x (n + m) = (map (\<lambda>i. x [^] i) (rev [n..< n + m])) @ exp_base x n"
unfolding exp_base_def by (metis map_append rev_append upt_add_eq_append zero_le)
lemma (in ring) drop_exp_base:
shows "drop n (exp_base x m) = exp_base x (m - n)"
proof -
have ?thesis if "n > m"
using that by (simp add: exp_base_def)
moreover have ?thesis if "n \<le> m"
using exp_base_append[of x "m - n" n] that by auto
ultimately show ?thesis
by linarith
qed
lemma (in ring) combine_eq_eval:
shows "combine Ks (exp_base x (length Ks)) = eval Ks x"
unfolding exp_base_def by (induct Ks) (auto)
lemma (in domain) pmod_image_characterization:
assumes "subfield K R" and "p \<in> carrier (K[X])" and "p \<noteq> []"
shows "(\<lambda>q. q pmod p) ` carrier (K[X]) = { q \<in> carrier (K[X]). length q \<le> degree p }"
proof -
interpret UP: principal_domain "K[X]"
using univ_poly_is_principal[OF assms(1)] .
show ?thesis
proof (intro no_atp(10)[OF subsetI subsetI])
fix q assume "q \<in> { q \<in> carrier (K[X]). length q \<le> degree p }"
then have "q \<in> carrier (K[X])" and "length q \<le> degree p"
by simp+
show "q \<in> (\<lambda>q. q pmod p) ` carrier (K[X])"
proof (cases "q = []")
case True
have "p pmod p = q"
unfolding True pmod_zero_iff_pdivides[OF assms(1,2,2)]
using assms(1-2) pdivides_iff_shell by auto
thus ?thesis
using assms(2) by blast
next
case False
with \<open>length q \<le> degree p\<close> have "degree q < degree p"
using le_eq_less_or_eq by fastforce
with \<open>q \<in> carrier (K[X])\<close> show ?thesis
using pmod_const(2)[OF assms(1) _ assms(2), of q] by (metis imageI)
qed
next
fix q assume "q \<in> (\<lambda>q. q pmod p) ` carrier (K[X])"
then obtain q' where "q' \<in> carrier (K[X])" and "q = q' pmod p"
by auto
thus "q \<in> { q \<in> carrier (K[X]). length q \<le> degree p }"
using long_division_closed(2)[OF assms(1) _ assms(2), of q']
pmod_degree[OF assms(1) _ assms(2-3), of q']
by auto
qed
qed
lemma (in domain) Span_var_pow_base:
assumes "subfield K R"
shows "ring.Span (K[X]) (poly_of_const ` K) (ring.exp_base (K[X]) X n) =
{ q \<in> carrier (K[X]). length q \<le> n }" (is "?lhs = ?rhs")
proof -
note subring = subfieldE(1)[OF assms]
note subfield = univ_poly_subfield_of_consts[OF assms]
interpret UP: domain "K[X]"
using univ_poly_is_domain[OF subring] .
show ?thesis
proof (intro no_atp(10)[OF subsetI subsetI])
fix q assume "q \<in> { q \<in> carrier (K[X]). length q \<le> n }"
then have q: "q \<in> carrier (K[X])" "length q \<le> n"
by simp+
let ?repl = "replicate (n - length q) \<zero>\<^bsub>K[X]\<^esub>"
let ?map = "map poly_of_const q"
let ?comb = "UP.combine"
define Ks where "Ks = ?repl @ ?map"
have "q = ?comb ?map (UP.exp_base X (length q))"
using q eval_rewrite[OF subring q(1)] unfolding sym[OF UP.combine_eq_eval] by auto
moreover from \<open>length q \<le> n\<close>
have "?comb (?repl @ Ks) (UP.exp_base X n) = ?comb Ks (UP.exp_base X (length q))"
if "set Ks \<subseteq> carrier (K[X])" for Ks
using UP.combine_prepend_replicate[OF that UP.exp_base_closed[OF var_closed(1)[OF subring]]]
unfolding UP.drop_exp_base by auto
moreover have "set ?map \<subseteq> carrier (K[X])"
using map_norm_in_poly_ring_carrier[OF subring q(1)]
unfolding sym[OF univ_poly_carrier] polynomial_def by auto
moreover have "?repl = map poly_of_const (replicate (n - length q) \<zero>)"
unfolding poly_of_const_def univ_poly_zero by (induct "n - length q") (auto)
hence "set ?repl \<subseteq> poly_of_const ` K"
using subringE(2)[OF subring] by auto
moreover from \<open>q \<in> carrier (K[X])\<close> have "set q \<subseteq> K"
unfolding sym[OF univ_poly_carrier] polynomial_def by auto
hence "set ?map \<subseteq> poly_of_const ` K"
by auto
ultimately have "q = ?comb Ks (UP.exp_base X n)" and "set Ks \<subseteq> poly_of_const ` K"
by (simp add: Ks_def)+
thus "q \<in> UP.Span (poly_of_const ` K) (UP.exp_base X n)"
using UP.Span_eq_combine_set[OF subfield UP.exp_base_closed[OF var_closed(1)[OF subring]]] by auto
next
fix q assume "q \<in> UP.Span (poly_of_const ` K) (UP.exp_base X n)"
thus "q \<in> { q \<in> carrier (K[X]). length q \<le> n }"
proof (induction n arbitrary: q)
case 0 thus ?case
unfolding UP.exp_base_def by (auto simp add: univ_poly_zero)
next
case (Suc n)
then obtain k p where k: "k \<in> K" and p: "p \<in> UP.Span (poly_of_const ` K) (UP.exp_base X n)"
