(* Title: HOL/Number_Theory/Pocklington.thy Author: Amine Chaieb *) section ‹Pocklington's Theorem for Primes› theory Pocklington imports Residues begin subsection ‹Lemmas about previously defined terms› lemma prime_nat_iff'': "prime (p::nat) ⟷ p ≠ 0 ∧ p ≠ 1 ∧ (∀m. 0 < m ∧ m < p ⟶ coprime p m)" unfolding prime_nat_iff proof safe fix m assume p: "p > 0" "p ≠ 1" and m: "m dvd p" "m ≠ p" and *: "∀m. m > 0 ∧ m < p ⟶ coprime p m" from p m have "m ≠ 0" by (intro notI) auto moreover from p m have "m < p" by (auto dest: dvd_imp_le) ultimately have "coprime p m" using * by blast with m show "m = 1" by simp qed (auto simp: prime_nat_iff simp del: One_nat_def intro!: prime_imp_coprime dest: dvd_imp_le) lemma finite_number_segment: "card { m. 0 < m ∧ m < n } = n - 1" proof - have "{ m. 0 < m ∧ m < n } = {1..<n}" by auto then show ?thesis by simp qed subsection ‹Some basic theorems about solving congruences› lemma cong_solve: fixes n :: nat assumes an: "coprime a n" shows "∃x. [a * x = b] (mod n)" proof (cases "a = 0") case True with an show ?thesis by (simp add: cong_nat_def) next case False from bezout_add_strong_nat [OF this] obtain d x y where dxy: "d dvd a" "d dvd n" "a * x = n * y + d" by blast from dxy(1,2) have d1: "d = 1" by (metis assms coprime_nat) with dxy(3) have "a * x * b = (n * y + 1) * b" by simp then have "a * (x * b) = n * (y * b) + b" by (auto simp: algebra_simps) then have "a * (x * b) mod n = (n * (y * b) + b) mod n" by simp then have "a * (x * b) mod n = b mod n" by (simp add: mod_add_left_eq) then have "[a * (x * b) = b] (mod n)" by (simp only: cong_nat_def) then show ?thesis by blast qed lemma cong_solve_unique: fixes n :: nat assumes an: "coprime a n" and nz: "n ≠ 0" shows "∃!x. x < n ∧ [a * x = b] (mod n)" proof - from cong_solve[OF an] obtain x where x: "[a * x = b] (mod n)" by blast let ?P = "λx. x < n ∧ [a * x = b] (mod n)" let ?x = "x mod n" from x have *: "[a * ?x = b] (mod n)" by (simp add: cong_nat_def mod_mult_right_eq[of a x n]) from mod_less_divisor[ of n x] nz * have Px: "?P ?x" by simp have "y = ?x" if Py: "y < n" "[a * y = b] (mod n)" for y proof - from Py(2) * have "[a * y = a * ?x] (mod n)" by (simp add: cong_nat_def) then have "[y = ?x] (mod n)" by (metis an cong_mult_lcancel_nat) with mod_less[OF Py(1)] mod_less_divisor[ of n x] nz show ?thesis by (simp add: cong_nat_def) qed with Px show ?thesis by blast qed lemma cong_solve_unique_nontrivial: fixes p :: nat assumes p: "prime p" and pa: "coprime p a" and x0: "0 < x" and xp: "x < p" shows "∃!y. 0 < y ∧ y < p ∧ [x * y = a] (mod p)" proof - from pa have ap: "coprime a p" by (metis gcd.commute) have px: "coprime x p" by (metis gcd.commute p prime_nat_iff'' x0 xp) obtain y where y: "y < p" "[x * y = a] (mod p)" "∀z. z < p ∧ [x * z = a] (mod p) ⟶ z = y" by (metis cong_solve_unique neq0_conv p prime_gt_0_nat px) have "y ≠ 0" proof assume "y = 0" with y(2) have "p dvd a" by (auto dest: cong_dvd_eq_nat) then show False by (metis gcd_nat.absorb1 not_prime_1 p pa) qed with y show ?thesis by blast qed lemma cong_unique_inverse_prime: fixes p :: nat assumes "prime p" and "0 < x" and "x < p" shows "∃!y. 0 < y ∧ y < p ∧ [x * y = 1] (mod p)" by (rule cong_solve_unique_nontrivial) (use assms in simp_all) lemma