Theory WilsonRuss

theory WilsonRuss
imports EulerFermat
```(*  Title:      HOL/Old_Number_Theory/WilsonRuss.thy
Author:     Thomas M. Rasmussen
*)

section ‹Wilson's Theorem according to Russinoff›

theory WilsonRuss
imports EulerFermat
begin

text ‹
Wilson's Theorem following quite closely Russinoff's approach
using Boyer-Moore (using finite sets instead of lists, though).
›

subsection ‹Definitions and lemmas›

definition inv :: "int => int => int"
where "inv p a = (a^(nat (p - 2))) mod p"

fun wset :: "int ⇒ int => int set" where
"wset a p =
(if 1 < a then
let ws = wset (a - 1) p
in (if a ∈ ws then ws else insert a (insert (inv p a) ws)) else {})"

text ‹\medskip @{term [source] inv}›

lemma inv_is_inv_aux: "1 < m ==> Suc (nat (m - 2)) = nat (m - 1)"
by simp

lemma inv_is_inv:
"zprime p ⟹ 0 < a ⟹ a < p ==> [a * inv p a = 1] (mod p)"
apply (unfold inv_def)
apply (subst zcong_zmod)
apply (subst mod_mult_right_eq [symmetric])
apply (subst zcong_zmod [symmetric])
apply (subst power_Suc [symmetric])
using Little_Fermat inv_is_inv_aux zdvd_not_zless apply auto
done

lemma inv_distinct:
"zprime p ⟹ 1 < a ⟹ a < p - 1 ==> a ≠ inv p a"
apply safe
apply (cut_tac a = a and p = p in zcong_square)
apply (cut_tac [3] a = a and p = p in inv_is_inv, auto)
apply (subgoal_tac "a = 1")
apply (rule_tac [2] m = p in zcong_zless_imp_eq)
apply (subgoal_tac [7] "a = p - 1")
apply (rule_tac [8] m = p in zcong_zless_imp_eq, auto)
done

lemma inv_not_0:
"zprime p ⟹ 1 < a ⟹ a < p - 1 ==> inv p a ≠ 0"
apply safe
apply (cut_tac a = a and p = p in inv_is_inv)
apply (unfold zcong_def, auto)
done

lemma inv_not_1:
"zprime p ⟹ 1 < a ⟹ a < p - 1 ==> inv p a ≠ 1"
apply safe
apply (cut_tac a = a and p = p in inv_is_inv)
prefer 4
apply simp
apply (subgoal_tac "a = 1")
apply (rule_tac [2] zcong_zless_imp_eq, auto)
done

lemma inv_not_p_minus_1_aux:
"[a * (p - 1) = 1] (mod p) = [a = p - 1] (mod p)"
apply (unfold zcong_def)
apply (simp add: diff_diff_eq diff_diff_eq2 right_diff_distrib)
apply (rule_tac s = "p dvd -((a + 1) + (p * -a))" in trans)
apply (subst dvd_minus_iff)
apply (subst zdvd_reduce)
apply (rule_tac s = "p dvd (a + 1) + (p * -1)" in trans)
apply (subst zdvd_reduce, auto)
done

lemma inv_not_p_minus_1:
"zprime p ⟹ 1 < a ⟹ a < p - 1 ==> inv p a ≠ p - 1"
apply safe
apply (cut_tac a = a and p = p in inv_is_inv, auto)
apply (subgoal_tac "a = p - 1")
apply (rule_tac [2] zcong_zless_imp_eq, auto)
done

lemma inv_g_1:
"zprime p ⟹ 1 < a ⟹ a < p - 1 ==> 1 < inv p a"
apply (case_tac "0≤ inv p a")
apply (subgoal_tac "inv p a ≠ 1")
apply (subgoal_tac "inv p a ≠ 0")
apply (subst order_less_le)
apply (subst order_less_le)
apply (rule_tac [2] inv_not_0)
apply (rule_tac [5] inv_not_1, auto)
apply (unfold inv_def zprime_def, simp)
done

lemma inv_less_p_minus_1:
"zprime p ⟹ 1 < a ⟹ a < p - 1 ==> inv p a < p - 1"
apply (case_tac "inv p a < p")
apply (subst order_less_le)
apply (unfold inv_def zprime_def, simp)
done

lemma inv_inv_aux: "5 ≤ p ==>
nat (p - 2) * nat (p - 2) = Suc (nat (p - 1) * nat (p - 3))"
apply (subst of_nat_eq_iff [where 'a = int, symmetric])
done

lemma zcong_zpower_zmult:
"[x^y = 1] (mod p) ⟹ [x^(y * z) = 1] (mod p)"
apply (induct z)
apply (subgoal_tac "zcong (x^y * x^(y * z)) (1 * 1) p")
apply (rule_tac [2] zcong_zmult, simp_all)
done

lemma inv_inv: "zprime p ⟹
5 ≤ p ⟹ 0 < a ⟹ a < p ==> inv p (inv p a) = a"
apply (unfold inv_def)
apply (subst power_mod)
apply (subst power_mult [symmetric])
apply (rule zcong_zless_imp_eq)
prefer 5
apply (subst zcong_zmod)
apply (subst mod_mod_trivial)
apply (subst zcong_zmod [symmetric])
apply (subst inv_inv_aux)
apply (subgoal_tac [2]
"zcong (a * a^(nat (p - 1) * nat (p - 3))) (a * 1) p")
apply (rule_tac [3] zcong_zmult)
apply (rule_tac [4] zcong_zpower_zmult)
apply (erule_tac [4] Little_Fermat)
apply (rule_tac [4] zdvd_not_zless, simp_all)
done

text ‹\medskip @{term wset}›

declare wset.simps [simp del]

lemma wset_induct:
assumes "!!a p. P {} a p"
and "!!a p. 1 < (a::int) ⟹
P (wset (a - 1) p) (a - 1) p ==> P (wset a p) a p"
shows "P (wset u v) u v"
apply (rule wset.induct)
apply (case_tac "1 < a")
apply (rule assms)
done

lemma wset_mem_imp_or [rule_format]:
"1 < a ⟹ b ∉ wset (a - 1) p
==> b ∈ wset a p --> b = a ∨ b = inv p a"
apply (subst wset.simps)
apply (unfold Let_def, simp)
done

lemma wset_mem_mem [simp]: "1 < a ==> a ∈ wset a p"
apply (subst wset.simps)
apply (unfold Let_def, simp)
done

lemma wset_subset: "1 < a ⟹ b ∈ wset (a - 1) p ==> b ∈ wset a p"
apply (subst wset.simps)
apply (unfold Let_def, auto)
done

lemma wset_g_1 [rule_format]:
"zprime p --> a < p - 1 --> b ∈ wset a p --> 1 < b"
apply (induct a p rule: wset_induct, auto)
apply (case_tac "b = a")
apply (case_tac [2] "b = inv p a")
apply (subgoal_tac [3] "b = a ∨ b = inv p a")
apply (rule_tac [4] wset_mem_imp_or)
prefer 2
apply simp
apply (rule inv_g_1, auto)
done

lemma wset_less [rule_format]:
"zprime p --> a < p - 1 --> b ∈ wset a p --> b < p - 1"
apply (induct a p rule: wset_induct, auto)
apply (case_tac "b = a")
apply (case_tac [2] "b = inv p a")
apply (subgoal_tac [3] "b = a ∨ b = inv p a")
apply (rule_tac [4] wset_mem_imp_or)
prefer 2
apply simp
apply (rule inv_less_p_minus_1, auto)
done

lemma wset_mem [rule_format]:
"zprime p -->
a < p - 1 --> 1 < b --> b ≤ a --> b ∈ wset a p"
apply (induct a p rule: wset.induct, auto)
apply (rule_tac wset_subset)
apply (simp (no_asm_simp))
apply auto
done

lemma wset_mem_inv_mem [rule_format]:
"zprime p --> 5 ≤ p --> a < p - 1 --> b ∈ wset a p
--> inv p b ∈ wset a p"
apply (induct a p rule: wset_induct, auto)
apply (case_tac "b = a")
apply (subst wset.simps)
apply (unfold Let_def)
apply (rule_tac [3] wset_subset, auto)
apply (case_tac "b = inv p a")
apply (simp (no_asm_simp))
apply (subst inv_inv)
apply (subgoal_tac [6] "b = a ∨ b = inv p a")
apply (rule_tac [7] wset_mem_imp_or, auto)
done

lemma wset_inv_mem_mem:
"zprime p ⟹ 5 ≤ p ⟹ a < p - 1 ⟹ 1 < b ⟹ b < p - 1
⟹ inv p b ∈ wset a p ⟹ b ∈ wset a p"
apply (rule_tac s = "inv p (inv p b)" and t = b in subst)
apply (rule_tac [2] wset_mem_inv_mem)
apply (rule inv_inv, simp_all)
done

lemma wset_fin: "finite (wset a p)"
apply (induct a p rule: wset_induct)
prefer 2
apply (subst wset.simps)
apply (unfold Let_def, auto)
done

lemma wset_zcong_prod_1 [rule_format]:
"zprime p -->
5 ≤ p --> a < p - 1 --> [(∏x∈wset a p. x) = 1] (mod p)"
apply (induct a p rule: wset_induct)
prefer 2
apply (subst wset.simps)
apply (auto, unfold Let_def, auto)
apply (subst setprod.insert)
apply (tactic ‹stac @{context} @{thm setprod.insert} 3›)
apply (subgoal_tac [5]
"zcong (a * inv p a * (∏x∈wset (a - 1) p. x)) (1 * 1) p")
prefer 5
apply (rule_tac [5] zcong_zmult)
apply (rule_tac [5] inv_is_inv)
apply (tactic "clarify_tac @{context} 4")
apply (subgoal_tac [4] "a ∈ wset (a - 1) p")
apply (rule_tac [5] wset_inv_mem_mem)
apply (rule inv_distinct, auto)
done

lemma d22set_eq_wset: "zprime p ==> d22set (p - 2) = wset (p - 2) p"
apply safe
apply (erule wset_mem)
apply (rule_tac [2] d22set_g_1)
apply (rule_tac [3] d22set_le)
apply (rule_tac [4] d22set_mem)
apply (erule_tac [4] wset_g_1)
prefer 6
apply (subgoal_tac "p - 2 + 1 = p - 1")
apply (simp (no_asm_simp))
apply (erule wset_less, auto)
done

subsection ‹Wilson›

lemma prime_g_5: "zprime p ⟹ p ≠ 2 ⟹ p ≠ 3 ==> 5 ≤ p"
apply (unfold zprime_def dvd_def)
apply (case_tac "p = 4", auto)
apply (rule notE)
prefer 2
apply assumption
apply (simp (no_asm))
apply (rule_tac x = 2 in exI)
apply (safe, arith)
apply (rule_tac x = 2 in exI, auto)
done

theorem Wilson_Russ:
"zprime p ==> [zfact (p - 1) = -1] (mod p)"
apply (subgoal_tac "[(p - 1) * zfact (p - 2) = -1 * 1] (mod p)")
apply (rule_tac [2] zcong_zmult)
apply (simp only: zprime_def)
apply (subst zfact.simps)
apply (rule_tac t = "p - 1 - 1" and s = "p - 2" in subst, auto)
apply (simp only: zcong_def)
apply (simp (no_asm_simp))
apply (case_tac "p = 2")