(* Author: Bernhard Haeupler
Proving equalities in commutative rings done "right" in Isabelle/HOL.
*)
section {* Proving equalities in commutative rings *}
theory Commutative_Ring
imports Main
begin
text {* Syntax of multivariate polynomials (pol) and polynomial expressions. *}
datatype 'a pol =
Pc 'a
| Pinj nat "'a pol"
| PX "'a pol" nat "'a pol"
datatype 'a polex =
Pol "'a pol"
| Add "'a polex" "'a polex"
| Sub "'a polex" "'a polex"
| Mul "'a polex" "'a polex"
| Pow "'a polex" nat
| Neg "'a polex"
text {* Interpretation functions for the shadow syntax. *}
primrec Ipol :: "'a::{comm_ring_1} list \<Rightarrow> 'a pol \<Rightarrow> 'a"
where
"Ipol l (Pc c) = c"
| "Ipol l (Pinj i P) = Ipol (drop i l) P"
| "Ipol l (PX P x Q) = Ipol l P * (hd l)^x + Ipol (drop 1 l) Q"
primrec Ipolex :: "'a::{comm_ring_1} list \<Rightarrow> 'a polex \<Rightarrow> 'a"
where
"Ipolex l (Pol P) = Ipol l P"
| "Ipolex l (Add P Q) = Ipolex l P + Ipolex l Q"
| "Ipolex l (Sub P Q) = Ipolex l P - Ipolex l Q"
| "Ipolex l (Mul P Q) = Ipolex l P * Ipolex l Q"
| "Ipolex l (Pow p n) = Ipolex l p ^ n"
| "Ipolex l (Neg P) = - Ipolex l P"
text {* Create polynomial normalized polynomials given normalized inputs. *}
definition mkPinj :: "nat \<Rightarrow> 'a pol \<Rightarrow> 'a pol"
where
"mkPinj x P = (case P of
Pc c \<Rightarrow> Pc c |
Pinj y P \<Rightarrow> Pinj (x + y) P |
PX p1 y p2 \<Rightarrow> Pinj x P)"
definition mkPX :: "'a::comm_ring pol \<Rightarrow> nat \<Rightarrow> 'a pol \<Rightarrow> 'a pol"
where
"mkPX P i Q =
(case P of
Pc c \<Rightarrow> if c = 0 then mkPinj 1 Q else PX P i Q
| Pinj j R \<Rightarrow> PX P i Q
| PX P2 i2 Q2 \<Rightarrow> if Q2 = Pc 0 then PX P2 (i + i2) Q else PX P i Q)"
text {* Defining the basic ring operations on normalized polynomials *}
lemma pol_size_nz[simp]: "size (p :: 'a pol) \<noteq> 0"
by (cases p) simp_all
function add :: "'a::comm_ring pol \<Rightarrow> 'a pol \<Rightarrow> 'a pol" (infixl "\<oplus>" 65)
where
"Pc a \<oplus> Pc b = Pc (a + b)"
| "Pc c \<oplus> Pinj i P = Pinj i (P \<oplus> Pc c)"
| "Pinj i P \<oplus> Pc c = Pinj i (P \<oplus> Pc c)"
| "Pc c \<oplus> PX P i Q = PX P i (Q \<oplus> Pc c)"
| "PX P i Q \<oplus> Pc c = PX P i (Q \<oplus> Pc c)"
| "Pinj x P \<oplus> Pinj y Q =
(if x = y then mkPinj x (P \<oplus> Q)
else (if x > y then mkPinj y (Pinj (x - y) P \<oplus> Q)
else mkPinj x (Pinj (y - x) Q \<oplus> P)))"
| "Pinj x P \<oplus> PX Q y R =
(if x = 0 then P \<oplus> PX Q y R
else (if x = 1 then PX Q y (R \<oplus> P)
else PX Q y (R \<oplus> Pinj (x - 1) P)))"
| "PX P x R \<oplus> Pinj y Q =
(if y = 0 then PX P x R \<oplus> Q
else (if y = 1 then PX P x (R \<oplus> Q)
else PX P x (R \<oplus> Pinj (y - 1) Q)))"
| "PX P1 x P2 \<oplus> PX Q1 y Q2 =
(if x = y then mkPX (P1 \<oplus> Q1) x (P2 \<oplus> Q2)
else (if x > y then mkPX (PX P1 (x - y) (Pc 0) \<oplus> Q1) y (P2 \<oplus> Q2)
else mkPX (PX Q1 (y-x) (Pc 0) \<oplus> P1) x (P2 \<oplus> Q2)))"
by pat_completeness auto
termination by (relation "measure (\<lambda>(x, y). size x + size y)") auto
function mul :: "'a::{comm_ring} pol \<Rightarrow> 'a pol \<Rightarrow> 'a pol" (infixl "\<otimes>" 70)
where
"Pc a \<otimes> Pc b = Pc (a * b)"
| "Pc c \<otimes> Pinj i P =
(if c = 0 then Pc 0 else mkPinj i (P \<otimes> Pc c))"