chinese_remainder_coprime_unique: fixes a :: nat assumes ab: "coprime a b" and az: "a ≠ 0" and bz: "b ≠ 0" and ma: "coprime m a" and nb: "coprime n b" shows "∃!x. coprime x (a * b) ∧ x < a * b ∧ [x = m] (mod a) ∧ [x = n] (mod b)" proof - let ?P = "λx. x < a * b ∧ [x = m] (mod a) ∧ [x = n] (mod b)" from binary_chinese_remainder_unique_nat[OF ab az bz] obtain x where x: "x < a * b" "[x = m] (mod a)" "[x = n] (mod b)" "∀y. ?P y ⟶ y = x" by blast from ma nb x have "coprime x a" "coprime x b" by (metis cong_gcd_eq_nat)+ then have "coprime x (a*b)" by (metis coprime_mul_eq) with x show ?thesis by blast qed subsection ‹Lucas's theorem› lemma lucas_coprime_lemma: fixes n :: nat assumes m: "m ≠ 0" and am: "[a^m = 1] (mod n)" shows "coprime a n" proof - consider "n = 1" | "n = 0" | "n > 1" by arith then show ?thesis proof cases case 1 then show ?thesis by simp next case 2 with am m show ?thesis by (metis am cong_0_nat gcd_nat.right_neutral power_eq_one_eq_nat) next case 3 from m obtain m' where m': "m = Suc m'" by (cases m) blast+ have "d = 1" if d: "d dvd a" "d dvd n" for d proof - from am mod_less[OF ‹n > 1›] have am1: "a^m mod n = 1" by (simp add: cong_nat_def) from dvd_mult2[OF d(1), of "a^m'"] have dam: "d dvd a^m" by (simp add: m') from dvd_mod_iff[OF d(2), of "a^m"] dam am1 show ?thesis by simp qed then show ?thesis by blast qed qed lemma lucas_weak: fixes n :: nat assumes n: "n ≥ 2" and an: "[a ^ (n - 1) = 1] (mod n)" and nm: "∀m. 0 < m ∧ m < n - 1 ⟶ ¬ [a ^ m = 1] (mod n)" shows "prime n" proof (rule totient_imp_prime) show "totient n = n - 1" proof (rule ccontr) have "[a ^ totient n = 1] (mod n)" by (rule euler_theorem, rule lucas_coprime_lemma [of "n - 1"]) (use n an in auto) moreover assume "totient n ≠ n - 1" then have "totient n > 0" "totient n < n - 1" using ‹n ≥ 2› and totient_less[of n] by simp_all ultimately show False using nm by auto qed qed (use n in auto) lemma nat_exists_least_iff: "(∃(n::nat). P n) ⟷ (∃n. P n ∧ (∀m < n. ¬ P m))" by (metis ex_least_nat_le not_less0) lemma nat_exists_least_iff': "(∃(n::nat). P n) ⟷ P (Least P) ∧ (∀m < (Least P). ¬ P m)" (is "?lhs ⟷ ?rhs") proof show ?lhs if ?rhs using that by blast show ?rhs if ?lhs proof - from ‹?lhs› obtain n where n: "P n" by blast let ?x = "Least P" have "¬ P m" if "m < ?x" for m by (rule not_less_Least[OF that]) with LeastI_ex[OF ‹?lhs›] show ?thesis by blast qed qed theorem lucas: assumes n2: "n ≥ 2" and an1: "[a^(n - 1) = 1] (mod n)" and pn: "∀p. prime p ∧ p dvd n - 1 ⟶ [a^((n - 1) div p) ≠ 1] (mod n)" shows "prime n" proof- from n2 have n01: "n ≠ 0" "n ≠ 1" "n - 1 ≠ 0" by arith+ from mod_less_divisor[of n 1] n01 have onen: "1 mod n = 1" by simp from lucas_coprime_lemma[OF n01(3) an1] cong_imp_coprime_nat an1 have an: "coprime a n" "coprime (a^(n - 1)) n" by (auto simp add: coprime_exp gcd.commute) have False if H0: "∃m. 0 < m ∧ m < n - 1 ∧ [a ^ m = 1] (mod n)" (is "EX m. ?P m") proof - from H0[unfolded nat_exists_least_iff[of ?P]] obtain m where m: "0 < m" "m < n - 1" "[a ^ m = 1] (mod n)" "∀k <m. ¬?P k" by blast have False if nm1: "(n - 1) mod m > 0" proof - from mod_less_divisor[OF m(1)] have th0:"(n - 1) mod m < m" by blast let ?y = "a^ ((n - 1) div m * m)" note mdeq = div_mult_mod_eq[of "(n - 