src/HOL/Number_Theory/Residues.thy
author wenzelm
Sun Mar 13 22:55:50 2011 +0100 (2011-03-13)
changeset 41959 b460124855b8
parent 41541 1fa4725c4656
child 44872 a98ef45122f3
permissions -rw-r--r--
tuned headers;
     1 (*  Title:      HOL/Number_Theory/Residues.thy
     2     Author:     Jeremy Avigad
     3 
     4 An algebraic treatment of residue rings, and resulting proofs of
     5 Euler's theorem and Wilson's theorem.
     6 *)
     7 
     8 header {* Residue rings *}
     9 
    10 theory Residues
    11 imports
    12   UniqueFactorization
    13   Binomial
    14   MiscAlgebra
    15 begin
    16 
    17 
    18 (*
    19  
    20   A locale for residue rings
    21 
    22 *)
    23 
    24 definition residue_ring :: "int => int ring" where
    25   "residue_ring m == (| 
    26     carrier =       {0..m - 1}, 
    27     mult =          (%x y. (x * y) mod m),
    28     one =           1,
    29     zero =          0,
    30     add =           (%x y. (x + y) mod m) |)"
    31 
    32 locale residues =
    33   fixes m :: int and R (structure)
    34   assumes m_gt_one: "m > 1"
    35   defines "R == residue_ring m"
    36 
    37 context residues begin
    38 
    39 lemma abelian_group: "abelian_group R"
    40   apply (insert m_gt_one)
    41   apply (rule abelian_groupI)
    42   apply (unfold R_def residue_ring_def)
    43   apply (auto simp add: mod_add_right_eq [symmetric] add_ac)
    44   apply (case_tac "x = 0")
    45   apply force
    46   apply (subgoal_tac "(x + (m - x)) mod m = 0")
    47   apply (erule bexI)
    48   apply auto
    49   done
    50 
    51 lemma comm_monoid: "comm_monoid R"
    52   apply (insert m_gt_one)
    53   apply (unfold R_def residue_ring_def)
    54   apply (rule comm_monoidI)
    55   apply auto
    56   apply (subgoal_tac "x * y mod m * z mod m = z * (x * y mod m) mod m")
    57   apply (erule ssubst)
    58   apply (subst zmod_zmult1_eq [symmetric])+
    59   apply (simp_all only: mult_ac)
    60   done
    61 
    62 lemma cring: "cring R"
    63   apply (rule cringI)
    64   apply (rule abelian_group)
    65   apply (rule comm_monoid)
    66   apply (unfold R_def residue_ring_def, auto)
    67   apply (subst mod_add_eq [symmetric])
    68   apply (subst mult_commute)
    69   apply (subst zmod_zmult1_eq [symmetric])
    70   apply (simp add: field_simps)
    71   done
    72 
    73 end
    74 
    75 sublocale residues < cring
    76   by (rule cring)
    77 
    78 
    79 context residues
    80 begin
    81 
    82 (* These lemmas translate back and forth between internal and 
    83    external concepts *)
    84 
    85 lemma res_carrier_eq: "carrier R = {0..m - 1}"
    86   by (unfold R_def residue_ring_def, auto)
    87 
    88 lemma res_add_eq: "x \<oplus> y = (x + y) mod m"
    89   by (unfold R_def residue_ring_def, auto)
    90 
    91 lemma res_mult_eq: "x \<otimes> y = (x * y) mod m"
    92   by (unfold R_def residue_ring_def, auto)
    93 
    94 lemma res_zero_eq: "\<zero> = 0"
    95   by (unfold R_def residue_ring_def, auto)
    96 
    97 lemma res_one_eq: "\<one> = 1"
    98   by (unfold R_def residue_ring_def units_of_def, auto)
    99 
   100 lemma res_units_eq: "Units R = { x. 0 < x & x < m & coprime x m}"
   101   apply (insert m_gt_one)
   102   apply (unfold Units_def R_def residue_ring_def)
   103   apply auto
   104   apply (subgoal_tac "x ~= 0")
   105   apply auto
   106   apply (rule invertible_coprime_int)
   107   apply (subgoal_tac "x ~= 0")
   108   apply auto
   109   apply (subst (asm) coprime_iff_invertible'_int)
   110   apply (rule m_gt_one)
   111   apply (auto simp add: cong_int_def mult_commute)
   112   done
   113 
   114 lemma res_neg_eq: "\<ominus> x = (- x) mod m"
   115   apply (insert m_gt_one)
   116   apply (unfold R_def a_inv_def m_inv_def residue_ring_def)
   117   apply auto
   118   apply (rule the_equality)
   119   apply auto
   120   apply (subst mod_add_right_eq [symmetric])
   121   apply auto
   122   apply (subst mod_add_left_eq [symmetric])
   123   apply auto
   124   apply (subgoal_tac "y mod m = - x mod m")
