# Theory Gauss

theory Gauss
imports Euler
```(*  Title:      HOL/Old_Number_Theory/Gauss.thy
*)

section ‹Gauss' Lemma›

theory Gauss
imports Euler
begin

locale GAUSS =
fixes p :: "int"
fixes a :: "int"

assumes p_prime: "zprime p"
assumes p_g_2: "2 < p"
assumes p_a_relprime: "~[a = 0](mod p)"
assumes a_nonzero:    "0 < a"
begin

definition "A = {(x::int). 0 < x & x ≤ ((p - 1) div 2)}"
definition "B = (%x. x * a) ` A"
definition "C = StandardRes p ` B"
definition "D = C ∩ {x. x ≤ ((p - 1) div 2)}"
definition "E = C ∩ {x. ((p - 1) div 2) < x}"
definition "F = (%x. (p - x)) ` E"

subsection ‹Basic properties of p›

lemma p_odd: "p ∈ zOdd"
by (auto simp add: p_prime p_g_2 zprime_zOdd_eq_grt_2)

lemma p_g_0: "0 < p"
using p_g_2 by auto

lemma int_nat: "int (nat ((p - 1) div 2)) = (p - 1) div 2"
using ListMem.insert p_g_2 by (auto simp add: pos_imp_zdiv_nonneg_iff)

lemma p_minus_one_l: "(p - 1) div 2 < p"
proof -
have "(p - 1) div 2 ≤ (p - 1) div 1"
by (rule zdiv_mono2) (auto simp add: p_g_0)
also have "… = p - 1" by simp
finally show ?thesis by simp
qed

lemma p_eq: "p = (2 * (p - 1) div 2) + 1"
using div_mult_self1_is_id [of 2 "p - 1"] by auto

lemma (in -) zodd_imp_zdiv_eq: "x ∈ zOdd ==> 2 * (x - 1) div 2 = 2 * ((x - 1) div 2)"
apply (frule odd_minus_one_even)
apply (subgoal_tac "2 ≠ 0")
apply (frule_tac b = "2 :: int" and a = "x - 1" in div_mult_self1_is_id)
done

lemma p_eq2: "p = (2 * ((p - 1) div 2)) + 1"
apply (insert p_eq p_prime p_g_2 zprime_zOdd_eq_grt_2 [of p], auto)
apply (frule zodd_imp_zdiv_eq, auto)
done

subsection ‹Basic Properties of the Gauss Sets›

lemma finite_A: "finite (A)"

lemma finite_B: "finite (B)"
by (auto simp add: B_def finite_A)

lemma finite_C: "finite (C)"
by (auto simp add: C_def finite_B)

lemma finite_D: "finite (D)"
by (auto simp add: D_def finite_C)

lemma finite_E: "finite (E)"
by (auto simp add: E_def finite_C)

lemma finite_F: "finite (F)"
by (auto simp add: F_def finite_E)

lemma C_eq: "C = D ∪ E"
by (auto simp add: C_def D_def E_def)

lemma A_card_eq: "card A = nat ((p - 1) div 2)"
apply (insert int_nat)
apply (erule subst)
done

lemma inj_on_xa_A: "inj_on (%x. x * a) A"
using a_nonzero by (simp add: A_def inj_on_def)

lemma A_res: "ResSet p A"
apply (auto simp add: A_def ResSet_def)
apply (rule_tac m = p in zcong_less_eq)
apply (insert p_g_2, auto)
done

lemma B_res: "ResSet p B"
apply (insert p_g_2 p_a_relprime p_minus_one_l)
apply (rule ResSet_image)
proof -
fix x fix y
assume a: "[x * a = y * a] (mod p)"
assume b: "0 < x"
assume c: "x ≤ (p - 1) div 2"
assume d: "0 < y"
assume e: "y ≤ (p - 1) div 2"
from a p_a_relprime p_prime a_nonzero zcong_cancel [of p a x y]
have "[x = y](mod p)"
by (simp add: zprime_imp_zrelprime zcong_def p_g_0 order_le_less)
with zcong_less_eq [of x y p] p_minus_one_l
order_le_less_trans [of x "(p - 1) div 2" p]
