Theory Commutative_Ring (Isabelle2005: October 2005)

# Theory Commutative_Ring

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theory Commutative_Ring
imports Main
uses (comm_ring.ML)
begin

    (*  ID:         $Id: Commutative_Ring.thy,v 1.1 2005/09/20 12:10:29 wenzelm Exp $
        Author:     Bernhard Haeupler

    Proving equalities in commutative rings done "right" in Isabelle/HOL.
    *)

    header {* Proving equalities in commutative rings *}

    theory Commutative_Ring
    imports Main
    uses ("comm_ring.ML")
    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. *}

    consts
      Ipol :: "'a::{comm_ring,recpower} list => 'a pol => 'a"
      Ipolex :: "'a::{comm_ring,recpower} list => 'a polex => 'a"

    primrec
      "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 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. *}

    constdefs
      mkPinj :: "nat => 'a pol => 'a pol"
      "mkPinj x P ≡ (case P of
        Pc c => Pc c |
        Pinj y P => Pinj (x + y) P |
        PX p1 y p2 => Pinj x P)"

    constdefs
      mkPX :: "'a::{comm_ring,recpower} pol => nat => 'a pol => 'a pol"
      "mkPX P i Q == (case P of
        Pc c => (if (c = 0) then (mkPinj 1 Q) else (PX P i Q)) |
        Pinj j R => PX P i Q |
        PX P2 i2 Q2 => (if (Q2 = (Pc 0)) then (PX P2 (i+i2) Q) else (PX P i Q)) )"

    text {* Defining the basic ring operations on normalized polynomials *}

    consts
      add :: "'a::{comm_ring,recpower} pol × 'a pol => 'a pol"
      mul :: "'a::{comm_ring,recpower} pol × 'a pol => 'a pol"
      neg :: "'a::{comm_ring,recpower} pol => 'a pol"
      sqr :: "'a::{comm_ring,recpower} pol => 'a pol"
      pow :: "'a::{comm_ring,recpower} pol × nat => 'a pol"

    text {* Addition *}
    recdef add "measure (λ(x, y). size x + size y)"
      "add (Pc a, Pc b) = Pc (a + b)"
      "add (Pc c, Pinj i P) = Pinj i (add (P, Pc c))"
      "add (Pinj i P, Pc c) = Pinj i (add (P, Pc c))"
      "add (Pc c, PX P i Q) = PX P i (add (Q, Pc c))"
      "add (PX P i Q, Pc c) = PX P i (add (Q, Pc c))"
      "add (Pinj x P, Pinj y Q) =
      (if x=y then mkPinj x (add (P, Q))
       else (if x>y then mkPinj y (add (Pinj (x-y) P, Q))
             else mkPinj x (add (Pinj (y-x) Q, P)) ))"
      "add (Pinj x P, PX Q y R) =
      (if x=0 then add(P, PX Q y R)
       else (if x=1 then PX Q y (add (R, P))
             else PX Q y (add (R, Pinj (x - 1) P))))"
      "add (PX P x R, Pinj y Q) =
      (if y=0 then add(PX P x R, Q)
       else (if y=1 then PX P x (add (R, Q))
             else PX P x (add (R, Pinj (y - 1) Q))))"
      "add (PX P1 x P2, PX Q1 y Q2) =
      (if x=y then mkPX (add (P1, Q1)) x (add (P2, Q2))
      else (if x>y then mkPX (add (PX P1 (x-y) (Pc 0), Q1)) y (add (P2,Q2))
            else mkPX (add (PX Q1 (y-x) (Pc 0), P1)) x (add (P2,Q2)) ))"

