src/HOL/Library/Fundamental_Theorem_Algebra.thy
 author blanchet Wed Sep 24 15:45:55 2014 +0200 (2014-09-24) changeset 58425 246985c6b20b parent 57862 8f074e6e22fc child 58881 b9556a055632 permissions -rw-r--r--
simpler proof
```     1 (* Author: Amine Chaieb, TU Muenchen *)
```
```     2
```
```     3 header{*Fundamental Theorem of Algebra*}
```
```     4
```
```     5 theory Fundamental_Theorem_Algebra
```
```     6 imports Polynomial Complex_Main
```
```     7 begin
```
```     8
```
```     9 subsection {* More lemmas about module of complex numbers *}
```
```    10
```
```    11 text{* The triangle inequality for cmod *}
```
```    12 lemma complex_mod_triangle_sub: "cmod w \<le> cmod (w + z) + norm z"
```
```    13   using complex_mod_triangle_ineq2[of "w + z" "-z"] by auto
```
```    14
```
```    15 subsection {* Basic lemmas about polynomials *}
```
```    16
```
```    17 lemma poly_bound_exists:
```
```    18   fixes p :: "'a::{comm_semiring_0,real_normed_div_algebra} poly"
```
```    19   shows "\<exists>m. m > 0 \<and> (\<forall>z. norm z \<le> r \<longrightarrow> norm (poly p z) \<le> m)"
```
```    20 proof (induct p)
```
```    21   case 0
```
```    22   then show ?case by (rule exI[where x=1]) simp
```
```    23 next
```
```    24   case (pCons c cs)
```
```    25   from pCons.hyps obtain m where m: "\<forall>z. norm z \<le> r \<longrightarrow> norm (poly cs z) \<le> m"
```
```    26     by blast
```
```    27   let ?k = " 1 + norm c + \<bar>r * m\<bar>"
```
```    28   have kp: "?k > 0"
```
```    29     using abs_ge_zero[of "r*m"] norm_ge_zero[of c] by arith
```
```    30   {
```
```    31     fix z :: 'a
```
```    32     assume H: "norm z \<le> r"
```
```    33     from m H have th: "norm (poly cs z) \<le> m"
```
```    34       by blast
```
```    35     from H have rp: "r \<ge> 0"
```
```    36       using norm_ge_zero[of z] by arith
```
```    37     have "norm (poly (pCons c cs) z) \<le> norm c + norm (z * poly cs z)"
```
```    38       using norm_triangle_ineq[of c "z* poly cs z"] by simp
```
```    39     also have "\<dots> \<le> norm c + r * m"
```
```    40       using mult_mono[OF H th rp norm_ge_zero[of "poly cs z"]]
```
```    41       by (simp add: norm_mult)
```
```    42     also have "\<dots> \<le> ?k"
```
```    43       by simp
```
```    44     finally have "norm (poly (pCons c cs) z) \<le> ?k" .
```
```    45   }
```
```    46   with kp show ?case by blast
```
```    47 qed
```
```    48
```
```    49
```
```    50 text{* Offsetting the variable in a polynomial gives another of same degree *}
```
```    51
```
```    52 definition offset_poly :: "'a::comm_semiring_0 poly \<Rightarrow> 'a \<Rightarrow> 'a poly"
```
```    53   where "offset_poly p h = fold_coeffs (\<lambda>a q. smult h q + pCons a q) p 0"
```
```    54
```
```    55 lemma offset_poly_0: "offset_poly 0 h = 0"
```
```    56   by (simp add: offset_poly_def)
```
```    57
```
```    58 lemma offset_poly_pCons:
```
```    59   "offset_poly (pCons a p) h =
```
```    60     smult h (offset_poly p h) + pCons a (offset_poly p h)"
```
```    61   by (cases "p = 0 \<and> a = 0") (auto simp add: offset_poly_def)
```
```    62
```
```    63 lemma offset_poly_single: "offset_poly [:a:] h = [:a:]"
```
```    64   by (simp add: offset_poly_pCons offset_poly_0)
```
```    65
```
```    66 lemma poly_offset_poly: "poly (offset_poly p h) x = poly p (h + x)"
```
```    67   apply (induct p)
```
```    68   apply (simp add: offset_poly_0)
```
```    69   apply (simp add: offset_poly_pCons algebra_simps)
```
```    70   done
```
```    71
```
```    72 lemma offset_poly_eq_0_lemma: "smult c p + pCons a p = 0 \<Longrightarrow> p = 0"
```
```    73   by (induct p arbitrary: a) (simp, force)
```
```    74
```
```    75 lemma offset_poly_eq_0_iff: "offset_poly p h = 0 \<longleftrightarrow> p = 0"
```
```    76   apply (safe intro!: offset_poly_0)
```
```    77   apply (induct p)
```
```    78   apply simp
```
```    79   apply (simp add: offset_poly_pCons)
```
```    80   apply (frule offset_poly_eq_0_lemma, simp)
```
```    81   done
```
```    82
```
```    83 lemma degree_offset_poly: "degree (offset_poly p h) = degree p"
```
```    84   apply (induct p)
```
```    85   apply (simp add: offset_poly_0)
```
```    86   apply (case_tac "p = 0")
```
```    87   apply (simp add: offset_poly_0 offset_poly_pCons)
```
```    88   apply (simp add: offset_poly_pCons)
```
```    89   apply (subst degree_add_eq_right)
```
```    90   apply (rule le_less_trans [OF degree_smult_le])
```
```    91   apply (simp add: offset_poly_eq_0_iff)
```
```    92   apply (simp add: offset_poly_eq_0_iff)
```
```    93   done
```
```    94
```
```    95 definition "psize p = (if p = 0 then 0 else Suc (degree p))"
```
```    96
```
```    97 lemma psize_eq_0_iff [simp]: "psize p = 0 \<longleftrightarrow> p = 0"
```
```    98   unfolding psize_def by simp
```
```    99
```
```   100 lemma poly_offset:
```
```   101   fixes p :: "'a::comm_ring_1 poly"
```
```   102   shows "\<exists>q. psize q = psize p \<and> (\<forall>x. poly q x = poly p (a + x))"
```
```   103 proof (intro exI conjI)
```
```   104   show "psize (offset_poly p a) = psize p"
```
```   105     unfolding psize_def
```
```   106     by (simp add: offset_poly_eq_0_iff degree_offset_poly)
```
```   107   show "\<forall>x. poly (offset_poly p a) x = poly p (a + x)"
```
```   108     by (simp add: poly_offset_poly)
```
```   109 qed
```
```   110
```
```   111 text{* An alternative useful formulation of completeness of the reals *}
```
```   112 lemma real_sup_exists:
```
```   113   assumes ex: "\<exists>x. P x"
```
```   114     and bz: "\<exists>z. \<forall>x. P x \<longrightarrow> x < z"
```
```   115   shows "\<exists>s::real. \<forall>y. (\<exists>x. P x \<and> y < x) \<longleftrightarrow> y < s"
```
```   116 proof
```
```   117   from bz have "bdd_above (Collect P)"
```
```   118     by (force intro: less_imp_le)
```
```   119   then show "\<forall>y. (\<exists>x. P x \<and> y < x) \<longleftrightarrow> y < Sup (Collect P)"
```
```   120     using ex bz by (subst less_cSup_iff) auto
```
```   121 qed
```
```   122
```
```   123 subsection {* Fundamental theorem of algebra *}
```
```   124 lemma  unimodular_reduce_norm:
```
```   125   assumes md: "cmod z = 1"
```
```   126   shows "cmod (z + 1) < 1 \<or> cmod (z - 1) < 1 \<or> cmod (z + ii) < 1 \<or> cmod (z - ii) < 1"
```
```   127 proof -
```
```   128   obtain x y where z: "z = Complex x y "
```
```   129     by (cases z) auto
```
```   130   from md z have xy: "x\<^sup>2 + y\<^sup>2 = 1"
```
```   131     by (simp add: cmod_def)
```
```   132   {
```
```   133     assume C: "cmod (z + 1) \<ge> 1" "cmod (z - 1) \<ge> 1" "cmod (z + ii) \<ge> 1" "cmod (z - ii) \<ge> 1"
```
```   134     from C z xy have "2 * x \<le> 1" "2 * x \<ge> -1" "2 * y \<le> 1" "2 * y \<ge> -1"
```
```   135       by (simp_all add: cmod_def power2_eq_square algebra_simps)
```
```   136     then have "abs (2 * x) \<le> 1" "abs (2 * y) \<le> 1"
```
```   137       by simp_all
```
```   138     then have "(abs (2 * x))\<^sup>2 \<le> 1\<^sup>2" "(abs (2 * y))\<^sup>2 \<le> 1\<^sup>2"
```
```   139       by - (rule power_mono, simp, simp)+
```
```   140     then have th0: "4 * x\<^sup>2 \<le> 1" "4 * y\<^sup>2 \<le> 1"
```
```   141       by (simp_all add: power_mult_distrib)
```
```   142     from add_mono[OF th0] xy have False by simp
```
```   143   }
```
```   144   then show ?thesis
```
```   145     unfolding linorder_not_le[symmetric] by blast
```
```   146 qed
```
```   147
```
```   148 text{* Hence we can always reduce modulus of @{text "1 + b z^n"} if nonzero *}
```
```   149 lemma reduce_poly_simple:
```
```   150   assumes b: "b \<noteq> 0"
```
```   151     and n: "n \<noteq> 0"
```
```   152   shows "\<exists>z. cmod (1 + b * z^n) < 1"
```
```   153   using n
```
```   154 proof (induct n rule: nat_less_induct)
```
```   155   fix n
```
```   156   assume IH: "\<forall>m<n. m \<noteq> 0 \<longrightarrow> (\<exists>z. cmod (1 + b * z ^ m) < 1)"
```
```   157   assume n: "n \<noteq> 0"
```
```   158   let ?P = "\<lambda>z n. cmod (1 + b * z ^ n) < 1"
```
```   159   {
```
```   160     assume e: "even n"
```
```   161     then have "\<exists>m. n = 2 * m"
```
```   162       by presburger
```
```   163     then obtain m where m: "n = 2 * m"
```
```   164       by blast
```
```   165     from n m have "m \<noteq> 0" "m < n"
```
```   166       by presburger+
```
```   167     with IH[rule_format, of m] obtain z where z: "?P z m"
```
```   168       by blast
```
```   169     from z have "?P (csqrt z) n"
```
```   170       by (simp add: m power_mult power2_csqrt)
```
```   171     then have "\<exists>z. ?P z n" ..