| "Pinj i P \<otimes> Pc c =
(if c = 0 then Pc 0 else mkPinj i (P \<otimes> Pc c))"
| "Pc c \<otimes> PX P i Q =
(if c = 0 then Pc 0 else mkPX (P \<otimes> Pc c) i (Q \<otimes> Pc c))"
| "PX P i Q \<otimes> Pc c =
(if c = 0 then Pc 0 else mkPX (P \<otimes> Pc c) i (Q \<otimes> Pc c))"
| "Pinj x P \<otimes> Pinj y Q =
(if x = y then mkPinj x (P \<otimes> Q) else
(if x > y then mkPinj y (Pinj (x-y) P \<otimes> Q)
else mkPinj x (Pinj (y - x) Q \<otimes> P)))"
| "Pinj x P \<otimes> PX Q y R =
(if x = 0 then P \<otimes> PX Q y R else
(if x = 1 then mkPX (Pinj x P \<otimes> Q) y (R \<otimes> P)
else mkPX (Pinj x P \<otimes> Q) y (R \<otimes> Pinj (x - 1) P)))"
| "PX P x R \<otimes> Pinj y Q =
(if y = 0 then PX P x R \<otimes> Q else
(if y = 1 then mkPX (Pinj y Q \<otimes> P) x (R \<otimes> Q)
else mkPX (Pinj y Q \<otimes> P) x (R \<otimes> Pinj (y - 1) Q)))"
| "PX P1 x P2 \<otimes> PX Q1 y Q2 =
mkPX (P1 \<otimes> Q1) (x + y) (P2 \<otimes> Q2) \<oplus>
(mkPX (P1 \<otimes> mkPinj 1 Q2) x (Pc 0) \<oplus>
(mkPX (Q1 \<otimes> mkPinj 1 P2) y (Pc 0)))"
by pat_completeness auto
termination by (relation "measure (\<lambda>(x, y). size x + size y)")
(auto simp add: mkPinj_def split: pol.split)
text {* Negation*}
primrec neg :: "'a::{comm_ring} pol \<Rightarrow> 'a pol"
where
"neg (Pc c) = Pc (-c)"
| "neg (Pinj i P) = Pinj i (neg P)"
| "neg (PX P x Q) = PX (neg P) x (neg Q)"
text {* Substraction *}
definition sub :: "'a::{comm_ring} pol \<Rightarrow> 'a pol \<Rightarrow> 'a pol" (infixl "\<ominus>" 65)
where "sub P Q = P \<oplus> neg Q"
text {* Square for Fast Exponentation *}
primrec sqr :: "'a::{comm_ring_1} pol \<Rightarrow> 'a pol"
where
"sqr (Pc c) = Pc (c * c)"
| "sqr (Pinj i P) = mkPinj i (sqr P)"
| "sqr (PX A x B) =
mkPX (sqr A) (x + x) (sqr B) \<oplus> mkPX (Pc (1 + 1) \<otimes> A \<otimes> mkPinj 1 B) x (Pc 0)"
text {* Fast Exponentation *}
fun pow :: "nat \<Rightarrow> 'a::{comm_ring_1} pol \<Rightarrow> 'a pol"
where
pow_if [simp del]: "pow n P =
(if n = 0 then Pc 1 else if even n then pow (n div 2) (sqr P)
else P \<otimes> pow (n div 2) (sqr P))"
lemma pow_simps [simp]:
"pow 0 P = Pc 1"
"pow (2 * n) P = pow n (sqr P)"
"pow (Suc (2 * n)) P = P \<otimes> pow n (sqr P)"
by (simp_all add: pow_if)
lemma even_pow:
"even n \<Longrightarrow> pow n P = pow (n div 2) (sqr P)"
by (erule evenE) simp
lemma odd_pow:
"odd n \<Longrightarrow> pow n P = P \<otimes> pow (n div 2) (sqr P)"
by (erule oddE) simp
text {* Normalization of polynomial expressions *}
primrec norm :: "'a::{comm_ring_1} polex \<Rightarrow> 'a pol"
where
"norm (Pol P) = P"
| "norm (Add P Q) = norm P \<oplus> norm Q"
| "norm (Sub P Q) = norm P \<ominus> norm Q"
| "norm (Mul P Q) = norm P \<otimes> norm Q"
| "norm (Pow P n) = pow n (norm P)"
| "norm (Neg P) = neg (norm P)"
text {* mkPinj preserve semantics *}
lemma mkPinj_ci: "Ipol l (mkPinj a B) = Ipol l (Pinj a B)"
by (induct B) (auto simp add: mkPinj_def algebra_simps)
text {* mkPX preserves semantics *}
lemma mkPX_ci: "Ipol l (mkPX A b C) = Ipol l (PX A b C)"
by (cases A) (auto simp add: mkPX_def mkPinj_ci power_add algebra_simps)
text {* Correctness theorems for the implemented operations *}
text {* Negation *}
lemma neg_ci: "Ipol l (neg P) = -(Ipol l P)"