    text {* Multiplication *}
    recdef mul "measure (λ(x, y). size x + size y)"
      "mul (Pc a, Pc b) = Pc (a*b)"
      "mul (Pc c, Pinj i P) = (if c=0 then Pc 0 else mkPinj i (mul (P, Pc c)))"
      "mul (Pinj i P, Pc c) = (if c=0 then Pc 0 else mkPinj i (mul (P, Pc c)))"
      "mul (Pc c, PX P i Q) =
      (if c=0 then Pc 0 else mkPX (mul (P, Pc c)) i (mul (Q, Pc c)))"
      "mul (PX P i Q, Pc c) =
      (if c=0 then Pc 0 else mkPX (mul (P, Pc c)) i (mul (Q, Pc c)))"
      "mul (Pinj x P, Pinj y Q) =
      (if x=y then mkPinj x (mul (P, Q))
       else (if x>y then mkPinj y (mul (Pinj (x-y) P, Q))
             else mkPinj x (mul (Pinj (y-x) Q, P)) ))"
      "mul (Pinj x P, PX Q y R) =
      (if x=0 then mul(P, PX Q y R)
       else (if x=1 then mkPX (mul (Pinj x P, Q)) y (mul (R, P))
             else mkPX (mul (Pinj x P, Q)) y (mul (R, Pinj (x - 1) P))))"
      "mul (PX P x R, Pinj y Q) =
      (if y=0 then mul(PX P x R, Q)
       else (if y=1 then mkPX (mul (Pinj y Q, P)) x (mul (R, Q))
             else mkPX (mul (Pinj y Q, P)) x (mul (R, Pinj (y - 1) Q))))"
      "mul (PX P1 x P2, PX Q1 y Q2) =
      add (mkPX (mul (P1, Q1)) (x+y) (mul (P2, Q2)),
      add (mkPX (mul (P1, mkPinj 1 Q2)) x (Pc 0), mkPX (mul (Q1, mkPinj 1 P2)) y (Pc 0)) )"
    (hints simp add: mkPinj_def split: pol.split)

    text {* Negation*}
    primrec
      "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 *}
    constdefs
      sub :: "'a::{comm_ring,recpower} pol => 'a pol => 'a pol"
      "sub p q ≡ add (p, neg q)"

    text {* Square for Fast Exponentation *}
    primrec
      "sqr (Pc c) = Pc (c * c)"
      "sqr (Pinj i P) = mkPinj i (sqr P)"
      "sqr (PX A x B) = add (mkPX (sqr A) (x + x) (sqr B),
        mkPX (mul (mul (Pc (1 + 1), A), mkPinj 1 B)) x (Pc 0))"

    text {* Fast Exponentation *}
    lemma pow_wf:"odd n ==> (n::nat) div 2 < n" by (cases n) auto
    recdef pow "measure (λ(x, y). y)"
      "pow (p, 0) = Pc 1"
      "pow (p, n) = (if even n then (pow (sqr p, n div 2)) else mul (p, pow (sqr p, n div 2)))"
    (hints simp add: pow_wf)

    lemma pow_if:
      "pow (p,n) =
       (if n = 0 then Pc 1 else if even n then pow (sqr p, n div 2)
        else mul (p, pow (sqr p, n div 2)))"
      by (cases n) simp_all

    (*
    lemma number_of_nat_B0: "(number_of (w BIT bit.B0) ::nat) = 2* (number_of w)"
    by simp

    lemma number_of_nat_even: "even (number_of (w BIT bit.B0)::nat)"
    by simp

    lemma pow_even : "pow (p, number_of(w BIT bit.B0)) = pow (sqr p, number_of w)"
      ( is "pow(?p,?n) = pow (_,?n2)")
    proof-
      have "even ?n" by simp
      hence "pow (p, ?n) = pow (sqr p, ?n div 2)"
        apply simp
        apply (cases "IntDef.neg (number_of w)")
        apply simp
        done
    *)

    text {* Normalization of polynomial expressions *}

    consts norm :: "'a::{comm_ring,recpower} polex => 'a pol"
    primrec
      "norm (Pol P) = P"
      "norm (Add P Q) = add (norm P, norm Q)"
      "norm (Sub p q) = sub (norm p) (norm q)"
      "norm (Mul P Q) = mul (norm P, norm Q)"
      "norm (Pow p n) = pow (norm p, n)"
      "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 ring_eq_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 ring_eq_simps)

    text {* Correctness theorems for the implemented operations *}

    text {* Negation *}
    lemma neg_ci: "!!l. Ipol l (neg P) = -(Ipol l P)"
      by (induct P) auto

    text {* Addition *}
    lemma add_ci: "!!l. Ipol l (add (P, Q)) = Ipol l P + Ipol l Q"
    proof (induct P Q 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 ring_eq_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 ring_eq_simps)
      qed
    next
      case (7 x P Q y R)
      have "x = 0 ∨ x = 1 ∨ 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: ring_eq_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 ∨ y = 1 ∨ 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 ring_eq_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 ring_eq_simps)
      next
        assume "x = y"
        with 9 show ?case by (simp add: mkPX_ci ring_eq_simps)
      qed
    qed (auto simp add: ring_eq_simps)

    text {* Multiplication *}
    lemma mul_ci: "!!l. Ipol l (mul (P, Q)) = Ipol l P * Ipol l Q"
      by (induct P Q rule: mul.induct)
        (simp_all add: mkPX_ci mkPinj_ci ring_eq_simps add_ci power_add)