   125   apply simp
   126   apply (subst zmod_eq_dvd_iff)
   127   apply auto
   128   done
   129 
   130 lemma finite [iff]: "finite(carrier R)"
   131   by (subst res_carrier_eq, auto)
   132 
   133 lemma finite_Units [iff]: "finite(Units R)"
   134   by (subst res_units_eq, auto)
   135 
   136 (* The function a -> a mod m maps the integers to the 
   137    residue classes. The following lemmas show that this mapping 
   138    respects addition and multiplication on the integers. *)
   139 
   140 lemma mod_in_carrier [iff]: "a mod m : carrier R"
   141   apply (unfold res_carrier_eq)
   142   apply (insert m_gt_one, auto)
   143   done
   144 
   145 lemma add_cong: "(x mod m) \<oplus> (y mod m) = (x + y) mod m"
   146   by (unfold R_def residue_ring_def, auto, arith)
   147 
   148 lemma mult_cong: "(x mod m) \<otimes> (y mod m) = (x * y) mod m"
   149   apply (unfold R_def residue_ring_def, auto)
   150   apply (subst zmod_zmult1_eq [symmetric])
   151   apply (subst mult_commute)
   152   apply (subst zmod_zmult1_eq [symmetric])
   153   apply (subst mult_commute)
   154   apply auto
   155   done
   156 
   157 lemma zero_cong: "\<zero> = 0"
   158   apply (unfold R_def residue_ring_def, auto)
   159   done
   160 
   161 lemma one_cong: "\<one> = 1 mod m"
   162   apply (insert m_gt_one)
   163   apply (unfold R_def residue_ring_def, auto)
   164   done
   165 
   166 (* revise algebra library to use 1? *)
   167 lemma pow_cong: "(x mod m) (^) n = x^n mod m"
   168   apply (insert m_gt_one)
   169   apply (induct n)
   170   apply (auto simp add: nat_pow_def one_cong)
   171   apply (subst mult_commute)
   172   apply (rule mult_cong)
   173   done
   174 
   175 lemma neg_cong: "\<ominus> (x mod m) = (- x) mod m"
   176   apply (rule sym)
   177   apply (rule sum_zero_eq_neg)
   178   apply auto
   179   apply (subst add_cong)
   180   apply (subst zero_cong)
   181   apply auto
   182   done
   183 
   184 lemma (in residues) prod_cong: 
   185   "finite A \<Longrightarrow> (\<Otimes> i:A. (f i) mod m) = (PROD i:A. f i) mod m"
   186   apply (induct set: finite)
   187   apply (auto simp: one_cong mult_cong)
   188   done
   189 
   190 lemma (in residues) sum_cong:
   191   "finite A \<Longrightarrow> (\<Oplus> i:A. (f i) mod m) = (SUM i: A. f i) mod m"
   192   apply (induct set: finite)
   193   apply (auto simp: zero_cong add_cong)
   194   done
   195 
   196 lemma mod_in_res_units [simp]: "1 < m \<Longrightarrow> coprime a m \<Longrightarrow> 
   197     a mod m : Units R"
   198   apply (subst res_units_eq, auto)
   199   apply (insert pos_mod_sign [of m a])
   200   apply (subgoal_tac "a mod m ~= 0")
   201   apply arith
   202   apply auto
   203   apply (subst (asm) gcd_red_int)
   204   apply (subst gcd_commute_int, assumption)
   205   done
   206 
   207 lemma res_eq_to_cong: "((a mod m) = (b mod m)) = [a = b] (mod (m::int))" 
   208   unfolding cong_int_def by auto
   209 
   210 (* Simplifying with these will translate a ring equation in R to a 
   211    congruence. *)
   212 
   213 lemmas res_to_cong_simps = add_cong mult_cong pow_cong one_cong
   214     prod_cong sum_cong neg_cong res_eq_to_cong
   215 
   216 (* Other useful facts about the residue ring *)
   217 
   218 lemma one_eq_neg_one: "\<one> = \<ominus> \<one> \<Longrightarrow> m = 2"
   219   apply (simp add: res_one_eq res_neg_eq)
   220   apply (insert m_gt_one)
   221   apply (subgoal_tac "~(m > 2)")
   222   apply arith
   223   apply (rule notI)
   224   apply (subgoal_tac "-1 mod m = m - 1")
   225   apply force
   226   apply (subst mod_add_self2 [symmetric])
   227   apply (subst mod_pos_pos_trivial)
   228   apply auto
   229   done
   230 
   231 end
   232 
   233 
   234 (* prime residues *)
   235 
   236 locale residues_prime =
   237   fixes p :: int and R (structure)
   238   assumes p_prime [intro]: "prime p"
   239   defines "R == residue_ring p"
   240 
   241 sublocale residues_prime < residues p
   242   apply (unfold R_def residues_def)
   243   using p_prime apply auto
   244   done
   245 