1)" m] have yn: "coprime ?y n" by (metis an(1) coprime_exp gcd.commute) have "?y mod n = (a^m)^((n - 1) div m) mod n" by (simp add: algebra_simps power_mult) also have "… = (a^m mod n)^((n - 1) div m) mod n" using power_mod[of "a^m" n "(n - 1) div m"] by simp also have "… = 1" using m(3)[unfolded cong_nat_def onen] onen by (metis power_one) finally have *: "?y mod n = 1" . have **: "[?y * a ^ ((n - 1) mod m) = ?y* 1] (mod n)" using an1[unfolded cong_nat_def onen] onen div_mult_mod_eq[of "(n - 1)" m, symmetric] by (simp add:power_add[symmetric] cong_nat_def * del: One_nat_def) have "[a ^ ((n - 1) mod m) = 1] (mod n)" by (metis cong_mult_rcancel_nat mult.commute ** yn) with m(4)[rule_format, OF th0] nm1 less_trans[OF mod_less_divisor[OF m(1), of "n - 1"] m(2)] show ?thesis by blast qed then have "(n - 1) mod m = 0" by auto then have mn: "m dvd n - 1" by presburger then obtain r where r: "n - 1 = m * r" unfolding dvd_def by blast from n01 r m(2) have r01: "r ≠ 0" "r ≠ 1" by auto obtain p where p: "prime p" "p dvd r" by (metis prime_factor_nat r01(2)) then have th: "prime p ∧ p dvd n - 1" unfolding r by (auto intro: dvd_mult) from r have "(a ^ ((n - 1) div p)) mod n = (a^(m*r div p)) mod n" by (simp add: power_mult) also have "… = (a^(m*(r div p))) mod n" using div_mult1_eq[of m r p] p(2)[unfolded dvd_eq_mod_eq_0] by simp also have "… = ((a^m)^(r div p)) mod n" by (simp add: power_mult) also have "… = ((a^m mod n)^(r div p)) mod n" using power_mod .. also from m(3) onen have "… = 1" by (simp add: cong_nat_def) finally have "[(a ^ ((n - 1) div p))= 1] (mod n)" using onen by (simp add: cong_nat_def) with pn th show ?thesis by blast qed then have "∀m. 0 < m ∧ m < n - 1 ⟶ ¬ [a ^ m = 1] (mod n)" by blast then show ?thesis by (rule lucas_weak[OF n2 an1]) qed subsection ‹Definition of the order of a number mod n (0 in non-coprime case)› definition "ord n a = (if coprime n a then Least (λd. d > 0 ∧ [a ^d = 1] (mod n)) else 0)" text ‹This has the expected properties.› lemma coprime_ord: fixes n::nat assumes "coprime n a" shows "ord n a > 0 ∧ [a ^(ord n a) = 1] (mod n) ∧ (∀m. 0 < m ∧ m < ord n a ⟶ [a^ m ≠ 1] (mod n))" proof- let ?P = "λd. 0 < d ∧ [a ^ d = 1] (mod n)" from bigger_prime[of a] obtain p where p: "prime p" "a < p" by blast from assms have o: "ord n a = Least ?P" by (simp add: ord_def) have ex: "∃m>0. ?P m" proof (cases "n ≥ 2") case True moreover from assms have "coprime a n" by (simp add: ac_simps) then have "[a ^ totient n = 1] (mod n)" by (rule euler_theorem) ultimately show ?thesis by (auto intro: exI [where x = "totient n"]) next case False then have "n = 0 ∨ n = 1" by auto with assms show ?thesis by auto qed from nat_exists_least_iff'[of ?P] ex assms show ?thesis unfolding o[symmetric] by auto qed text ‹With the special value ‹0› for non-coprime case, it's more convenient.