```
```   172   }
```
```   173   moreover
```
```   174   {
```
```   175     assume o: "odd n"
```
```   176     have th0: "cmod (complex_of_real (cmod b) / b) = 1"
```
```   177       using b by (simp add: norm_divide)
```
```   178     from o have "\<exists>m. n = Suc (2 * m)"
```
```   179       by presburger+
```
```   180     then obtain m where m: "n = Suc (2 * m)"
```
```   181       by blast
```
```   182     from unimodular_reduce_norm[OF th0] o
```
```   183     have "\<exists>v. cmod (complex_of_real (cmod b) / b + v^n) < 1"
```
```   184       apply (cases "cmod (complex_of_real (cmod b) / b + 1) < 1")
```
```   185       apply (rule_tac x="1" in exI)
```
```   186       apply simp
```
```   187       apply (cases "cmod (complex_of_real (cmod b) / b - 1) < 1")
```
```   188       apply (rule_tac x="-1" in exI)
```
```   189       apply simp
```
```   190       apply (cases "cmod (complex_of_real (cmod b) / b + ii) < 1")
```
```   191       apply (cases "even m")
```
```   192       apply (rule_tac x="ii" in exI)
```
```   193       apply (simp add: m power_mult)
```
```   194       apply (rule_tac x="- ii" in exI)
```
```   195       apply (simp add: m power_mult)
```
```   196       apply (cases "even m")
```
```   197       apply (rule_tac x="- ii" in exI)
```
```   198       apply (simp add: m power_mult)
```
```   199       apply (auto simp add: m power_mult)
```
```   200       apply (rule_tac x="ii" in exI)
```
```   201       apply (auto simp add: m power_mult)
```
```   202       done
```
```   203     then obtain v where v: "cmod (complex_of_real (cmod b) / b + v^n) < 1"
```
```   204       by blast
```
```   205     let ?w = "v / complex_of_real (root n (cmod b))"
```
```   206     from odd_real_root_pow[OF o, of "cmod b"]
```
```   207     have th1: "?w ^ n = v^n / complex_of_real (cmod b)"
```
```   208       by (simp add: power_divide of_real_power[symmetric])
```
```   209     have th2:"cmod (complex_of_real (cmod b) / b) = 1"
```
```   210       using b by (simp add: norm_divide)
```
```   211     then have th3: "cmod (complex_of_real (cmod b) / b) \<ge> 0"
```
```   212       by simp
```
```   213     have th4: "cmod (complex_of_real (cmod b) / b) *
```
```   214         cmod (1 + b * (v ^ n / complex_of_real (cmod b))) <
```
```   215         cmod (complex_of_real (cmod b) / b) * 1"
```
```   216       apply (simp only: norm_mult[symmetric] distrib_left)
```
```   217       using b v
```
```   218       apply (simp add: th2)
```
```   219       done
```
```   220     from mult_less_imp_less_left[OF th4 th3]
```
```   221     have "?P ?w n" unfolding th1 .
```
```   222     then have "\<exists>z. ?P z n" ..
```
```   223   }
```
```   224   ultimately show "\<exists>z. ?P z n" by blast
```
```   225 qed
```
```   226
```
```   227 text{* Bolzano-Weierstrass type property for closed disc in complex plane. *}
```
```   228
```
```   229 lemma metric_bound_lemma: "cmod (x - y) \<le> \<bar>Re x - Re y\<bar> + \<bar>Im x - Im y\<bar>"
```
```   230   using real_sqrt_sum_squares_triangle_ineq[of "Re x - Re y" 0 0 "Im x - Im y"]
```
```   231   unfolding cmod_def by simp
```
```   232
```
```   233 lemma bolzano_weierstrass_complex_disc:
```
```   234   assumes r: "\<forall>n. cmod (s n) \<le> r"
```
```   235   shows "\<exists>f z. subseq f \<and> (\<forall>e >0. \<exists>N. \<forall>n \<ge> N. cmod (s (f n) - z) < e)"
```
```   236 proof-
```
```   237   from seq_monosub[of "Re \<circ> s"]
```
```   238   obtain f where f: "subseq f" "monoseq (\<lambda>n. Re (s (f n)))"
```
```   239     unfolding o_def by blast
```
```   240   from seq_monosub[of "Im \<circ> s \<circ> f"]
```
```   241   obtain g where g: "subseq g" "monoseq (\<lambda>n. Im (s (f (g n))))"
```
```   242     unfolding o_def by blast
```
```   243   let ?h = "f \<circ> g"
```
```   244   from r[rule_format, of 0] have rp: "r \<ge> 0"
```
```   245     using norm_ge_zero[of "s 0"] by arith
```
```   246   have th: "\<forall>n. r + 1 \<ge> \<bar>Re (s n)\<bar>"
```
```   247   proof
```
```   248     fix n
```
```   249     from abs_Re_le_cmod[of "s n"] r[rule_format, of n]
```
```   250     show "\<bar>Re (s n)\<bar> \<le> r + 1" by arith
```
```   251   qed
```
```   252   have conv1: "convergent (\<lambda>n. Re (s (f n)))"
```
```   253     apply (rule Bseq_monoseq_convergent)
```
```   254     apply (simp add: Bseq_def)
```
```   255     apply (metis gt_ex le_less_linear less_trans order.trans th)
```
```   256     apply (rule f(2))
```
```   257     done
```
```   258   have th: "\<forall>n. r + 1 \<ge> \<bar>Im (s n)\<bar>"
```
```   259   proof
```
```   260     fix n
```
```   261     from abs_Im_le_cmod[of "s n"] r[rule_format, of n]
```
```   262     show "\<bar>Im (s n)\<bar> \<le> r + 1"
```
```   263       by arith
```
```   264   qed
```
```   265
```
```   266   have conv2: "convergent (\<lambda>n. Im (s (f (g n))))"
```
```   267     apply (rule Bseq_monoseq_convergent)
```
```   268     apply (simp add: Bseq_def)
```
```   269     apply (metis gt_ex le_less_linear less_trans order.trans th)
```
```   270     apply (rule g(2))
```
```   271     done
```
```   272
```
```   273   from conv1[unfolded convergent_def] obtain x where "LIMSEQ (\<lambda>n. Re (s (f n))) x"
```
```   274     by blast
```
```   275   then have x: "\<forall>r>0. \<exists>n0. \<forall>n\<ge>n0. \<bar>Re (s (f n)) - x\<bar> < r"
```
```   276     unfolding LIMSEQ_iff real_norm_def .
```
```   277
```
```   278   from conv2[unfolded convergent_def] obtain y where "LIMSEQ (\<lambda>n. Im (s (f (g n)))) y"
```
```   279     by blast
```
```   280   then have y: "\<forall>r>0. \<exists>n0. \<forall>n\<ge>n0. \<bar>Im (s (f (g n))) - y\<bar> < r"
```
```   281     unfolding LIMSEQ_iff real_norm_def .