   246 context residues_prime begin
   247 
   248 lemma is_field: "field R"
   249   apply (rule cring.field_intro2)
   250   apply (rule cring)
   251   apply (auto simp add: res_carrier_eq res_one_eq res_zero_eq
   252     res_units_eq)
   253   apply (rule classical)
   254   apply (erule notE)
   255   apply (subst gcd_commute_int)
   256   apply (rule prime_imp_coprime_int)
   257   apply (rule p_prime)
   258   apply (rule notI)
   259   apply (frule zdvd_imp_le)
   260   apply auto
   261   done
   262 
   263 lemma res_prime_units_eq: "Units R = {1..p - 1}"
   264   apply (subst res_units_eq)
   265   apply auto
   266   apply (subst gcd_commute_int)
   267   apply (rule prime_imp_coprime_int)
   268   apply (rule p_prime)
   269   apply (rule zdvd_not_zless)
   270   apply auto
   271   done
   272 
   273 end
   274 
   275 sublocale residues_prime < field
   276   by (rule is_field)
   277 
   278 
   279 (*
   280   Test cases: Euler's theorem and Wilson's theorem.
   281 *)
   282 
   283 
   284 subsection{* Euler's theorem *}
   285 
   286 (* the definition of the phi function *)
   287 
   288 definition phi :: "int => nat" where
   289   "phi m == card({ x. 0 < x & x < m & gcd x m = 1})" 
   290 
   291 lemma phi_zero [simp]: "phi 0 = 0"
   292   apply (subst phi_def)
   293 (* Auto hangs here. Once again, where is the simplification rule 
   294    1 == Suc 0 coming from? *)
   295   apply (auto simp add: card_eq_0_iff)
   296 (* Add card_eq_0_iff as a simp rule? delete card_empty_imp? *)
   297   done
   298 
   299 lemma phi_one [simp]: "phi 1 = 0"
   300   apply (auto simp add: phi_def card_eq_0_iff)
   301   done
   302 
   303 lemma (in residues) phi_eq: "phi m = card(Units R)"
   304   by (simp add: phi_def res_units_eq)
   305 
   306 lemma (in residues) euler_theorem1: 
   307   assumes a: "gcd a m = 1"
   308   shows "[a^phi m = 1] (mod m)"
   309 proof -
   310   from a m_gt_one have [simp]: "a mod m : Units R"
   311     by (intro mod_in_res_units)
   312   from phi_eq have "(a mod m) (^) (phi m) = (a mod m) (^) (card (Units R))"
   313     by simp
   314   also have "\<dots> = \<one>" 
   315     by (intro units_power_order_eq_one, auto)
   316   finally show ?thesis
   317     by (simp add: res_to_cong_simps)
   318 qed
   319 
   320 (* In fact, there is a two line proof!
   321 
   322 lemma (in residues) euler_theorem1: 
   323   assumes a: "gcd a m = 1"
   324   shows "[a^phi m = 1] (mod m)"
   325 proof -
   326   have "(a mod m) (^) (phi m) = \<one>"
   327     by (simp add: phi_eq units_power_order_eq_one a m_gt_one)
   328   thus ?thesis
   329     by (simp add: res_to_cong_simps)
   330 qed
   331 
   332 *)
   333 
   334 (* outside the locale, we can relax the restriction m > 1 *)
   335 
   336 lemma euler_theorem:
   337   assumes "m >= 0" and "gcd a m = 1"
   338   shows "[a^phi m = 1] (mod m)"
   339 proof (cases)
   340   assume "m = 0 | m = 1"
   341   thus ?thesis by auto
   342 next
   343   assume "~(m = 0 | m = 1)"
   344   with assms show ?thesis
   345     by (intro residues.euler_theorem1, unfold residues_def, auto)
   346 qed
   347 
   348 lemma (in residues_prime) phi_prime: "phi p = (nat p - 1)"
   349   apply (subst phi_eq)
   350   apply (subst res_prime_units_eq)
   351   apply auto
   352   done
   353 
   354 lemma phi_prime: "prime p \<Longrightarrow> phi p = (nat p - 1)"
   355   apply (rule residues_prime.phi_prime)
   356   apply (erule residues_prime.intro)
   357   done
   358 
   359 lemma fermat_theorem:
   360   assumes "prime p" and "~ (p dvd a)"
   361   shows "[a^(nat p - 1) = 1] (mod p)"
   362 proof -
   363   from assms have "[a^phi p = 1] (mod p)"
   364     apply (intro euler_theorem)
   365     (* auto should get this next part. matching across
   366        substitutions is needed. *)
   367     apply (frule prime_gt_1_int, arith)
   368     apply (subst gcd_commute_int, erule prime_imp_coprime_int, assumption)
   369     done
   370   also have "phi p = nat p - 1"
   371     by (rule phi_prime, rule assms)
   372   finally show ?thesis .