› lemma ord_works: "[a ^ (ord n a) = 1] (mod n) ∧ (∀m. 0 < m ∧ m < ord n a ⟶ ¬ [a^ m = 1] (mod n))" for n :: nat by (cases "coprime n a") (use coprime_ord[of n a] in ‹auto simp add: ord_def cong_nat_def›) lemma ord: "[a^(ord n a) = 1] (mod n)" for n :: nat using ord_works by blast lemma ord_minimal: "0 < m ⟹ m < ord n a ⟹ ¬ [a^m = 1] (mod n)" for n :: nat using ord_works by blast lemma ord_eq_0: "ord n a = 0 ⟷ ¬ coprime n a" for n :: nat by (cases "coprime n a") (simp add: coprime_ord, simp add: ord_def) lemma divides_rexp: "x dvd y ⟹ x dvd (y ^ Suc n)" for x y :: nat by (simp add: dvd_mult2[of x y]) lemma ord_divides:"[a ^ d = 1] (mod n) ⟷ ord n a dvd d" (is "?lhs ⟷ ?rhs") for n :: nat proof assume ?rhs then obtain k where "d = ord n a * k" unfolding dvd_def by blast then have "[a ^ d = (a ^ (ord n a) mod n)^k] (mod n)" by (simp add : cong_nat_def power_mult power_mod) also have "[(a ^ (ord n a) mod n)^k = 1] (mod n)" using ord[of a n, unfolded cong_nat_def] by (simp add: cong_nat_def power_mod) finally show ?lhs . next assume ?lhs show ?rhs proof (cases "coprime n a") case prem: False then have o: "ord n a = 0" by (simp add: ord_def) show ?thesis proof (cases d) case 0 with o prem show ?thesis by (simp add: cong_nat_def) next case (Suc d') then have d0: "d ≠ 0" by simp from prem obtain p where p: "p dvd n" "p dvd a" "p ≠ 1" by auto from ‹?lhs› obtain q1 q2 where q12: "a ^ d + n * q1 = 1 + n * q2" by (metis prem d0 gcd.commute lucas_coprime_lemma) then have "a ^ d + n * q1 - n * q2 = 1" by simp with dvd_diff_nat [OF dvd_add [OF divides_rexp]] dvd_mult2 Suc p have "p dvd 1" by metis with p(3) have False by simp then show ?thesis .. qed next case H: True let ?o = "ord n a" let ?q = "d div ord n a" let ?r = "d mod ord n a" have eqo: "[(a^?o)^?q = 1] (mod n)" by (metis cong_exp_nat ord power_one) from H have onz: "?o ≠ 0" by (simp add: ord_eq_0) then have op: "?o > 0" by simp from div_mult_mod_eq[of d "ord n a"] ‹?lhs› have "[a^(?o*?q + ?r) = 1] (mod n)" by (simp add: cong_nat_def mult.commute) then have "[(a^?o)^?q * (a^?r) = 1] (mod n)" by (simp add: cong_nat_def power_mult[symmetric] power_add[symmetric]) then have th: "[a^?r = 1] (mod n)" using eqo mod_mult_left_eq[of "(a^?o)^?q" "a^?r" n] by (simp add: cong_nat_def del: One_nat_def) (metis mod_mult_left_eq nat_mult_1) show ?thesis proof (cases "?r = 0") case True then show ?thesis by (simp add: dvd_eq_mod_eq_0) next case False with mod_less_divisor[OF op, of d] have r0o:"?r >0 ∧ ?r < ?o" by simp from conjunct2[OF ord_works[of a n], rule_format, OF r0o] th show ?thesis by blast qed qed qed lemma order_divides_totient: "ord n a dvd totient n" if "coprime n a" by (metis euler_theorem gcd.commute ord_divides that) lemma order_divides_expdiff: fixes n::nat and a::nat assumes na: "coprime n a" shows "[a^d = a^e] (mod n) ⟷ [d = e] (mod (ord n a))" proof - have th: "[a^d = a^e] (mod n) ⟷ [d = e] (mod (ord n a))" if na: "coprime n a" and ed: "(e::nat) ≤ d" for n a d e :: nat proof - from na ed have "∃c. d = e + c" by presburger then obtain c where c: "d = e + c" .. from na have an: "coprime a n" by (metis gcd.commute) have aen: "coprime (a^e) n" by (metis coprime_exp gcd.commute na) have acn: "coprime (a^c) n" by (metis coprime_exp gcd.commute na) from c have "[a^d = a^e] (mod n) ⟷ [a^(e + c) = a^(e + 0)] (mod n)" by simp also have "… ⟷ [a^e* a^c = a^e *a^0] (mod n)" by (simp add: power_add) also have "… ⟷ [a ^ c = 1] (mod n)" using cong_mult_lcancel_nat [OF aen, of "a^c" "a^0"] by simp also have "… ⟷ ord n a dvd c" by (simp only: ord_divides) also have "… ⟷ [e + c = e + 0] (mod ord n a)" using cong_add_lcancel_nat by (metis