```
```   282   let ?w = "Complex x y"
```
```   283   from f(1) g(1) have hs: "subseq ?h"
```
```   284     unfolding subseq_def by auto
```
```   285   {
```
```   286     fix e :: real
```
```   287     assume ep: "e > 0"
```
```   288     then have e2: "e/2 > 0"
```
```   289       by simp
```
```   290     from x[rule_format, OF e2] y[rule_format, OF e2]
```
```   291     obtain N1 N2 where N1: "\<forall>n\<ge>N1. \<bar>Re (s (f n)) - x\<bar> < e / 2"
```
```   292       and N2: "\<forall>n\<ge>N2. \<bar>Im (s (f (g n))) - y\<bar> < e / 2"
```
```   293       by blast
```
```   294     {
```
```   295       fix n
```
```   296       assume nN12: "n \<ge> N1 + N2"
```
```   297       then have nN1: "g n \<ge> N1" and nN2: "n \<ge> N2"
```
```   298         using seq_suble[OF g(1), of n] by arith+
```
```   299       from add_strict_mono[OF N1[rule_format, OF nN1] N2[rule_format, OF nN2]]
```
```   300       have "cmod (s (?h n) - ?w) < e"
```
```   301         using metric_bound_lemma[of "s (f (g n))" ?w] by simp
```
```   302     }
```
```   303     then have "\<exists>N. \<forall>n\<ge>N. cmod (s (?h n) - ?w) < e"
```
```   304       by blast
```
```   305   }
```
```   306   with hs show ?thesis by blast
```
```   307 qed
```
```   308
```
```   309 text{* Polynomial is continuous. *}
```
```   310
```
```   311 lemma poly_cont:
```
```   312   fixes p :: "'a::{comm_semiring_0,real_normed_div_algebra} poly"
```
```   313   assumes ep: "e > 0"
```
```   314   shows "\<exists>d >0. \<forall>w. 0 < norm (w - z) \<and> norm (w - z) < d \<longrightarrow> norm (poly p w - poly p z) < e"
```
```   315 proof -
```
```   316   obtain q where q: "degree q = degree p" "\<And>x. poly q x = poly p (z + x)"
```
```   317   proof
```
```   318     show "degree (offset_poly p z) = degree p"
```
```   319       by (rule degree_offset_poly)
```
```   320     show "\<And>x. poly (offset_poly p z) x = poly p (z + x)"
```
```   321       by (rule poly_offset_poly)
```
```   322   qed
```
```   323   have th: "\<And>w. poly q (w - z) = poly p w"
```
```   324     using q(2)[of "w - z" for w] by simp
```
```   325   show ?thesis unfolding th[symmetric]
```
```   326   proof (induct q)
```
```   327     case 0
```
```   328     then show ?case
```
```   329       using ep by auto
```
```   330   next
```
```   331     case (pCons c cs)
```
```   332     from poly_bound_exists[of 1 "cs"]
```
```   333     obtain m where m: "m > 0" "\<And>z. norm z \<le> 1 \<Longrightarrow> norm (poly cs z) \<le> m"
```
```   334       by blast
```
```   335     from ep m(1) have em0: "e/m > 0"
```
```   336       by (simp add: field_simps)
```
```   337     have one0: "1 > (0::real)"
```
```   338       by arith
```
```   339     from real_lbound_gt_zero[OF one0 em0]
```
```   340     obtain d where d: "d > 0" "d < 1" "d < e / m"
```
```   341       by blast
```
```   342     from d(1,3) m(1) have dm: "d * m > 0" "d * m < e"
```
```   343       by (simp_all add: field_simps)
```
```   344     show ?case
```
```   345     proof (rule ex_forward[OF real_lbound_gt_zero[OF one0 em0]], clarsimp simp add: norm_mult)
```
```   346       fix d w
```
```   347       assume H: "d > 0" "d < 1" "d < e/m" "w \<noteq> z" "norm (w - z) < d"
```
```   348       then have d1: "norm (w-z) \<le> 1" "d \<ge> 0"
```
```   349         by simp_all
```
```   350       from H(3) m(1) have dme: "d*m < e"
```
```   351         by (simp add: field_simps)
```
```   352       from H have th: "norm (w - z) \<le> d"
```
```   353         by simp
```
```   354       from mult_mono[OF th m(2)[OF d1(1)] d1(2) norm_ge_zero] dme
```
```   355       show "norm (w - z) * norm (poly cs (w - z)) < e"
```
```   356         by simp
```
```   357     qed
```
```   358   qed
```
```   359 qed
```
```   360
```
```   361 text{* Hence a polynomial attains minimum on a closed disc
```
```   362   in the complex plane. *}
```
```   363 lemma poly_minimum_modulus_disc: "\<exists>z. \<forall>w. cmod w \<le> r \<longrightarrow> cmod (poly p z) \<le> cmod (poly p w)"
```
```   364 proof -
```
```   365   {
```
```   366     assume "\<not> r \<ge> 0"
```
```   367     then have ?thesis
```
```   368       by (metis norm_ge_zero order.trans)
```
```   369   }
```
```   370   moreover
```
```   371   {
```
```   372     assume rp: "r \<ge> 0"
```
```   373     from rp have "cmod 0 \<le> r \<and> cmod (poly p 0) = - (- cmod (poly p 0))"
```
```   374       by simp
```
```   375     then have mth1: "\<exists>x z. cmod z \<le> r \<and> cmod (poly p z) = - x"
```
```   376       by blast
```
```   377     {
```
```   378       fix x z
```
```   379       assume H: "cmod z \<le> r" "cmod (poly p z) = - x" "\<not> x < 1"
```
```   380       then have "- x < 0 "
```
```   381         by arith
```
```   382       with H(2) norm_ge_zero[of "poly p z"] have False
```
```   383         by simp
```
```   384     }
```
```   385     then have mth2: "\<exists>z. \<forall>x. (\<exists>z. cmod z \<le> r \<and> cmod (poly p z) = - x) \<longrightarrow> x < z"
```
```   386       by blast
```
```   387     from real_sup_exists[OF mth1 mth2] obtain s where
```
```   388       s: "\<forall>y. (\<exists>x. (\<exists>z. cmod z \<le> r \<and> cmod (poly p z) = - x) \<and> y < x) \<longleftrightarrow> y < s" by blast
```
```   389     let ?m = "- s"
```
```   390     {
```
```   391       fix y
```
```   392       from s[rule_format, of "-y"]
```
```   393       have "(\<exists>z x. cmod z \<le> r \<and> - (- cmod (poly p z)) < y) \<longleftrightarrow> ?m < y"
```
```   394         unfolding minus_less_iff[of y ] equation_minus_iff by blast
```
```   395     }
```
```   396     note s1 = this[unfolded minus_minus]
```
```   397     from s1[of ?m] have s1m: "\<And>z x. cmod z \<le> r \<Longrightarrow> cmod (poly p z) \<ge> ?m"
```
```   398       by auto
```
```   399     {
```
```   400       fix n :: nat
```
```   401       from s1[rule_format, of "?m + 1/real (Suc n)"]
```
```   402       have "\<exists>z. cmod z \<le> r \<and> cmod (poly p z) < - s + 1 / real (Suc n)"
```
```   403         by simp
```
```   404     }
```
```   405     then have th: "\<forall>n. \<exists>z. cmod z \<le> r \<and> cmod (poly p z) < - s + 1 / real (Suc n)" ..
```
```   406     from choice[OF th] obtain g where
```
```   407         g: "\<forall>n. cmod (g n) \<le> r" "\<forall>n. cmod (poly p (g n)) <?m + 1 /real(Suc n)"
```
```   408       by blast
```
```   409     from bolzano_weierstrass_complex_disc[OF g(1)]
```
```   410     obtain f z where fz: "subseq f" "\<forall>e>0. \<exists>N. \<forall>n\<ge>N. cmod (g (f n) - z) < e"
```
```   411       by blast
```
```   412     {
```
```   413       fix w
```
```   414       assume wr: "cmod w \<le> r"
```
```   415       let ?e = "\<bar>cmod (poly p z) - ?m\<bar>"
```
```   416       {
```
```   417         assume e: "?e > 0"
```
```   418         then have e2: "?e/2 > 0"
```
```   419           by simp
```
```   420         from poly_cont[OF e2, of z p] obtain d where
```
```   421             d: "d > 0" "\<forall>w. 0<cmod (w - z)\<and> cmod(w - z) < d \<longrightarrow> cmod(poly p w - poly p z) < ?e/2"
```
```   422           by blast
```
```   423         {
```
```   424           fix w
```
```   425           assume w: "cmod (w - z) < d"
```
```   426           have "cmod(poly p w - poly p z) < ?e / 2"
```
```   427             using d(2)[rule_format, of w] w e by (cases "w = z") simp_all
```
```   428         }
```
```   429         note th1 = this
```
```   430
```
```   431         from fz(2) d(1) obtain N1 where N1: "\<forall>n\<ge>N1. cmod (g (f n) - z) < d"
```
```   432           by blast
```
```   433         from reals_Archimedean2[of "2/?e"] obtain N2 :: nat where N2: "2/?e < real N2"
```
```   434           by blast
```
```   435         have th2: "cmod (poly p (g (f (N1 + N2))) - poly p z) < ?e/2"
```
```   436           using N1[rule_format, of "N1 + N2"] th1 by simp
```
```   437         {
```
```   438           fix a b e2 m :: real
```
```   439           have "a < e2 \<Longrightarrow> \<bar>b - m\<bar> < e2 \<Longrightarrow> 2 * e2 \<le> \<bar>b - m\<bar> + a \<Longrightarrow> False"
```
```   440             by arith
```
```   441         }
```
```   442         note th0 = this
```
```   443         have ath: "\<And>m x e::real. m \<le> x \<Longrightarrow> x < m + e \<Longrightarrow> \<bar>x - m\<bar> < e"
```
```   444           by arith
```
```   445         from s1m[OF g(1)[rule_format]] have th31: "?m \<le> cmod(poly p (g (f (N1 + N2))))" .