order_le_less_trans [of y "(p - 1) div 2" p] show "x = y"
by (simp add: b c d e p_minus_one_l p_g_0)
qed

lemma SR_B_inj: "inj_on (StandardRes p) B"
apply (auto simp add: B_def StandardRes_def inj_on_def A_def)
proof -
fix x fix y
assume a: "x * a mod p = y * a mod p"
assume b: "0 < x"
assume c: "x ≤ (p - 1) div 2"
assume d: "0 < y"
assume e: "y ≤ (p - 1) div 2"
assume f: "x ≠ y"
from a have "[x * a = y * a](mod p)"
with p_a_relprime p_prime a_nonzero zcong_cancel [of p a x y]
have "[x = y](mod p)"
by (simp add: zprime_imp_zrelprime zcong_def p_g_0 order_le_less)
with zcong_less_eq [of x y p] p_minus_one_l
order_le_less_trans [of x "(p - 1) div 2" p]
order_le_less_trans [of y "(p - 1) div 2" p] have "x = y"
by (simp add: b c d e p_minus_one_l p_g_0)
then have False
then show "a = 0"
by simp
qed

lemma inj_on_pminusx_E: "inj_on (%x. p - x) E"
apply (auto simp add: E_def C_def B_def A_def)
apply (rule_tac g = "%x. -1 * (x - p)" in inj_on_inverseI)
apply auto
done

lemma A_ncong_p: "x ∈ A ==> ~[x = 0](mod p)"
apply (frule_tac m = p in zcong_not_zero)
apply (insert p_minus_one_l)
apply auto
done

lemma A_greater_zero: "x ∈ A ==> 0 < x"

lemma B_ncong_p: "x ∈ B ==> ~[x = 0](mod p)"
apply (frule A_ncong_p)
apply (insert p_a_relprime p_prime a_nonzero)
apply (frule_tac a = xa and b = a in zcong_zprime_prod_zero_contra)
done

lemma B_greater_zero: "x ∈ B ==> 0 < x"
using a_nonzero by (auto simp add: B_def A_greater_zero)

lemma C_ncong_p: "x ∈ C ==>  ~[x = 0](mod p)"
apply (frule B_ncong_p)
apply (subgoal_tac "[xa = StandardRes p xa](mod p)")
apply (frule_tac a = xa and b = "StandardRes p xa" and c = 0 in zcong_trans)
apply auto
done

lemma C_greater_zero: "y ∈ C ==> 0 < y"
proof -
fix x
assume a: "x ∈ B"
from p_g_0 have "0 ≤ StandardRes p x"
moreover have "~[x = 0] (mod p)"
then have "StandardRes p x ≠ 0"
ultimately show "0 < StandardRes p x"
qed

lemma D_ncong_p: "x ∈ D ==> ~[x = 0](mod p)"
by (auto simp add: D_def C_ncong_p)

lemma E_ncong_p: "x ∈ E ==> ~[x = 0](mod p)"
by (auto simp add: E_def C_ncong_p)

lemma F_ncong_p: "x ∈ F ==> ~[x = 0](mod p)"
proof -
fix x assume a: "x ∈ E" assume b: "[p - x = 0] (mod p)"
from E_ncong_p have "~[x = 0] (mod p)"
moreover from a have "0 < x"
by (simp add: a E_def C_greater_zero)
moreover from a have "x < p"
by (auto simp add: E_def C_def p_g_0 StandardRes_ubound)
ultimately have "~[p - x = 0] (mod p)"
from this show False by (simp add: b)
qed

lemma F_subset: "F ⊆ {x. 0 < x & x ≤ ((p - 1) div 2)}"
apply (auto simp add: F_def E_def)
apply (insert p_g_0)
apply (frule_tac x = xa in StandardRes_ubound)
apply (frule_tac x = x in StandardRes_ubound)
apply (subgoal_tac "xa = StandardRes p xa")
apply (auto simp add: C_def StandardRes_prop2 StandardRes_prop1)
proof -
from zodd_imp_zdiv_eq p_prime p_g_2 zprime_zOdd_eq_grt_2 have