cong_dvd_eq_nat dvd_0_right cong_dvd_modulus_nat cong_mult_self_nat nat_mult_1) finally show ?thesis using c by simp qed consider "e ≤ d" | "d ≤ e" by arith then show ?thesis proof cases case 1 with na show ?thesis by (rule th) next case 2 from th[OF na this] show ?thesis by (metis cong_sym_nat) qed qed subsection ‹Another trivial primality characterization› lemma prime_prime_factor: "prime n ⟷ n ≠ 1 ∧ (∀p. prime p ∧ p dvd n ⟶ p = n)" (is "?lhs ⟷ ?rhs") for n :: nat proof (cases "n = 0 ∨ n = 1") case True then show ?thesis by (metis bigger_prime dvd_0_right not_prime_1 not_prime_0) next case False show ?thesis proof assume "prime n" then show ?rhs by (metis not_prime_1 prime_nat_iff) next assume ?rhs with False show "prime n" by (auto simp: prime_nat_iff) (metis One_nat_def prime_factor_nat prime_nat_iff) qed qed lemma prime_divisor_sqrt: "prime n ⟷ n ≠ 1 ∧ (∀d. d dvd n ∧ d⇧^{2}≤ n ⟶ d = 1)" for n :: nat proof - consider "n = 0" | "n = 1" | "n ≠ 0" "n ≠ 1" by blast then show ?thesis proof cases case 1 then show ?thesis by simp next case 2 then show ?thesis by simp next case n: 3 then have np: "n > 1" by arith { fix d assume d: "d dvd n" "d⇧^{2}≤ n" and H: "∀m. m dvd n ⟶ m = 1 ∨ m = n" from H d have d1n: "d = 1 ∨ d = n" by blast then have "d = 1" proof assume dn: "d = n" from n have "n⇧^{2}> n * 1" by (simp add: power2_eq_square) with dn d(2) show ?thesis by simp qed } moreover { fix d assume d: "d dvd n" and H: "∀d'. d' dvd n ∧ d'⇧^{2}≤ n ⟶ d' = 1" from d n have "d ≠ 0" by (metis dvd_0_left_iff) then have dp: "d > 0" by simp from d[unfolded dvd_def] obtain e where e: "n= d*e" by blast from n dp e have ep:"e > 0" by simp from dp ep have "d⇧^{2}≤ n ∨ e⇧^{2}≤ n" by (auto simp add: e power2_eq_square mult_le_cancel_left) then have "d = 1 ∨ d = n" proof assume "d⇧^{2}≤ n" with H[rule_format, of d] d have "d = 1" by blast then show ?thesis .. next assume h: "e⇧^{2}≤ n" from e have "e dvd n" by (simp add: dvd_def mult.commute) with H[rule_format, of e] h have "e = 1" by simp with e have "d = n" by simp then show ?thesis .. qed } ultimately show ?thesis unfolding prime_nat_iff using np n(2) by blast qed qed lemma prime_prime_factor_sqrt: "prime (n::nat) ⟷ n ≠ 0 ∧ n ≠ 1 ∧ (∄p. prime p ∧ p dvd n ∧ p⇧^{2}≤ n)" (is "?lhs ⟷?rhs") proof - consider "n = 0" | "n = 1" | "n ≠ 0" "n ≠ 1" by blast then show ?thesis proof cases case 1 then show ?thesis by (metis not_prime_0) next case 2 then show ?thesis by (metis not_prime_1) next case n: 3 show ?thesis proof assume ?lhs from this[unfolded prime_divisor_sqrt] n show ?rhs by (metis prime_prime_factor) next assume ?rhs { fix d assume d: "d dvd n" "d⇧^{2}≤ n" "d ≠ 1" then obtain p where p: "prime p" "p dvd d" by (metis prime_factor_nat) from d(1) n have dp: "d > 0" by (metis dvd_0_left neq0_conv) from mult_mono[OF dvd_imp_le[OF p(2) dp] dvd_imp_le[OF p(2) dp]] d(2) have "p⇧^{2}≤ n" unfolding power2_eq_square by arith with ‹?rhs› n p(1) dvd_trans[OF p(2) d(1)] have False by blast } with n prime_divisor_sqrt show ?lhs by auto qed qed qed subsection ‹Pocklington theorem› lemma pocklington_lemma: fixes p :: nat assumes n: "n ≥ 2" and nqr: "n - 1 = q * r" and an: "[a^ (n - 1) = 1] (mod n)" and aq: "∀p. prime p ∧ p dvd q ⟶ coprime (a ^ ((n - 1) div