```
```   446         from seq_suble[OF fz(1), of "N1 + N2"]
```
```   447         have th00: "real (Suc (N1 + N2)) \<le> real (Suc (f (N1 + N2)))"
```
```   448           by simp
```
```   449         have th000: "0 \<le> (1::real)" "(1::real) \<le> 1" "real (Suc (N1 + N2)) > 0"
```
```   450           using N2 by auto
```
```   451         from frac_le[OF th000 th00]
```
```   452         have th00: "?m + 1 / real (Suc (f (N1 + N2))) \<le> ?m + 1 / real (Suc (N1 + N2))"
```
```   453           by simp
```
```   454         from g(2)[rule_format, of "f (N1 + N2)"]
```
```   455         have th01:"cmod (poly p (g (f (N1 + N2)))) < - s + 1 / real (Suc (f (N1 + N2)))" .
```
```   456         from order_less_le_trans[OF th01 th00]
```
```   457         have th32: "cmod (poly p (g (f (N1 + N2)))) < ?m + (1/ real(Suc (N1 + N2)))" .
```
```   458         from N2 have "2/?e < real (Suc (N1 + N2))"
```
```   459           by arith
```
```   460         with e2 less_imp_inverse_less[of "2/?e" "real (Suc (N1 + N2))"]
```
```   461         have "?e/2 > 1/ real (Suc (N1 + N2))"
```
```   462           by (simp add: inverse_eq_divide)
```
```   463         with ath[OF th31 th32]
```
```   464         have thc1: "\<bar>cmod (poly p (g (f (N1 + N2)))) - ?m\<bar> < ?e/2"
```
```   465           by arith
```
```   466         have ath2: "\<And>a b c m::real. \<bar>a - b\<bar> \<le> c \<Longrightarrow> \<bar>b - m\<bar> \<le> \<bar>a - m\<bar> + c"
```
```   467           by arith
```
```   468         have th22: "\<bar>cmod (poly p (g (f (N1 + N2)))) - cmod (poly p z)\<bar> \<le>
```
```   469             cmod (poly p (g (f (N1 + N2))) - poly p z)"
```
```   470           by (simp add: norm_triangle_ineq3)
```
```   471         from ath2[OF th22, of ?m]
```
```   472         have thc2: "2 * (?e/2) \<le>
```
```   473             \<bar>cmod(poly p (g (f (N1 + N2)))) - ?m\<bar> + cmod (poly p (g (f (N1 + N2))) - poly p z)"
```
```   474           by simp
```
```   475         from th0[OF th2 thc1 thc2] have False .
```
```   476       }
```
```   477       then have "?e = 0"
```
```   478         by auto
```
```   479       then have "cmod (poly p z) = ?m"
```
```   480         by simp
```
```   481       with s1m[OF wr] have "cmod (poly p z) \<le> cmod (poly p w)"
```
```   482         by simp
```
```   483     }
```
```   484     then have ?thesis by blast
```
```   485   }
```
```   486   ultimately show ?thesis by blast
```
```   487 qed
```
```   488
```
```   489 text {* Nonzero polynomial in z goes to infinity as z does. *}
```
```   490
```
```   491 lemma poly_infinity:
```
```   492   fixes p:: "'a::{comm_semiring_0,real_normed_div_algebra} poly"
```
```   493   assumes ex: "p \<noteq> 0"
```
```   494   shows "\<exists>r. \<forall>z. r \<le> norm z \<longrightarrow> d \<le> norm (poly (pCons a p) z)"
```
```   495   using ex
```
```   496 proof (induct p arbitrary: a d)
```
```   497   case 0
```
```   498   then show ?case by simp
```
```   499 next
```
```   500   case (pCons c cs a d)
```
```   501   show ?case
```
```   502   proof (cases "cs = 0")
```
```   503     case False
```
```   504     with pCons.hyps obtain r where r: "\<forall>z. r \<le> norm z \<longrightarrow> d + norm a \<le> norm (poly (pCons c cs) z)"
```
```   505       by blast
```
```   506     let ?r = "1 + \<bar>r\<bar>"
```
```   507     {
```
```   508       fix z :: 'a
```
```   509       assume h: "1 + \<bar>r\<bar> \<le> norm z"
```
```   510       have r0: "r \<le> norm z"
```
```   511         using h by arith
```
```   512       from r[rule_format, OF r0] have th0: "d + norm a \<le> 1 * norm(poly (pCons c cs) z)"
```
```   513         by arith
```
```   514       from h have z1: "norm z \<ge> 1"
```
```   515         by arith
```
```   516       from order_trans[OF th0 mult_right_mono[OF z1 norm_ge_zero[of "poly (pCons c cs) z"]]]
```
```   517       have th1: "d \<le> norm(z * poly (pCons c cs) z) - norm a"
```
```   518         unfolding norm_mult by (simp add: algebra_simps)
```
```   519       from norm_diff_ineq[of "z * poly (pCons c cs) z" a]
```
```   520       have th2: "norm (z * poly (pCons c cs) z) - norm a \<le> norm (poly (pCons a (pCons c cs)) z)"
```
```   521         by (simp add: algebra_simps)
```
```   522       from th1 th2 have "d \<le> norm (poly (pCons a (pCons c cs)) z)"
```
```   523         by arith
```
```   524     }
```
```   525     then show ?thesis by blast
```
```   526   next
```
```   527     case True
```
```   528     with pCons.prems have c0: "c \<noteq> 0"
```
```   529       by simp
```
```   530     {
```
```   531       fix z :: 'a
```
```   532       assume h: "(\<bar>d\<bar> + norm a) / norm c \<le> norm z"
```
```   533       from c0 have "norm c > 0"
```
```   534         by simp
```
```   535       from h c0 have th0: "\<bar>d\<bar> + norm a \<le> norm (z * c)"
```
```   536         by (simp add: field_simps norm_mult)
```
```   537       have ath: "\<And>mzh mazh ma. mzh \<le> mazh + ma \<Longrightarrow> \<bar>d\<bar> + ma \<le> mzh \<Longrightarrow> d \<le> mazh"
```
```   538         by arith
```
```   539       from norm_diff_ineq[of "z * c" a] have th1: "norm (z * c) \<le> norm (a + z * c) + norm a"
```
```   540         by (simp add: algebra_simps)
```
```   541       from ath[OF th1 th0] have "d \<le> norm (poly (pCons a (pCons c cs)) z)"
```
```   542         using True by simp
```
```   543     }
```
```   544     then show ?thesis by blast
```
```   545   qed
```
```   546 qed
```
```   547
```
```   548 text {* Hence polynomial's modulus attains its minimum somewhere. *}
```
```   549 lemma poly_minimum_modulus: "\<exists>z.\<forall>w. cmod (poly p z) \<le> cmod (poly p w)"
```
```   550 proof (induct p)
```
```   551   case 0
```
```   552   then show ?case by simp
```
```   553 next
```
```   554   case (pCons c cs)
```
```   555   show ?case
```
```   556   proof (cases "cs = 0")
```
```   557     case False
```
```   558     from poly_infinity[OF False, of "cmod (poly (pCons c cs) 0)" c]
```
```   559     obtain r where r: "\<And>z. r \<le> cmod z \<Longrightarrow> cmod (poly (pCons c cs) 0) \<le> cmod (poly (pCons c cs) z)"
```
```   560       by blast
```
```   561     have ath: "\<And>z r. r \<le> cmod z \<or> cmod z \<le> \<bar>r\<bar>"
```
```   562       by arith
```
```   563     from poly_minimum_modulus_disc[of "\<bar>r\<bar>" "pCons c cs"]
```
```   564     obtain v where v: "\<And>w. cmod w \<le> \<bar>r\<bar> \<Longrightarrow> cmod (poly (pCons c cs) v) \<le> cmod (poly (pCons c cs) w)"
```
```   565       by blast
```
```   566     {
```
```   567       fix z
```
```   568       assume z: "r \<le> cmod z"
```
```   569       from v[of 0] r[OF z] have "cmod (poly (pCons c cs) v) \<le> cmod (poly (pCons c cs) z)"
```
```   570         by simp
```
```   571     }
```
```   572     note v0 = this
```
```   573     from v0 v ath[of r] show ?thesis
```
```   574       by blast
```
```   575   next
```
```   576     case True
```
```   577     with pCons.hyps show ?thesis by simp
```
```   578   qed
```
```   579 qed
```
```   580
```
```   581 text{* Constant function (non-syntactic characterization). *}
```
```   582 definition "constant f \<longleftrightarrow> (\<forall>x y. f x = f y)"
```
```   583
```
```   584 lemma nonconstant_length: "\<not> constant (poly p) \<Longrightarrow> psize p \<ge> 2"
```
```   585   by (induct p) (auto simp: constant_def psize_def)
```
```   586
```
```   587 lemma poly_replicate_append: "poly (monom 1 n * p) (x::'a::comm_ring_1) = x^n * poly p x"
```
```   588   by (simp add: poly_monom)
```
```   589
```
```   590 text {* Decomposition of polynomial, skipping zero coefficients
```
```   591   after the first.  *}
```
```   592
```
```   593 lemma poly_decompose_lemma:
```
```   594   assumes nz: "\<not> (\<forall>z. z \<noteq> 0 \<longrightarrow> poly p z = (0::'a::idom))"
```
```   595   shows "\<exists>k a q. a \<noteq> 0 \<and> Suc (psize q + k) = psize p \<and> (\<forall>z. poly p z = z^k * poly (pCons a q) z)"
```
```   596   unfolding psize_def
```
```   597   using nz
```
```   598 proof (induct p)
```
```   599   case 0
```
```   600   then show ?case by simp
```
```   601 next
```
```   602   case (pCons c cs)
```
```   603   show ?case
```
```   604   proof (cases "c = 0")
```
```   605     case True
```
```   606     from pCons.hyps pCons.prems True show ?thesis
```
```   607       apply (auto)
```
```   608       apply (rule_tac x="k+1" in exI)
```
```   609       apply (rule_tac x="a" in exI, clarsimp)
```
```   610       apply (rule_tac x="q" in exI)
```
```   611       apply auto
```
```   612       done
```
```   613   next
```
```   614     case False
```
```   615     show ?thesis
```
```   616       apply (rule exI[where x=0])
```
```   617       apply (rule exI[where x=c], auto simp add: False)
```
```   618       done
```
```   619   qed
```
```   620 qed
```
```   621
```
```   622 lemma poly_decompose:
```
```   623   assumes nc: "\<not> constant (poly p)"
```
```   624   shows "\<exists>k a q. a \<noteq> (0::'a::idom) \<and> k \<noteq> 0 \<and>
```
```   625                psize q + k + 1 = psize p \<and>
```
```   626               (\<forall>z. poly p z = poly p 0 + z^k * poly (pCons a q) z)"
```
```   627   using nc
```
```   628 proof (induct p)
```
```   629   case 0
```
```   630   then show ?case
```
```   631     by (simp add: constant_def)
```
```   632 next
```
```   633   case (pCons c cs)
```
```   634   {
```
```   635     assume C: "\<forall>z. z \<noteq> 0 \<longrightarrow> poly cs z = 0"
```
```   636     {
```
```   637       fix x y
```
```   638       from C have "poly (pCons c cs) x = poly (pCons c cs) y"
```
```   639         by (cases "x = 0") auto
```
```   640     }
```
```   641     with pCons.prems have False
```
```   642       by (auto simp add: constant_def)
```
```   643   }
```
```   644   then have th: "\<not> (\<forall>z. z \<noteq> 0 \<longrightarrow> poly cs z = 0)" ..