   373 qed
   374 
   375 
   376 subsection {* Wilson's theorem *}
   377 
   378 lemma (in field) inv_pair_lemma: "x : Units R \<Longrightarrow> y : Units R \<Longrightarrow> 
   379     {x, inv x} ~= {y, inv y} \<Longrightarrow> {x, inv x} Int {y, inv y} = {}" 
   380   apply auto
   381   apply (erule notE)
   382   apply (erule inv_eq_imp_eq)
   383   apply auto
   384   apply (erule notE)
   385   apply (erule inv_eq_imp_eq)
   386   apply auto
   387   done
   388 
   389 lemma (in residues_prime) wilson_theorem1:
   390   assumes a: "p > 2"
   391   shows "[fact (p - 1) = - 1] (mod p)"
   392 proof -
   393   let ?InversePairs = "{ {x, inv x} | x. x : Units R - {\<one>, \<ominus> \<one>}}" 
   394   have UR: "Units R = {\<one>, \<ominus> \<one>} Un (Union ?InversePairs)"
   395     by auto
   396   have "(\<Otimes>i: Units R. i) = 
   397     (\<Otimes>i: {\<one>, \<ominus> \<one>}. i) \<otimes> (\<Otimes>i: Union ?InversePairs. i)"
   398     apply (subst UR)
   399     apply (subst finprod_Un_disjoint)
   400     apply (auto intro:funcsetI)
   401     apply (drule sym, subst (asm) inv_eq_one_eq)
   402     apply auto
   403     apply (drule sym, subst (asm) inv_eq_neg_one_eq)
   404     apply auto
   405     done
   406   also have "(\<Otimes>i: {\<one>, \<ominus> \<one>}. i) = \<ominus> \<one>"
   407     apply (subst finprod_insert)
   408     apply auto
   409     apply (frule one_eq_neg_one)
   410     apply (insert a, force)
   411     done
   412   also have "(\<Otimes>i:(Union ?InversePairs). i) = 
   413       (\<Otimes>A: ?InversePairs. (\<Otimes>y:A. y))"
   414     apply (subst finprod_Union_disjoint)
   415     apply force
   416     apply force
   417     apply clarify
   418     apply (rule inv_pair_lemma)
   419     apply auto
   420     done
   421   also have "\<dots> = \<one>"
   422     apply (rule finprod_one)
   423     apply auto
   424     apply (subst finprod_insert)
   425     apply auto
   426     apply (frule inv_eq_self)
   427     apply (auto)
   428     done
   429   finally have "(\<Otimes>i: Units R. i) = \<ominus> \<one>"
   430     by simp
   431   also have "(\<Otimes>i: Units R. i) = (\<Otimes>i: Units R. i mod p)"
   432     apply (rule finprod_cong')
   433     apply (auto)
   434     apply (subst (asm) res_prime_units_eq)
   435     apply auto
   436     done
   437   also have "\<dots> = (PROD i: Units R. i) mod p"
   438     apply (rule prod_cong)
   439     apply auto
   440     done
   441   also have "\<dots> = fact (p - 1) mod p"
   442     apply (subst fact_altdef_int)
   443     apply (insert assms, force)
   444     apply (subst res_prime_units_eq, rule refl)
   445     done
   446   finally have "fact (p - 1) mod p = \<ominus> \<one>".
   447   thus ?thesis
   448     by (simp add: res_to_cong_simps)
   449 qed
   450 
   451 lemma wilson_theorem: "prime (p::int) \<Longrightarrow> [fact (p - 1) = - 1] (mod p)"
   452   apply (frule prime_gt_1_int)
   453   apply (case_tac "p = 2")
   454   apply (subst fact_altdef_int, simp)
   455   apply (subst cong_int_def)
   456   apply simp
   457   apply (rule residues_prime.wilson_theorem1)
   458   apply (rule residues_prime.intro)
   459   apply auto
   460 done
   461 
   462 
   463 end