    text {* Substraction *}
    lemma sub_ci: "Ipol l (sub p q) = Ipol l p - Ipol l q"
      by (simp add: add_ci neg_ci sub_def)

    text {* Square *}
    lemma sqr_ci:"!!ls. Ipol ls (sqr p) = Ipol ls p * Ipol ls p"
      by (induct p) (simp_all add: add_ci mkPinj_ci mkPX_ci mul_ci ring_eq_simps power_add)

    text {* Power *}
    lemma even_pow:"even n ==> pow (p, n) = pow (sqr p, n div 2)" by (induct n) simp_all

    lemma pow_ci: "!!p. Ipol ls (pow (p, n)) = (Ipol ls p) ^ n"
    proof (induct n rule: nat_less_induct)
      case (1 k)
      have two:"2 = Suc (Suc 0)" by simp
      show ?case
      proof (cases k)
        case (Suc l)
        show ?thesis
        proof cases
          assume EL: "even l"
          have "Suc l div 2 = l div 2"
            by (simp add: nat_number even_nat_plus_one_div_two [OF EL])
          moreover
          from Suc have "l < k" by simp
          with 1 have "∀p. Ipol ls (pow (p, l)) = Ipol ls p ^ l" by simp
          moreover
          note Suc EL even_nat_plus_one_div_two [OF EL]
          ultimately show ?thesis by (auto simp add: mul_ci power_Suc even_pow)
        next
          assume OL: "odd l"
          with prems have "[|∀m<Suc l. ∀p. Ipol ls (pow (p, m)) = Ipol ls p ^ m; k = Suc l; odd l|] ==> ∀p. Ipol ls (sqr p) ^ (Suc l div 2) = Ipol ls p ^ Suc l"
          proof(cases l)
            case (Suc w)
            from prems have EW: "even w" by simp
            from two have two_times:"(2 * (w div 2))= w"
              by (simp only: even_nat_div_two_times_two[OF EW])
            have A: "!!p. (Ipol ls p * Ipol ls p) = (Ipol ls p) ^ (Suc (Suc 0))"
              by (simp add: power_Suc)
            from A two [symmetric] have "ALL p.(Ipol ls p * Ipol ls p) = (Ipol ls p) ^ 2"
              by simp
            with prems show ?thesis
              by (auto simp add: power_mult[symmetric, of _ 2 _] two_times mul_ci sqr_ci)
          qed simp
          with prems show ?thesis by simp
        qed
      next
        case 0
        then show ?thesis by simp
      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 eq: "norm P1  = norm P2"
      shows "Ipolex l P1 = Ipolex l P2"
    proof -
      from eq have "Ipol l (norm P1) = Ipol l (norm P2)" by simp
      thus ?thesis by (simp only: norm_ci)
    qed

    text {* Code generation *}
    (*
    Does not work, since no generic ring operations implementation is there
    generate_code ("ring.ML") test = "norm"*)

    use "comm_ring.ML"
    setup "CommRing.setup"

    end

lemma pow_wf:

      odd n ==> n div 2 < n

lemma pow_if:

      pow (p, n) =
      (if n = 0 then Pc (1::'a)
       else if even n then pow (sqr p, n div 2) else mul (p, pow (sqr p, n div 2)))

lemma mkPinj_ci:

      Ipol l (mkPinj a B) = Ipol l (Pinj a B)

lemma mkPX_ci:

      Ipol l (mkPX A b C) = Ipol l (PX A b C)

lemma neg_ci:

      Ipol l (Commutative_Ring.neg P) = - Ipol l P

lemma add_ci:

      Ipol l (add (P, Q)) = Ipol l P + Ipol l Q

lemma mul_ci:

      Ipol l (mul (P, Q)) = Ipol l P * Ipol l Q

lemma sub_ci:

      Ipol l (sub p q) = Ipol l p - Ipol l q

lemma sqr_ci:

      Ipol ls (sqr p) = Ipol ls p * Ipol ls p

lemma even_pow:

      even n ==> pow (p, n) = pow (sqr p, n div 2)

lemma pow_ci:

      Ipol ls (pow (p, n)) = Ipol ls p ^ n

lemma norm_ci:

      Ipolex l Pe = Ipol l (norm Pe)

lemma norm_eq:

      norm P1.0 = norm P2.0 ==> Ipolex l P1.0 = Ipolex l P2.0