```
```   645   from poly_decompose_lemma[OF th]
```
```   646   show ?case
```
```   647     apply clarsimp
```
```   648     apply (rule_tac x="k+1" in exI)
```
```   649     apply (rule_tac x="a" in exI)
```
```   650     apply simp
```
```   651     apply (rule_tac x="q" in exI)
```
```   652     apply (auto simp add: psize_def split: if_splits)
```
```   653     done
```
```   654 qed
```
```   655
```
```   656 text{* Fundamental theorem of algebra *}
```
```   657
```
```   658 lemma fundamental_theorem_of_algebra:
```
```   659   assumes nc: "\<not> constant (poly p)"
```
```   660   shows "\<exists>z::complex. poly p z = 0"
```
```   661   using nc
```
```   662 proof (induct "psize p" arbitrary: p rule: less_induct)
```
```   663   case less
```
```   664   let ?p = "poly p"
```
```   665   let ?ths = "\<exists>z. ?p z = 0"
```
```   666
```
```   667   from nonconstant_length[OF less(2)] have n2: "psize p \<ge> 2" .
```
```   668   from poly_minimum_modulus obtain c where c: "\<forall>w. cmod (?p c) \<le> cmod (?p w)"
```
```   669     by blast
```
```   670
```
```   671   show ?ths
```
```   672   proof (cases "?p c = 0")
```
```   673     case True
```
```   674     then show ?thesis by blast
```
```   675   next
```
```   676     case False
```
```   677     note pc0 = this
```
```   678     from poly_offset[of p c] obtain q where q: "psize q = psize p" "\<forall>x. poly q x = ?p (c + x)"
```
```   679       by blast
```
```   680     {
```
```   681       assume h: "constant (poly q)"
```
```   682       from q(2) have th: "\<forall>x. poly q (x - c) = ?p x"
```
```   683         by auto
```
```   684       {
```
```   685         fix x y
```
```   686         from th have "?p x = poly q (x - c)"
```
```   687           by auto
```
```   688         also have "\<dots> = poly q (y - c)"
```
```   689           using h unfolding constant_def by blast
```
```   690         also have "\<dots> = ?p y"
```
```   691           using th by auto
```
```   692         finally have "?p x = ?p y" .
```
```   693       }
```
```   694       with less(2) have False
```
```   695         unfolding constant_def by blast
```
```   696     }
```
```   697     then have qnc: "\<not> constant (poly q)"
```
```   698       by blast
```
```   699     from q(2) have pqc0: "?p c = poly q 0"
```
```   700       by simp
```
```   701     from c pqc0 have cq0: "\<forall>w. cmod (poly q 0) \<le> cmod (?p w)"
```
```   702       by simp
```
```   703     let ?a0 = "poly q 0"
```
```   704     from pc0 pqc0 have a00: "?a0 \<noteq> 0"
```
```   705       by simp
```
```   706     from a00 have qr: "\<forall>z. poly q z = poly (smult (inverse ?a0) q) z * ?a0"
```
```   707       by simp
```
```   708     let ?r = "smult (inverse ?a0) q"
```
```   709     have lgqr: "psize q = psize ?r"
```
```   710       using a00
```
```   711       unfolding psize_def degree_def
```
```   712       by (simp add: poly_eq_iff)
```
```   713     {
```
```   714       assume h: "\<And>x y. poly ?r x = poly ?r y"
```
```   715       {
```
```   716         fix x y
```
```   717         from qr[rule_format, of x] have "poly q x = poly ?r x * ?a0"
```
```   718           by auto
```
```   719         also have "\<dots> = poly ?r y * ?a0"
```
```   720           using h by simp
```
```   721         also have "\<dots> = poly q y"
```
```   722           using qr[rule_format, of y] by simp
```
```   723         finally have "poly q x = poly q y" .
```
```   724       }
```
```   725       with qnc have False
```
```   726         unfolding constant_def by blast
```
```   727     }
```
```   728     then have rnc: "\<not> constant (poly ?r)"
```
```   729       unfolding constant_def by blast
```
```   730     from qr[rule_format, of 0] a00 have r01: "poly ?r 0 = 1"
```
```   731       by auto
```
```   732     {
```
```   733       fix w
```
```   734       have "cmod (poly ?r w) < 1 \<longleftrightarrow> cmod (poly q w / ?a0) < 1"
```
```   735         using qr[rule_format, of w] a00 by (simp add: divide_inverse ac_simps)
```
```   736       also have "\<dots> \<longleftrightarrow> cmod (poly q w) < cmod ?a0"
```
```   737         using a00 unfolding norm_divide by (simp add: field_simps)
```
```   738       finally have "cmod (poly ?r w) < 1 \<longleftrightarrow> cmod (poly q w) < cmod ?a0" .