"2 * (p - 1) div 2 = 2 * ((p - 1) div 2)"
by simp
with p_eq2 show " !!x. [| (p - 1) div 2 < StandardRes p x; x ∈ B |]
==> p - StandardRes p x ≤ (p - 1) div 2"
by simp
qed

lemma D_subset: "D ⊆ {x. 0 < x & x ≤ ((p - 1) div 2)}"
by (auto simp add: D_def C_greater_zero)

lemma F_eq: "F = {x. ∃y ∈ A. ( x = p - (StandardRes p (y*a)) & (p - 1) div 2 < StandardRes p (y*a))}"
by (auto simp add: F_def E_def D_def C_def B_def A_def)

lemma D_eq: "D = {x. ∃y ∈ A. ( x = StandardRes p (y*a) & StandardRes p (y*a) ≤ (p - 1) div 2)}"
by (auto simp add: D_def C_def B_def A_def)

lemma D_leq: "x ∈ D ==> x ≤ (p - 1) div 2"

lemma F_ge: "x ∈ F ==> x ≤ (p - 1) div 2"
apply (auto simp add: F_eq A_def)
proof -
fix y
assume "(p - 1) div 2 < StandardRes p (y * a)"
then have "p - StandardRes p (y * a) < p - ((p - 1) div 2)"
by arith
also from p_eq2 have "... = 2 * ((p - 1) div 2) + 1 - ((p - 1) div 2)"
by auto
also have "2 * ((p - 1) div 2) + 1 - (p - 1) div 2 = (p - 1) div 2 + 1"
by arith
finally show "p - StandardRes p (y * a) ≤ (p - 1) div 2"
using zless_add1_eq [of "p - StandardRes p (y * a)" "(p - 1) div 2"] by auto
qed

lemma all_A_relprime: "∀x ∈ A. zgcd x p = 1"
using p_prime p_minus_one_l by (auto simp add: A_def zless_zprime_imp_zrelprime)

lemma A_prod_relprime: "zgcd (setprod id A) p = 1"
by(rule all_relprime_prod_relprime[OF finite_A all_A_relprime])

subsection ‹Relationships Between Gauss Sets›

lemma B_card_eq_A: "card B = card A"
using finite_A by (simp add: finite_A B_def inj_on_xa_A card_image)

lemma B_card_eq: "card B = nat ((p - 1) div 2)"

lemma F_card_eq_E: "card F = card E"
using finite_E by (simp add: F_def inj_on_pminusx_E card_image)

lemma C_card_eq_B: "card C = card B"
apply (insert finite_B)
apply (subgoal_tac "inj_on (StandardRes p) B")
apply (simp add: B_def C_def card_image)
apply (rule StandardRes_inj_on_ResSet)
done

lemma D_E_disj: "D ∩ E = {}"
by (auto simp add: D_def E_def)

lemma C_card_eq_D_plus_E: "card C = card D + card E"
by (auto simp add: C_eq card_Un_disjoint D_E_disj finite_D finite_E)

lemma C_prod_eq_D_times_E: "setprod id E * setprod id D = setprod id C"
apply (insert D_E_disj finite_D finite_E C_eq)
apply (frule setprod.union_disjoint [of D E id])
apply auto
done

lemma C_B_zcong_prod: "[setprod id C = setprod id B] (mod p)"
apply (insert finite_B SR_B_inj)
apply (frule setprod.reindex [of "StandardRes p" B id])
apply auto
apply (rule setprod_same_function_zcong)
apply (auto simp add: StandardRes_prop1 zcong_sym p_g_0)
done

lemma F_Un_D_subset: "(F ∪ D) ⊆ A"
apply (rule Un_least)
apply (auto simp add: A_def F_subset D_subset)
done

lemma F_D_disj: "(F ∩ D) = {}"
apply (auto simp add: F_eq D_eq)
proof -
fix y fix ya
assume "p - StandardRes p (y * a) = StandardRes p (ya * a)"
then have "p = StandardRes p (y * a) + StandardRes p (ya * a)"
by arith