p) - 1) n" and pp: "prime p" and pn: "p dvd n" shows "[p = 1] (mod q)" proof - have p01: "p ≠ 0" "p ≠ 1" using pp by (auto intro: prime_gt_0_nat) obtain k where k: "a ^ (q * r) - 1 = n * k" by (metis an cong_to_1_nat dvd_def nqr) from pn[unfolded dvd_def] obtain l where l: "n = p * l" by blast have a0: "a ≠ 0" proof assume "a = 0" with n have "a^ (n - 1) = 0" by (simp add: power_0_left) with n an mod_less[of 1 n] show False by (simp add: power_0_left cong_nat_def) qed with n nqr have aqr0: "a ^ (q * r) ≠ 0" by simp then have "(a ^ (q * r) - 1) + 1 = a ^ (q * r)" by simp with k l have "a ^ (q * r) = p * l * k + 1" by simp then have "a ^ (r * q) + p * 0 = 1 + p * (l * k)" by (simp add: ac_simps) then have odq: "ord p (a^r) dvd q" unfolding ord_divides[symmetric] power_mult[symmetric] by (metis an cong_dvd_modulus_nat mult.commute nqr pn) from odq[unfolded dvd_def] obtain d where d: "q = ord p (a^r) * d" by blast have d1: "d = 1" proof (rule ccontr) assume d1: "d ≠ 1" obtain P where P: "prime P" "P dvd d" by (metis d1 prime_factor_nat) from d dvd_mult[OF P(2), of "ord p (a^r)"] have Pq: "P dvd q" by simp from aq P(1) Pq have caP:"coprime (a^ ((n - 1) div P) - 1) n" by blast from Pq obtain s where s: "q = P*s" unfolding dvd_def by blast from P(1) have P0: "P ≠ 0" by (metis not_prime_0) from P(2) obtain t where t: "d = P*t" unfolding dvd_def by blast from d s t P0 have s': "ord p (a^r) * t = s" by (metis mult.commute mult_cancel1 mult.assoc) have "ord p (a^r) * t*r = r * ord p (a^r) * t" by (metis mult.assoc mult.commute) then have exps: "a^(ord p (a^r) * t*r) = ((a ^ r) ^ ord p (a^r)) ^ t" by (simp only: power_mult) then have "[((a ^ r) ^ ord p (a^r)) ^ t= 1] (mod p)" by (metis cong_exp_nat ord power_one) then have pd0: "p dvd a^(ord p (a^r) * t*r) - 1" by (metis cong_to_1_nat exps) from nqr s s' have "(n - 1) div P = ord p (a^r) * t*r" using P0 by simp with caP have "coprime (a^(ord p (a^r) * t*r) - 1) n" by simp with p01 pn pd0 coprime_common_divisor_nat show False by auto qed with d have o: "ord p (a^r) = q" by simp from pp totient_prime [of p] have totient_eq: "totient p = p - 1" by simp { fix d assume d: "d dvd p" "d dvd a" "d ≠ 1" from pp[unfolded prime_nat_iff] d have dp: "d = p" by blast from n have "n ≠ 0" by simp then have False using d dp pn by auto (metis One_nat_def Suc_pred an dvd_1_iff_1 gcd_greatest_iff lucas_coprime_lemma) } then have cpa: "coprime p a" by auto have arp: "coprime (a^r) p" by (metis coprime_exp cpa gcd.commute) from euler_theorem [OF arp, simplified ord_divides] o totient_eq have "q dvd (p - 1)" by simp then obtain d where d:"p - 1 = q * d" unfolding dvd_def by blast have "p ≠ 0" by (metis p01(1)) with d have "p + q * 0 = 1 + q * d" by simp then show ?thesis by (metis cong_iff_lin_nat mult.commute) qed theorem pocklington: assumes n: "n ≥ 2" and nqr: "n - 1 = q * r" and sqr: "n ≤ q⇧^{2}" and an: "[a^ (n - 1) = 1] (mod n)" and aq: "∀p. prime p ∧ p dvd q ⟶ coprime (a^ ((n - 1) div p) - 1) n" shows "prime n" unfolding prime_prime_factor_sqrt[of n] proof - let ?ths = "n ≠ 0 ∧ n ≠ 1 ∧ (∄p. prime p ∧ p dvd n ∧ p⇧^{2}≤ n)" from n have n01: "n ≠ 0" "n ≠ 1" by arith+ { fix p assume p: "prime p" "p dvd n" "p⇧^{2}≤ n" from p(3) sqr have "p^(Suc 1) ≤ q^(Suc 1)" by (simp add: power2_eq_square) then have pq: "p ≤ q" by (metis le0 power_le_imp_le_base) from pocklington_lemma[OF n nqr an aq p(1,2)] have *: "q dvd p - 1" by (metis cong_to_1_nat) have "p - 1 ≠ 0" using prime_ge_2_nat [OF p(1)] by arith with pq * have False by (simp add: nat_dvd_not_less) } with n01 show ?ths by blast qed text ‹Variant for application, to separate the exponentiation.