```
```   739     }
```
```   740     note mrmq_eq = this
```
```   741     from poly_decompose[OF rnc] obtain k a s where
```
```   742       kas: "a \<noteq> 0" "k \<noteq> 0" "psize s + k + 1 = psize ?r"
```
```   743         "\<forall>z. poly ?r z = poly ?r 0 + z^k* poly (pCons a s) z" by blast
```
```   744     {
```
```   745       assume "psize p = k + 1"
```
```   746       with kas(3) lgqr[symmetric] q(1) have s0: "s = 0"
```
```   747         by auto
```
```   748       {
```
```   749         fix w
```
```   750         have "cmod (poly ?r w) = cmod (1 + a * w ^ k)"
```
```   751           using kas(4)[rule_format, of w] s0 r01 by (simp add: algebra_simps)
```
```   752       }
```
```   753       note hth = this [symmetric]
```
```   754       from reduce_poly_simple[OF kas(1,2)] have "\<exists>w. cmod (poly ?r w) < 1"
```
```   755         unfolding hth by blast
```
```   756     }
```
```   757     moreover
```
```   758     {
```
```   759       assume kn: "psize p \<noteq> k + 1"
```
```   760       from kn kas(3) q(1) lgqr have k1n: "k + 1 < psize p"
```
```   761         by simp
```
```   762       have th01: "\<not> constant (poly (pCons 1 (monom a (k - 1))))"
```
```   763         unfolding constant_def poly_pCons poly_monom
```
```   764         using kas(1)
```
```   765         apply simp
```
```   766         apply (rule exI[where x=0])
```
```   767         apply (rule exI[where x=1])
```
```   768         apply simp
```
```   769         done
```
```   770       from kas(1) kas(2) have th02: "k + 1 = psize (pCons 1 (monom a (k - 1)))"
```
```   771         by (simp add: psize_def degree_monom_eq)
```
```   772       from less(1) [OF k1n [simplified th02] th01]
```
```   773       obtain w where w: "1 + w^k * a = 0"
```
```   774         unfolding poly_pCons poly_monom
```
```   775         using kas(2) by (cases k) (auto simp add: algebra_simps)
```
```   776       from poly_bound_exists[of "cmod w" s] obtain m where
```
```   777         m: "m > 0" "\<forall>z. cmod z \<le> cmod w \<longrightarrow> cmod (poly s z) \<le> m" by blast
```
```   778       have w0: "w \<noteq> 0"
```
```   779         using kas(2) w by (auto simp add: power_0_left)
```
```   780       from w have "(1 + w ^ k * a) - 1 = 0 - 1"
```
```   781         by simp
```
```   782       then have wm1: "w^k * a = - 1"
```
```   783         by simp
```
```   784       have inv0: "0 < inverse (cmod w ^ (k + 1) * m)"
```
```   785         using norm_ge_zero[of w] w0 m(1)
```
```   786         by (simp add: inverse_eq_divide zero_less_mult_iff)
```
```   787       with real_lbound_gt_zero[OF zero_less_one] obtain t where
```
```   788         t: "t > 0" "t < 1" "t < inverse (cmod w ^ (k + 1) * m)" by blast
```
```   789       let ?ct = "complex_of_real t"
```
```   790       let ?w = "?ct * w"
```
```   791       have "1 + ?w^k * (a + ?w * poly s ?w) = 1 + ?ct^k * (w^k * a) + ?w^k * ?w * poly s ?w"
```
```   792         using kas(1) by (simp add: algebra_simps power_mult_distrib)
```
```   793       also have "\<dots> = complex_of_real (1 - t^k) + ?w^k * ?w * poly s ?w"
```
```   794         unfolding wm1 by simp
```
```   795       finally have "cmod (1 + ?w^k * (a + ?w * poly s ?w)) =
```
```   796         cmod (complex_of_real (1 - t^k) + ?w^k * ?w * poly s ?w)"
```
```   797         by metis
```
```   798       with norm_triangle_ineq[of "complex_of_real (1 - t^k)" "?w^k * ?w * poly s ?w"]
```
```   799       have th11: "cmod (1 + ?w^k * (a + ?w * poly s ?w)) \<le> \<bar>1 - t^k\<bar> + cmod (?w^k * ?w * poly s ?w)"
```
```   800         unfolding norm_of_real by simp
```
```   801       have ath: "\<And>x t::real. 0 \<le> x \<Longrightarrow> x < t \<Longrightarrow> t \<le> 1 \<Longrightarrow> \<bar>1 - t\<bar> + x < 1"
```
```   802         by arith
```
```   803       have "t * cmod w \<le> 1 * cmod w"
```
```   804         apply (rule mult_mono)
```
```   805         using t(1,2)
```
```   806         apply auto
```
```   807         done
```
```   808       then have tw: "cmod ?w \<le> cmod w"
```
```   809         using t(1) by (simp add: norm_mult)
```
```   810       from t inv0 have "t * (cmod w ^ (k + 1) * m) < 1"
```
```   811         by (simp add: field_simps)
```
```   812       with zero_less_power[OF t(1), of k] have th30: "t^k * (t* (cmod w ^ (k + 1) * m)) < t^k * 1"
```
```   813         by (metis comm_mult_strict_left_mono)
```
```   814       have "cmod (?w^k * ?w * poly s ?w) = t^k * (t* (cmod w ^ (k + 1) * cmod (poly s ?w)))"
```
```   815         using w0 t(1)
```
```   816         by (simp add: algebra_simps power_mult_distrib norm_power norm_mult)
```
```   817       then have "cmod (?w^k * ?w * poly s ?w) \<le> t^k * (t* (cmod w ^ (k + 1) * m))"
```
```   818         using t(1,2) m(2)[rule_format, OF tw] w0
```
```   819         by auto
```
```   820       with th30 have th120: "cmod (?w^k * ?w * poly s ?w) < t^k"
```
```   821         by simp
```
```   822       from power_strict_mono[OF t(2), of k] t(1) kas(2) have th121: "t^k \<le> 1"
```
```   823         by auto
```
```   824       from ath[OF norm_ge_zero[of "?w^k * ?w * poly s ?w"] th120 th121]
```
```   825       have th12: "\<bar>1 - t^k\<bar> + cmod (?w^k * ?w * poly s ?w) < 1" .
```
```   826       from th11 th12 have "cmod (1 + ?w^k * (a + ?w * poly s ?w)) < 1"
```
```   827         by arith
```
```   828       then have "cmod (poly ?r ?w) < 1"
```
```   829         unfolding kas(4)[rule_format, of ?w] r01 by simp
```
```   830       then have "\<exists>w. cmod (poly ?r w) < 1"
```
```   831         by blast
```
```   832     }
```
```   833     ultimately have cr0_contr: "\<exists>w. cmod (poly ?r w) < 1"
```
```   834       by blast
```
```   835     from cr0_contr cq0 q(2) show ?thesis
```
```   836       unfolding mrmq_eq not_less[symmetric] by auto
```
```   837   qed
```
```   838 qed
```
```   839
```
```   840 text {* Alternative version with a syntactic notion of constant polynomial. *}
```
```   841
```
```   842 lemma fundamental_theorem_of_algebra_alt:
```
```   843   assumes nc: "\<not> (\<exists>a l. a \<noteq> 0 \<and> l = 0 \<and> p = pCons a l)"
```
```   844   shows "\<exists>z. poly p z = (0::complex)"
```
```   845   using nc
```
```   846 proof (induct p)
```
```   847   case 0
```
```   848   then show ?case by simp
```
```   849 next
```
```   850   case (pCons c cs)
```
```   851   show ?case
```
```   852   proof (cases "c = 0")
```
```   853     case True
```
```   854     then show ?thesis by auto
```
```   855   next
```
```   856     case False
```
```   857     {
```
```   858       assume nc: "constant (poly (pCons c cs))"
```
```   859       from nc[unfolded constant_def, rule_format, of 0]
```
```   860       have "\<forall>w. w \<noteq> 0 \<longrightarrow> poly cs w = 0" by auto
```
```   861       then have "cs = 0"
```
```   862       proof (induct cs)
```
```   863         case 0
```
```   864         then show ?case by simp
```
```   865       next
```
```   866         case (pCons d ds)
```
```   867         show ?case
```
```   868         proof (cases "d = 0")
```
```   869           case True
```
```   870           then show ?thesis using pCons.prems pCons.hyps by simp
```
```   871         next
```
```   872           case False
```
```   873           from poly_bound_exists[of 1 ds] obtain m where
```
```   874             m: "m > 0" "\<forall>z. \<forall>z. cmod z \<le> 1 \<longrightarrow> cmod (poly ds z) \<le> m" by blast
```
```   875           have dm: "cmod d / m > 0"
```
```   876             using False m(1) by (simp add: field_simps)
```
```   877           from real_lbound_gt_zero[OF dm zero_less_one] obtain x where
```
```   878             x: "x > 0" "x < cmod d / m" "x < 1" by blast
```
```   879           let ?x = "complex_of_real x"
```
```   880           from x have cx: "?x \<noteq> 0"  "cmod ?x \<le> 1"
```
```   881             by simp_all
```
```   882           from pCons.prems[rule_format, OF cx(1)]
```
```   883           have cth: "cmod (?x*poly ds ?x) = cmod d"
```
```   884             by (simp add: eq_diff_eq[symmetric])
```
```   885           from m(2)[rule_format, OF cx(2)] x(1)
```
```   886           have th0: "cmod (?x*poly ds ?x) \<le> x*m"
```
```   887             by (simp add: norm_mult)
```
```   888           from x(2) m(1) have "x * m < cmod d"
```
```   889             by (simp add: field_simps)
```
```   890           with th0 have "cmod (?x*poly ds ?x) \<noteq> cmod d"
```
```   891             by auto
```
```   892           with cth show ?thesis
```
```   893             by blast
```
```   894         qed
```
```   895       qed
```
```   896     }
```
```   897     then have nc: "\<not> constant (poly (pCons c cs))"
```
```   898       using pCons.prems False by blast
```
```   899     from fundamental_theorem_of_algebra[OF nc] show ?thesis .
```
```   900   qed
```
```   901 qed
```
```   902
```
```   903
```
```   904 subsection{* Nullstellensatz, degrees and divisibility of polynomials *}
```
```   905
```
```   906 lemma nullstellensatz_lemma:
```
```   907   fixes p :: "complex poly"
```
```   908   assumes "\<forall>x. poly p x = 0 \<longrightarrow> poly q x = 0"
```
```   909     and "degree p = n"
```
```   910     and "n \<noteq> 0"
```
```   911   shows "p dvd (q ^ n)"
```
```   912   using assms
```
```   913 proof (induct n arbitrary: p q rule: nat_less_induct)
```
```   914   fix n :: nat
```
```   915   fix p q :: "complex poly"
```
```   916   assume IH: "\<forall>m<n. \<forall>p q.