and q: "q = ((poly_of_const k) \<otimes>\<^bsub>K[X]\<^esub> (X [^]\<^bsub>K[X]\<^esub> n)) \<oplus>\<^bsub>K[X]\<^esub> p"
unfolding UP.exp_base_def using UP.line_extension_mem_iff by auto
have p_in_carrier: "p \<in> carrier (K[X])" and "length p \<le> n"
using Suc(1)[OF p] by simp+
moreover from \<open>k \<in> K\<close> have "poly_of_const k \<in> carrier (K[X])"
unfolding poly_of_const_def sym[OF univ_poly_carrier] polynomial_def by auto
ultimately have "q \<in> carrier (K[X])"
unfolding q using var_pow_closed[OF subring, of n] by algebra
moreover have "poly_of_const k = \<zero>\<^bsub>K[X]\<^esub>" if "k = \<zero>"
unfolding poly_of_const_def that univ_poly_zero by simp
with \<open>p \<in> carrier (K[X])\<close> have "q = p" if "k = \<zero>"
unfolding q using var_pow_closed[OF subring, of n] that by algebra
with \<open>length p \<le> n\<close> have "length q \<le> Suc n" if "k = \<zero>"
using that by simp
moreover have "poly_of_const k = [ k ]" if "k \<noteq> \<zero>"
unfolding poly_of_const_def using that by simp
hence monom: "monom k n = (poly_of_const k) \<otimes>\<^bsub>K[X]\<^esub> (X [^]\<^bsub>K[X]\<^esub> n)" if "k \<noteq> \<zero>"
using that monom_eq_var_pow[OF subring] subfieldE(3)[OF assms] k by auto
with \<open>p \<in> carrier (K[X])\<close> and \<open>k \<in> K\<close> and \<open>length p \<le> n\<close>
have "length q = Suc n" if "k \<noteq> \<zero>"
using that poly_add_length_eq[OF subring monom_is_polynomial[OF subring, of k n], of p]
unfolding univ_poly_carrier monom_def univ_poly_add sym[OF monom[OF that]] q by auto
ultimately show ?case
by (cases "k = \<zero>", auto)
qed
qed
qed
lemma (in domain) var_pow_base_independent:
assumes "subfield K R"
shows "ring.independent (K[X]) (poly_of_const ` K) (ring.exp_base (K[X]) X n)"
proof -
note subring = subfieldE(1)[OF assms]
interpret UP: domain "K[X]"
using univ_poly_is_domain[OF subring] .
show ?thesis
proof (induction n, simp add: UP.exp_base_def)
case (Suc n)
have "X [^]\<^bsub>K[X]\<^esub> n \<notin> UP.Span (poly_of_const ` K) (ring.exp_base (K[X]) X n)"
unfolding sym[OF unitary_monom_eq_var_pow[OF subring]] monom_def
Span_var_pow_base[OF assms] by auto
moreover have "X [^]\<^bsub>K[X]\<^esub> n # UP.exp_base X n = UP.exp_base X (Suc n)"
unfolding UP.exp_base_def by simp
ultimately show ?case
using UP.li_Cons[OF var_pow_closed[OF subring, of n] _Suc] by simp
qed
qed
lemma (in domain) bounded_degree_dimension:
assumes "subfield K R"
shows "ring.dimension (K[X]) n (poly_of_const ` K) { q \<in> carrier (K[X]). length q \<le> n }"
proof -
interpret UP: domain "K[X]"
using univ_poly_is_domain[OF subfieldE(1)[OF assms]] .
have "length (UP.exp_base X n) = n"
unfolding UP.exp_base_def by simp
thus ?thesis
using UP.dimension_independent[OF var_pow_base_independent[OF assms], of n]
unfolding Span_var_pow_base[OF assms] by simp
qed
corollary (in domain) univ_poly_infinite_dimension:
assumes "subfield K R" shows "ring.infinite_dimension (K[X]) (poly_of_const ` K) (carrier (K[X]))"
proof (rule ccontr)
interpret UP: domain "K[X]"
using univ_poly_is_domain[OF subfieldE(1)[OF assms]] .
assume "\<not> UP.infinite_dimension (poly_of_const ` K) (carrier (K[X]))"
then obtain n where n: "UP.dimension n (poly_of_const ` K) (carrier (K[X]))"
by blast
show False
using UP.independent_length_le_dimension[OF univ_poly_subfield_of_consts[OF assms] n
var_pow_base_independent[OF assms, of "Suc n"]
UP.exp_base_closed[OF var_closed(1)[OF subfieldE(1)[OF assms]]]]
unfolding UP.exp_base_def by simp
qed
corollary (in domain) rupture_dimension:
assumes "subfield K R" and "p \<in> carrier (K[X])" and "degree p > 0"
shows "ring.dimension (Rupt K p) (degree p) ((rupture_surj K p) ` poly_of_const ` K) (carrier (Rupt K p))"
proof -
interpret UP: domain "K[X]"
using univ_poly_is_domain[OF subfieldE(1)[OF assms(1)]] .
interpret Hom: ring_hom_ring "K[X]" "Rupt K p" "rupture_surj K p"
using rupture_surj_hom(2)[OF subfieldE(1)[OF assms(1)] assms(2)] .
have not_nil: "p \<noteq> []"
using assms(3) by auto
show ?thesis
using Hom.inj_hom_dimension[OF univ_poly_subfield_of_consts rupture_one_not_zero
rupture_surj_inj_on] bounded_degree_dimension assms
unfolding sym[OF rupture_carrier_as_pmod_image[OF assms(1-2)]]
pmod_image_characterization[OF assms(1-2) not_nil]
by simp
qed
end