by (induct P arbitrary: l) auto
text {* Addition *}
lemma add_ci: "Ipol l (P \<oplus> Q) = Ipol l P + Ipol l Q"
proof (induct P Q arbitrary: l rule: add.induct)
case (6 x P y Q)
show ?case
proof (rule linorder_cases)
assume "x < y"
with 6 show ?case by (simp add: mkPinj_ci algebra_simps)
next
assume "x = y"
with 6 show ?case by (simp add: mkPinj_ci)
next
assume "x > y"
with 6 show ?case by (simp add: mkPinj_ci algebra_simps)
qed
next
case (7 x P Q y R)
have "x = 0 \<or> x = 1 \<or> x > 1" by arith
moreover
{ assume "x = 0" with 7 have ?case by simp }
moreover
{ assume "x = 1" with 7 have ?case by (simp add: algebra_simps) }
moreover
{ assume "x > 1" from 7 have ?case by (cases x) simp_all }
ultimately show ?case by blast
next
case (8 P x R y Q)
have "y = 0 \<or> y = 1 \<or> y > 1" by arith
moreover
{ assume "y = 0" with 8 have ?case by simp }
moreover
{ assume "y = 1" with 8 have ?case by simp }
moreover
{ assume "y > 1" with 8 have ?case by simp }
ultimately show ?case by blast
next
case (9 P1 x P2 Q1 y Q2)
show ?case
proof (rule linorder_cases)
assume a: "x < y" hence "EX d. d + x = y" by arith
with 9 a show ?case by (auto simp add: mkPX_ci power_add algebra_simps)
next
assume a: "y < x" hence "EX d. d + y = x" by arith
with 9 a show ?case by (auto simp add: power_add mkPX_ci algebra_simps)
next
assume "x = y"
with 9 show ?case by (simp add: mkPX_ci algebra_simps)
qed
qed (auto simp add: algebra_simps)
text {* Multiplication *}
lemma mul_ci: "Ipol l (P \<otimes> Q) = Ipol l P * Ipol l Q"
by (induct P Q arbitrary: l rule: mul.induct)
(simp_all add: mkPX_ci mkPinj_ci algebra_simps add_ci power_add)
text {* Substraction *}
lemma sub_ci: "Ipol l (P \<ominus> Q) = Ipol l P - Ipol l Q"
by (simp add: add_ci neg_ci sub_def)
text {* Square *}
lemma sqr_ci: "Ipol ls (sqr P) = Ipol ls P * Ipol ls P"
by (induct P arbitrary: ls)
(simp_all add: add_ci mkPinj_ci mkPX_ci mul_ci algebra_simps power_add)
text {* Power *}
lemma pow_ci: "Ipol ls (pow n P) = Ipol ls P ^ n"
proof (induct n arbitrary: P rule: less_induct)
case (less k)
show ?case
proof (cases "k = 0")
case True then show ?thesis by simp
next
case False then have "k > 0" by simp
then have "k div 2 < k" by arith
with less have *: "Ipol ls (pow (k div 2) (sqr P)) = Ipol ls (sqr P) ^ (k div 2)"
by simp
show ?thesis
proof (cases "even k")
case True with * show ?thesis
by (simp add: even_pow sqr_ci power_mult_distrib power_add [symmetric] mult_2 [symmetric] even_two_times_div_two)
next
case False with * show ?thesis
by (simp add: odd_pow mul_ci sqr_ci power_mult_distrib power_add [symmetric] mult_2 [symmetric] power_Suc [symmetric])
qed
qed
qed
text {* Normalization preserves semantics *}
lemma norm_ci: "Ipolex l Pe = Ipol l (norm Pe)"
by (induct Pe) (simp_all add: add_ci sub_ci mul_ci neg_ci pow_ci)
text {* Reflection lemma: Key to the (incomplete) decision procedure *}
lemma norm_eq:
assumes "norm P1 = norm P2"
shows "Ipolex l P1 = Ipolex l P2"
proof -
from assms have "Ipol l (norm P1) = Ipol l (norm P2)" by simp
then show ?thesis by (simp only: norm_ci)
qed
ML_file "commutative_ring_tac.ML"
method_setup comm_ring = {*
Scan.succeed (SIMPLE_METHOD' o Commutative_Ring_Tac.tac)
*} "reflective decision procedure for equalities over commutative rings"
end