› lemma pocklington_alt: assumes n: "n ≥ 2" and nqr: "n - 1 = q * r" and sqr: "n ≤ q⇧^{2}" and an: "[a^ (n - 1) = 1] (mod n)" and aq: "∀p. prime p ∧ p dvd q ⟶ (∃b. [a^((n - 1) div p) = b] (mod n) ∧ coprime (b - 1) n)" shows "prime n" proof - { fix p assume p: "prime p" "p dvd q" from aq[rule_format] p obtain b where b: "[a^((n - 1) div p) = b] (mod n)" "coprime (b - 1) n" by blast have a0: "a ≠ 0" proof assume a0: "a = 0" from n an have "[0 = 1] (mod n)" unfolding a0 power_0_left by auto then show False using n by (simp add: cong_nat_def dvd_eq_mod_eq_0[symmetric]) qed then have a1: "a ≥ 1" by arith from one_le_power[OF a1] have ath: "1 ≤ a ^ ((n - 1) div p)" . have b0: "b ≠ 0" proof assume b0: "b = 0" from p(2) nqr have "(n - 1) mod p = 0" by (metis mod_0 mod_mod_cancel mod_mult_self1_is_0) with div_mult_mod_eq[of "n - 1" p] have "(n - 1) div p * p= n - 1" by auto then have eq: "(a^((n - 1) div p))^p = a^(n - 1)" by (simp only: power_mult[symmetric]) have "p - 1 ≠ 0" using prime_ge_2_nat [OF p(1)] by arith then have pS: "Suc (p - 1) = p" by arith from b have d: "n dvd a^((n - 1) div p)" unfolding b0 by auto from divides_rexp[OF d, of "p - 1"] pS eq cong_dvd_eq_nat [OF an] n show False by simp qed then have b1: "b ≥ 1" by arith from cong_imp_coprime_nat[OF Cong.cong_diff_nat[OF cong_sym_nat [OF b(1)] cong_refl_nat[of 1] b1]] ath b1 b nqr have "coprime (a ^ ((n - 1) div p) - 1) n" by simp } then have "∀p. prime p ∧ p dvd q ⟶ coprime (a ^ ((n - 1) div p) - 1) n " by blast then show ?thesis by (rule pocklington[OF n nqr sqr an]) qed subsection ‹Prime factorizations› (* FIXME some overlap with material in UniqueFactorization, class unique_factorization *) definition "primefact ps n ⟷ foldr op * ps 1 = n ∧ (∀p∈ set ps. prime p)" lemma primefact: fixes n :: nat assumes n: "n ≠ 0" shows "∃ps. primefact ps n" proof - obtain xs where xs: "mset xs = prime_factorization n" using ex_mset [of "prime_factorization n"] by blast from assms have "n = prod_mset (prime_factorization n)" by (simp add: prod_mset_prime_factorization) also have "… = prod_mset (mset xs)" by (simp add: xs) also have "… = foldr op * xs 1" by (induct xs) simp_all finally have "foldr op * xs 1 = n" .. moreover from xs have "∀p∈#mset xs. prime p" by auto ultimately have "primefact xs n" by (auto simp: primefact_def) then show ?thesis .. qed lemma primefact_contains: fixes p :: nat assumes pf: "primefact ps n" and p: "prime p" and pn: "p dvd n" shows "p ∈ set ps" using pf p pn proof (induct ps arbitrary: p n) case Nil then show ?case by (auto simp: primefact_def) next case (Cons q qs) from Cons.prems[unfolded primefact_def] have q: "prime q" "q * foldr op * qs 1 = n" "∀p ∈set qs. prime p" and p: "prime p" "p dvd q * foldr op * qs 1" by simp_all consider "p dvd q" | "p dvd foldr op * qs 1" by (metis p prime_dvd_mult_eq_nat) then show ?case proof cases case 1 with p(1) q(1) have "p = q" unfolding prime_nat_iff by auto then show ?thesis by simp next case prem: 2 from q(3) have pqs: "primefact qs (foldr op * qs 1)" by (simp add: primefact_def) from Cons.hyps[OF pqs p(1) prem] show ?thesis by simp qed qed lemma primefact_variant: "primefact ps n ⟷ foldr op * ps 1 = n ∧ list_all prime ps" by (auto simp add: primefact_def list_all_iff) text ‹Variant of Lucas theorem.