```
```   917                  (\<forall>x. poly p x = (0::complex) \<longrightarrow> poly q x = 0) \<longrightarrow>
```
```   918                  degree p = m \<longrightarrow> m \<noteq> 0 \<longrightarrow> p dvd (q ^ m)"
```
```   919     and pq0: "\<forall>x. poly p x = 0 \<longrightarrow> poly q x = 0"
```
```   920     and dpn: "degree p = n"
```
```   921     and n0: "n \<noteq> 0"
```
```   922   from dpn n0 have pne: "p \<noteq> 0" by auto
```
```   923   let ?ths = "p dvd (q ^ n)"
```
```   924   {
```
```   925     fix a
```
```   926     assume a: "poly p a = 0"
```
```   927     {
```
```   928       assume oa: "order a p \<noteq> 0"
```
```   929       let ?op = "order a p"
```
```   930       from pne have ap: "([:- a, 1:] ^ ?op) dvd p" "\<not> [:- a, 1:] ^ (Suc ?op) dvd p"
```
```   931         using order by blast+
```
```   932       note oop = order_degree[OF pne, unfolded dpn]
```
```   933       {
```
```   934         assume q0: "q = 0"
```
```   935         then have ?ths using n0
```
```   936           by (simp add: power_0_left)
```
```   937       }
```
```   938       moreover
```
```   939       {
```
```   940         assume q0: "q \<noteq> 0"
```
```   941         from pq0[rule_format, OF a, unfolded poly_eq_0_iff_dvd]
```
```   942         obtain r where r: "q = [:- a, 1:] * r" by (rule dvdE)
```
```   943         from ap(1) obtain s where s: "p = [:- a, 1:] ^ ?op * s"
```
```   944           by (rule dvdE)
```
```   945         have sne: "s \<noteq> 0" using s pne by auto
```
```   946         {
```
```   947           assume ds0: "degree s = 0"
```
```   948           from ds0 obtain k where kpn: "s = [:k:]"
```
```   949             by (cases s) (auto split: if_splits)
```
```   950           from sne kpn have k: "k \<noteq> 0" by simp
```
```   951           let ?w = "([:1/k:] * ([:-a,1:] ^ (n - ?op))) * (r ^ n)"
```
```   952           have "q ^ n = p * ?w"
```
```   953             apply (subst r)
```
```   954             apply (subst s)
```
```   955             apply (subst kpn)
```
```   956             using k oop [of a]
```
```   957             apply (subst power_mult_distrib)
```
```   958             apply simp
```
```   959             apply (subst power_add [symmetric])
```
```   960             apply simp
```
```   961             done
```
```   962           then have ?ths
```
```   963             unfolding dvd_def by blast
```
```   964         }
```
```   965         moreover
```
```   966         {
```
```   967           assume ds0: "degree s \<noteq> 0"
```
```   968           from ds0 sne dpn s oa
```
```   969             have dsn: "degree s < n"
```
```   970               apply auto
```
```   971               apply (erule ssubst)
```
```   972               apply (simp add: degree_mult_eq degree_linear_power)
```
```   973               done
```
```   974             {
```
```   975               fix x assume h: "poly s x = 0"
```
```   976               {
```
```   977                 assume xa: "x = a"
```
```   978                 from h[unfolded xa poly_eq_0_iff_dvd] obtain u where u: "s = [:- a, 1:] * u"
```
```   979                   by (rule dvdE)
```
```   980                 have "p = [:- a, 1:] ^ (Suc ?op) * u"
```
```   981                   apply (subst s)
```
```   982                   apply (subst u)
```
```   983                   apply (simp only: power_Suc ac_simps)
```
```   984                   done
```
```   985                 with ap(2)[unfolded dvd_def] have False
```
```   986                   by blast
```
```   987               }
```
```   988               note xa = this
```
```   989               from h have "poly p x = 0"
```
```   990                 by (subst s) simp
```
```   991               with pq0 have "poly q x = 0"
```
```   992                 by blast
```
```   993               with r xa have "poly r x = 0"
```
```   994                 by auto
```
```   995             }
```
```   996             note impth = this
```
```   997             from IH[rule_format, OF dsn, of s r] impth ds0
```
```   998             have "s dvd (r ^ (degree s))"
```
```   999               by blast
```
```  1000             then obtain u where u: "r ^ (degree s) = s * u" ..
```
```  1001             then have u': "\<And>x. poly s x * poly u x = poly r x ^ degree s"
```
```  1002               by (simp only: poly_mult[symmetric] poly_power[symmetric])
```
```  1003             let ?w = "(u * ([:-a,1:] ^ (n - ?op))) * (r ^ (n - degree s))"
```
```  1004             from oop[of a] dsn have "q ^ n = p * ?w"
```
```  1005               apply -
```
```  1006               apply (subst s)
```
```  1007               apply (subst r)
```
```  1008               apply (simp only: power_mult_distrib)
```
```  1009               apply (subst mult.assoc [where b=s])
```
```  1010               apply (subst mult.assoc [where a=u])
```
```  1011               apply (subst mult.assoc [where b=u, symmetric])
```
```  1012               apply (subst u [symmetric])
```
```  1013               apply (simp add: ac_simps power_add [symmetric])
```
```  1014               done
```
```  1015             then have ?ths
```
```  1016               unfolding dvd_def by blast
```
```  1017         }
```
```  1018         ultimately have ?ths by blast
```
```  1019       }
```
```  1020       ultimately have ?ths by blast
```
```  1021     }
```
```  1022     then have ?ths using a order_root pne by blast
```
```  1023   }
```
```  1024   moreover
```
```  1025   {
```
```  1026     assume exa: "\<not> (\<exists>a. poly p a = 0)"
```
```  1027     from fundamental_theorem_of_algebra_alt[of p] exa
```
```  1028     obtain c where ccs: "c \<noteq> 0" "p = pCons c 0"
```
```  1029       by blast
```
```  1030     then have pp: "\<And>x. poly p x = c"
```
```  1031       by simp
```
```  1032     let ?w = "[:1/c:] * (q ^ n)"
```
```  1033     from ccs have "(q ^ n) = (p * ?w)"
```
```  1034       by simp
```
```  1035     then have ?ths
```
```  1036       unfolding dvd_def by blast
```
```  1037   }
```
```  1038   ultimately show ?ths by blast
```
```  1039 qed
```
```  1040
```
```  1041 lemma nullstellensatz_univariate:
```
```  1042   "(\<forall>x. poly p x = (0::complex) \<longrightarrow> poly q x = 0) \<longleftrightarrow>
```
```  1043     p dvd (q ^ (degree p)) \<or> (p = 0 \<and> q = 0)"
```
```  1044 proof -
```
```  1045   {
```
```  1046     assume pe: "p = 0"
```
```  1047     then have eq: "(\<forall>x. poly p x = (0::complex) \<longrightarrow> poly q x = 0) \<longleftrightarrow> q = 0"
```
```  1048       by (auto simp add: poly_all_0_iff_0)
```
```  1049     {
```
```  1050       assume "p dvd (q ^ (degree p))"
```
```  1051       then obtain r where r: "q ^ (degree p) = p * r" ..
```
```  1052       from r pe have False by simp
```
```  1053     }
```
```  1054     with eq pe have ?thesis by blast
```
```  1055   }
```
```  1056   moreover
```
```  1057   {
```
```  1058     assume pe: "p \<noteq> 0"
```
```  1059     {
```
```  1060       assume dp: "degree p = 0"
```
```  1061       then obtain k where k: "p = [:k:]" "k \<noteq> 0" using pe
```
```  1062         by (cases p) (simp split: if_splits)
```
```  1063       then have th1: "\<forall>x. poly p x \<noteq> 0"
```
```  1064         by simp
```
```  1065       from k dp have "q ^ (degree p) = p * [:1/k:]"
```
```  1066         by (simp add: one_poly_def)
```
```  1067       then have th2: "p dvd (q ^ (degree p))" ..
```
```  1068       from th1 th2 pe have ?thesis
```
```  1069         by blast
```
```  1070     }
```
```  1071     moreover
```
```  1072     {
```
```  1073       assume dp: "degree p \<noteq> 0"
```
```  1074       then obtain n where n: "degree p = Suc n "
```
```  1075         by (cases "degree p") auto
```
```  1076       {
```
```  1077         assume "p dvd (q ^ (Suc n))"
```
```  1078         then obtain u where u: "q ^ (Suc n) = p * u" ..