moreover have "p dvd p"
by auto
ultimately have "p dvd (StandardRes p (y * a) + StandardRes p (ya * a))"
by auto
then have a: "[StandardRes p (y * a) + StandardRes p (ya * a) = 0] (mod p)"
have "[y * a = StandardRes p (y * a)] (mod p)"
by (simp only: zcong_sym StandardRes_prop1)
moreover have "[ya * a = StandardRes p (ya * a)] (mod p)"
by (simp only: zcong_sym StandardRes_prop1)
ultimately have "[y * a + ya * a =
StandardRes p (y * a) + StandardRes p (ya * a)] (mod p)"
with a have "[y * a + ya * a = 0] (mod p)"
apply (elim zcong_trans)
by (simp only: zcong_refl)
also have "y * a + ya * a = a * (y + ya)"
finally have "[a * (y + ya) = 0] (mod p)" .
with p_prime a_nonzero zcong_zprime_prod_zero [of p a "y + ya"]
p_a_relprime
have a: "[y + ya = 0] (mod p)"
by auto
assume b: "y ∈ A" and c: "ya: A"
with A_def have "0 < y + ya"
by auto
moreover from b c A_def have "y + ya ≤ (p - 1) div 2 + (p - 1) div 2"
by auto
moreover from b c p_eq2 A_def have "y + ya < p"
by auto
ultimately show False
apply simp
apply (frule_tac m = p in zcong_not_zero)
done
qed

lemma F_Un_D_card: "card (F ∪ D) = nat ((p - 1) div 2)"
proof -
have "card (F ∪ D) = card E + card D"
by (auto simp add: finite_F finite_D F_D_disj
card_Un_disjoint F_card_eq_E)
then have "card (F ∪ D) = card C"
from this show "card (F ∪ D) = nat ((p - 1) div 2)"
qed

lemma F_Un_D_eq_A: "F ∪ D = A"
using finite_A F_Un_D_subset A_card_eq F_Un_D_card by (auto simp add: card_seteq)

lemma prod_D_F_eq_prod_A:
"(setprod id D) * (setprod id F) = setprod id A"
apply (insert F_D_disj finite_D finite_F)
apply (frule setprod.union_disjoint [of F D id])
done

lemma prod_F_zcong:
"[setprod id F = ((-1) ^ (card E)) * (setprod id E)] (mod p)"
proof -
have "setprod id F = setprod id (op - p ` E)"
then have "setprod id F = setprod (op - p) E"
apply simp
apply (insert finite_E inj_on_pminusx_E)
apply (frule setprod.reindex [of "minus p" E id])
apply auto
done
then have one:
"[setprod id F = setprod (StandardRes p o (op - p)) E] (mod p)"
apply simp
apply (insert p_g_0 finite_E StandardRes_prod)
by (auto)
moreover have a: "∀x ∈ E. [p - x = 0 - x] (mod p)"
apply clarify
apply (insert zcong_id [of p])
apply (rule_tac a = p and m = p and c = x and d = x in zcong_zdiff, auto)
done
moreover have b: "∀x ∈ E. [StandardRes p (p - x) = p - x](mod p)"
apply clarify
done
moreover have "∀x ∈ E. [StandardRes p (p - x) = - x](mod p)"
apply clarify
apply (insert a b)
apply (rule_tac b = "p - x" in zcong_trans, auto)
done
ultimately have c:
"[setprod (StandardRes p o (op - p)) E = setprod (uminus) E](mod p)"
apply simp
using finite_E p_g_0
setprod_same_function_zcong [of E "StandardRes p o (op - p)" uminus p]
by auto
then have two: "[setprod id F = setprod (uminus) E](mod p)"
apply (insert one c)
apply (rule zcong_trans [of "setprod id F"
"setprod (StandardRes p o op - p) E" p