› lemma lucas_primefact: assumes n: "n ≥ 2" and an: "[a^(n - 1) = 1] (mod n)" and psn: "foldr op * ps 1 = n - 1" and psp: "list_all (λp. prime p ∧ ¬ [a^((n - 1) div p) = 1] (mod n)) ps" shows "prime n" proof - { fix p assume p: "prime p" "p dvd n - 1" "[a ^ ((n - 1) div p) = 1] (mod n)" from psn psp have psn1: "primefact ps (n - 1)" by (auto simp add: list_all_iff primefact_variant) from p(3) primefact_contains[OF psn1 p(1,2)] psp have False by (induct ps) auto } with lucas[OF n an] show ?thesis by blast qed text ‹Variant of Pocklington theorem.› lemma pocklington_primefact: assumes n: "n ≥ 2" and qrn: "q*r = n - 1" and nq2: "n ≤ q⇧^{2}" and arnb: "(a^r) mod n = b" and psq: "foldr op * ps 1 = q" and bqn: "(b^q) mod n = 1" and psp: "list_all (λp. prime p ∧ coprime ((b^(q div p)) mod n - 1) n) ps" shows "prime n" proof - from bqn psp qrn have bqn: "a ^ (n - 1) mod n = 1" and psp: "list_all (λp. prime p ∧ coprime (a^(r *(q div p)) mod n - 1) n) ps" unfolding arnb[symmetric] power_mod by (simp_all add: power_mult[symmetric] algebra_simps) from n have n0: "n > 0" by arith from div_mult_mod_eq[of "a^(n - 1)" n] mod_less_divisor[OF n0, of "a^(n - 1)"] have an1: "[a ^ (n - 1) = 1] (mod n)" by (metis bqn cong_nat_def mod_mod_trivial) have "coprime (a ^ ((n - 1) div p) - 1) n" if p: "prime p" "p dvd q" for p proof - from psp psq have pfpsq: "primefact ps q" by (auto simp add: primefact_variant list_all_iff) from psp primefact_contains[OF pfpsq p] have p': "coprime (a ^ (r * (q div p)) mod n - 1) n" by (simp add: list_all_iff) from p prime_nat_iff have p01: "p ≠ 0" "p ≠ 1" "p = Suc (p - 1)" by auto from div_mult1_eq[of r q p] p(2) have eq1: "r* (q div p) = (n - 1) div p" unfolding qrn[symmetric] dvd_eq_mod_eq_0 by (simp add: mult.commute) have ath: "a ≤ b ⟹ a ≠ 0 ⟹ 1 ≤ a ∧ 1 ≤ b" for a b :: nat by arith { assume "a ^ ((n - 1) div p) mod n = 0" then obtain s where s: "a ^ ((n - 1) div p) = n * s" unfolding mod_eq_0_iff by blast then have eq0: "(a^((n - 1) div p))^p = (n*s)^p" by simp from qrn[symmetric] have qn1: "q dvd n - 1" by (auto simp: dvd_def) from dvd_trans[OF p(2) qn1] have npp: "(n - 1) div p * p = n - 1" by simp with eq0 have "a ^ (n - 1) = (n * s) ^ p" by (simp add: power_mult[symmetric]) with bqn p01 have "1 = (n * s)^(Suc (p - 1)) mod n" by simp also have "… = 0" by (simp add: mult.assoc) finally have False by simp } then have *: "a ^ ((n - 1) div p) mod n ≠ 0" by auto have "[a ^ ((n - 1) div p) mod n = a ^ ((n - 1) div p)] (mod n)" by (simp add: cong_nat_def) with ath[OF mod_less_eq_dividend *] have "[a ^ ((n - 1) div p) mod n - 1 = a ^ ((n - 1) div p) - 1] (mod n)" by (metis cong_diff_nat cong_refl_nat) then show ?thesis by (metis cong_imp_coprime_nat eq1 p') qed with pocklington[OF n qrn[symmetric] nq2 an1] show ?thesis by blast qed end