```
```  1079         {
```
```  1080           fix x
```
```  1081           assume h: "poly p x = 0" "poly q x \<noteq> 0"
```
```  1082           then have "poly (q ^ (Suc n)) x \<noteq> 0"
```
```  1083             by simp
```
```  1084           then have False using u h(1)
```
```  1085             by (simp only: poly_mult) simp
```
```  1086         }
```
```  1087       }
```
```  1088       with n nullstellensatz_lemma[of p q "degree p"] dp
```
```  1089       have ?thesis by auto
```
```  1090     }
```
```  1091     ultimately have ?thesis by blast
```
```  1092   }
```
```  1093   ultimately show ?thesis by blast
```
```  1094 qed
```
```  1095
```
```  1096 text {* Useful lemma *}
```
```  1097
```
```  1098 lemma constant_degree:
```
```  1099   fixes p :: "'a::{idom,ring_char_0} poly"
```
```  1100   shows "constant (poly p) \<longleftrightarrow> degree p = 0" (is "?lhs = ?rhs")
```
```  1101 proof
```
```  1102   assume l: ?lhs
```
```  1103   from l[unfolded constant_def, rule_format, of _ "0"]
```
```  1104   have th: "poly p = poly [:poly p 0:]"
```
```  1105     by auto
```
```  1106   then have "p = [:poly p 0:]"
```
```  1107     by (simp add: poly_eq_poly_eq_iff)
```
```  1108   then have "degree p = degree [:poly p 0:]"
```
```  1109     by simp
```
```  1110   then show ?rhs
```
```  1111     by simp
```
```  1112 next
```
```  1113   assume r: ?rhs
```
```  1114   then obtain k where "p = [:k:]"
```
```  1115     by (cases p) (simp split: if_splits)
```
```  1116   then show ?lhs
```
```  1117     unfolding constant_def by auto
```
```  1118 qed
```
```  1119
```
```  1120 lemma divides_degree:
```
```  1121   assumes pq: "p dvd (q:: complex poly)"
```
```  1122   shows "degree p \<le> degree q \<or> q = 0"
```
```  1123   by (metis dvd_imp_degree_le pq)
```
```  1124
```
```  1125 text {* Arithmetic operations on multivariate polynomials. *}
```
```  1126
```
```  1127 lemma mpoly_base_conv:
```
```  1128   fixes x :: "'a::comm_ring_1"
```
```  1129   shows "0 = poly 0 x" "c = poly [:c:] x" "x = poly [:0,1:] x"
```
```  1130   by simp_all
```
```  1131
```
```  1132 lemma mpoly_norm_conv:
```
```  1133   fixes x :: "'a::comm_ring_1"
```
```  1134   shows "poly [:0:] x = poly 0 x" "poly [:poly 0 y:] x = poly 0 x"
```
```  1135   by simp_all
```
```  1136
```
```  1137 lemma mpoly_sub_conv:
```
```  1138   fixes x :: "'a::comm_ring_1"
```
```  1139   shows "poly p x - poly q x = poly p x + -1 * poly q x"
```
```  1140   by simp
```
```  1141
```
```  1142 lemma poly_pad_rule: "poly p x = 0 \<Longrightarrow> poly (pCons 0 p) x = 0"
```
```  1143   by simp
```
```  1144
```
```  1145 lemma poly_cancel_eq_conv:
```
```  1146   fixes x :: "'a::field"
```
```  1147   shows "x = 0 \<Longrightarrow> a \<noteq> 0 \<Longrightarrow> y = 0 \<longleftrightarrow> a * y - b * x = 0"
```
```  1148   by auto
```
```  1149
```
```  1150 lemma poly_divides_pad_rule:
```
```  1151   fixes p:: "('a::comm_ring_1) poly"
```
```  1152   assumes pq: "p dvd q"
```
```  1153   shows "p dvd (pCons 0 q)"
```
```  1154 proof -
```
```  1155   have "pCons 0 q = q * [:0,1:]" by simp
```
```  1156   then have "q dvd (pCons 0 q)" ..
```
```  1157   with pq show ?thesis by (rule dvd_trans)
```
```  1158 qed
```
```  1159
```
```  1160 lemma poly_divides_conv0:
```
```  1161   fixes p:: "'a::field poly"
```
```  1162   assumes lgpq: "degree q < degree p"
```
```  1163     and lq: "p \<noteq> 0"
```
```  1164   shows "p dvd q \<longleftrightarrow> q = 0" (is "?lhs \<longleftrightarrow> ?rhs")
```
```  1165 proof
```
```  1166   assume r: ?rhs
```
```  1167   then have "q = p * 0" by simp
```
```  1168   then show ?lhs ..
```
```  1169 next
```
```  1170   assume l: ?lhs
```
```  1171   show ?rhs
```
```  1172   proof (cases "q = 0")
```
```  1173     case True
```
```  1174     then show ?thesis by simp
```
```  1175   next
```
```  1176     assume q0: "q \<noteq> 0"
```
```  1177     from l q0 have "degree p \<le> degree q"
```
```  1178       by (rule dvd_imp_degree_le)
```
```  1179     with lgpq show ?thesis by simp
```
```  1180   qed
```
```  1181 qed
```
```  1182
```
```  1183 lemma poly_divides_conv1:
```
```  1184   fixes p :: "'a::field poly"
```
```  1185   assumes a0: "a \<noteq> 0"
```
```  1186     and pp': "p dvd p'"
```
```  1187     and qrp': "smult a q - p' = r"
```
```  1188   shows "p dvd q \<longleftrightarrow> p dvd r" (is "?lhs \<longleftrightarrow> ?rhs")
```
```  1189 proof
```
```  1190   from pp' obtain t where t: "p' = p * t" ..
```
```  1191   {
```
```  1192     assume l: ?lhs
```
```  1193     then obtain u where u: "q = p * u" ..
```
```  1194     have "r = p * (smult a u - t)"
```
```  1195       using u qrp' [symmetric] t by (simp add: algebra_simps)
```
```  1196     then show ?rhs ..
```
```  1197   next
```
```  1198     assume r: ?rhs
```
```  1199     then obtain u where u: "r = p * u" ..
```
```  1200     from u [symmetric] t qrp' [symmetric] a0
```
```  1201     have "q = p * smult (1/a) (u + t)" by (simp add: algebra_simps)
```
```  1202     then show ?lhs ..
```
```  1203   }
```
```  1204 qed
```
```  1205
```
```  1206 lemma basic_cqe_conv1:
```
```  1207   "(\<exists>x. poly p x = 0 \<and> poly 0 x \<noteq> 0) \<longleftrightarrow> False"
```
```  1208   "(\<exists>x. poly 0 x \<noteq> 0) \<longleftrightarrow> False"
```
```  1209   "(\<exists>x. poly [:c:] x \<noteq> 0) \<longleftrightarrow> c \<noteq> 0"
```
```  1210   "(\<exists>x. poly 0 x = 0) \<longleftrightarrow> True"
```
```  1211   "(\<exists>x. poly [:c:] x = 0) \<longleftrightarrow> c = 0"
```
```  1212   by simp_all
```
```  1213
```
```  1214 lemma basic_cqe_conv2:
```
```  1215   assumes l: "p \<noteq> 0"
```
```  1216   shows "\<exists>x. poly (pCons a (pCons b p)) x = (0::complex)"
```
```  1217 proof -
```
```  1218   {
```
```  1219     fix h t
```
```  1220     assume h: "h \<noteq> 0" "t = 0" and "pCons a (pCons b p) = pCons h t"
```
```  1221     with l have False by simp
```
```  1222   }
```
```  1223   then have th: "\<not> (\<exists> h t. h \<noteq> 0 \<and> t = 0 \<and> pCons a (pCons b p) = pCons h t)"
```
```  1224     by blast
```
```  1225   from fundamental_theorem_of_algebra_alt[OF th] show ?thesis
```
```  1226     by auto
```
```  1227 qed
```
```  1228
```
```  1229 lemma  basic_cqe_conv_2b: "(\<exists>x. poly p x \<noteq> (0::complex)) \<longleftrightarrow> p \<noteq> 0"
```
```  1230   by (metis poly_all_0_iff_0)
```
```  1231
```
```  1232 lemma basic_cqe_conv3:
```
```  1233   fixes p q :: "complex poly"
```
```  1234   assumes l: "p \<noteq> 0"
```
```  1235   shows "(\<exists>x. poly (pCons a p) x = 0 \<and> poly q x \<noteq> 0) \<longleftrightarrow> \<not> (pCons a p) dvd (q ^ psize p)"
```
```  1236 proof -
```
```  1237   from l have dp: "degree (pCons a p) = psize p"
```
```  1238     by (simp add: psize_def)
```
```  1239   from nullstellensatz_univariate[of "pCons a p" q] l
```
```  1240   show ?thesis
```
```  1241     by (metis dp pCons_eq_0_iff)
```
```  1242 qed
```
```  1243
```
```  1244 lemma basic_cqe_conv4:
```
```  1245   fixes p q :: "complex poly"
```
```  1246   assumes h: "\<And>x. poly (q ^ n) x = poly r x"
```
```  1247   shows "p dvd (q ^ n) \<longleftrightarrow> p dvd r"
```
```  1248 proof -
```
```  1249   from h have "poly (q ^ n) = poly r"
```
```  1250     by auto
```
```  1251   then have "(q ^ n) = r"
```
```  1252     by (simp add: poly_eq_poly_eq_iff)
```
```  1253   then show "p dvd (q ^ n) \<longleftrightarrow> p dvd r"
```
```  1254     by simp
```
```  1255 qed
```
```  1256
```
```  1257 lemma poly_const_conv:
```
```  1258   fixes x :: "'a::comm_ring_1"
```
```  1259   shows "poly [:c:] x = y \<longleftrightarrow> c = y"
```
```  1260   by simp
```
```  1261
```
```  1262 end
```