"setprod uminus E"], auto)
done
also have "setprod uminus E = (setprod id E) * (-1)^(card E)"
using finite_E by (induct set: finite) auto
then have "setprod uminus E = (-1) ^ (card E) * (setprod id E)"
with two show ?thesis
by simp
qed

subsection ‹Gauss' Lemma›

lemma aux: "setprod id A * (- 1) ^ card E * a ^ card A * (- 1) ^ card E = setprod id A * a ^ card A"
by (auto simp add: finite_E neg_one_special)

theorem pre_gauss_lemma:
"[a ^ nat((p - 1) div 2) = (-1) ^ (card E)] (mod p)"
proof -
have "[setprod id A = setprod id F * setprod id D](mod p)"
by (auto simp add: prod_D_F_eq_prod_A mult.commute cong del:setprod.cong)
then have "[setprod id A = ((-1)^(card E) * setprod id E) *
setprod id D] (mod p)"
apply (rule zcong_trans)
apply (auto simp add: prod_F_zcong zcong_scalar cong del: setprod.cong)
done
then have "[setprod id A = ((-1)^(card E) * setprod id C)] (mod p)"
apply (rule zcong_trans)
apply (insert C_prod_eq_D_times_E, erule subst)
apply (subst mult.assoc, auto)
done
then have "[setprod id A = ((-1)^(card E) * setprod id B)] (mod p)"
apply (rule zcong_trans)
apply (simp add: C_B_zcong_prod zcong_scalar2 cong del:setprod.cong)
done
then have "[setprod id A = ((-1)^(card E) *
(setprod id ((%x. x * a) ` A)))] (mod p)"
then have "[setprod id A = ((-1)^(card E) * (setprod (%x. x * a) A))]
(mod p)"
by (simp add:finite_A inj_on_xa_A setprod.reindex cong del:setprod.cong)
moreover have "setprod (%x. x * a) A =
setprod (%x. a) A * setprod id A"
using finite_A by (induct set: finite) auto
ultimately have "[setprod id A = ((-1)^(card E) * (setprod (%x. a) A *
setprod id A))] (mod p)"
by simp
then have "[setprod id A = ((-1)^(card E) * a^(card A) *
setprod id A)](mod p)"
apply (rule zcong_trans)
apply (simp add: zcong_scalar2 zcong_scalar finite_A setprod_constant mult.assoc)
done
then have a: "[setprod id A * (-1)^(card E) =
((-1)^(card E) * a^(card A) * setprod id A * (-1)^(card E))](mod p)"
by (rule zcong_scalar)
then have "[setprod id A * (-1)^(card E) = setprod id A *
(-1)^(card E) * a^(card A) * (-1)^(card E)](mod p)"
apply (rule zcong_trans)
apply (simp add: a mult.commute mult.left_commute)
done
then have "[setprod id A * (-1)^(card E) = setprod id A *
a^(card A)](mod p)"
apply (rule zcong_trans)
apply (simp add: aux cong del:setprod.cong)
done
with this zcong_cancel2 [of p "setprod id A" "(- 1) ^ card E" "a ^ card A"]
p_g_0 A_prod_relprime have "[(- 1) ^ card E = a ^ card A](mod p)"
from this show ?thesis
qed

theorem gauss_lemma: "(Legendre a p) = (-1) ^ (card E)"
proof -
from Euler_Criterion p_prime p_g_2 have
"[(Legendre a p) = a^(nat (((p) - 1) div 2))] (mod p)"
by auto
moreover note pre_gauss_lemma
ultimately have "[(Legendre a p) = (-1) ^ (card E)] (mod p)"
by (rule zcong_trans)
moreover from p_a_relprime have "(Legendre a p) = 1 | (Legendre a p) = (-1)"