src/HOL/Parity.thy
 author haftmann Thu Oct 16 19:26:14 2014 +0200 (2014-10-16) changeset 58688 ddd124805260 parent 58687 5469874b0228 child 58689 ee5bf401cfa7 permissions -rw-r--r--
restructured
```     1 (*  Title:      HOL/Parity.thy
```
```     2     Author:     Jeremy Avigad
```
```     3     Author:     Jacques D. Fleuriot
```
```     4 *)
```
```     5
```
```     6 header {* Even and Odd for int and nat *}
```
```     7
```
```     8 theory Parity
```
```     9 imports Main
```
```    10 begin
```
```    11
```
```    12 subsection {* Preliminaries about divisibility on @{typ nat} and @{typ int} *}
```
```    13
```
```    14 lemma two_dvd_Suc_Suc_iff [simp]:
```
```    15   "2 dvd Suc (Suc n) \<longleftrightarrow> 2 dvd n"
```
```    16   using dvd_add_triv_right_iff [of 2 n] by simp
```
```    17
```
```    18 lemma two_dvd_Suc_iff:
```
```    19   "2 dvd Suc n \<longleftrightarrow> \<not> 2 dvd n"
```
```    20   by (induct n) auto
```
```    21
```
```    22 lemma two_dvd_diff_nat_iff:
```
```    23   fixes m n :: nat
```
```    24   shows "2 dvd m - n \<longleftrightarrow> m < n \<or> 2 dvd m + n"
```
```    25 proof (cases "n \<le> m")
```
```    26   case True
```
```    27   then have "m - n + n * 2 = m + n" by simp
```
```    28   moreover have "2 dvd m - n \<longleftrightarrow> 2 dvd m - n + n * 2" by simp
```
```    29   ultimately have "2 dvd m - n \<longleftrightarrow> 2 dvd m + n" by (simp only:)
```
```    30   then show ?thesis by auto
```
```    31 next
```
```    32   case False
```
```    33   then show ?thesis by simp
```
```    34 qed
```
```    35
```
```    36 lemma two_dvd_diff_iff:
```
```    37   fixes k l :: int
```
```    38   shows "2 dvd k - l \<longleftrightarrow> 2 dvd k + l"
```
```    39   using dvd_add_times_triv_right_iff [of 2 "k - l" l] by (simp add: ac_simps)
```
```    40
```
```    41 lemma two_dvd_abs_add_iff:
```
```    42   fixes k l :: int
```
```    43   shows "2 dvd \<bar>k\<bar> + l \<longleftrightarrow> 2 dvd k + l"
```
```    44   by (cases "k \<ge> 0") (simp_all add: two_dvd_diff_iff ac_simps)
```
```    45
```
```    46 lemma two_dvd_add_abs_iff:
```
```    47   fixes k l :: int
```
```    48   shows "2 dvd k + \<bar>l\<bar> \<longleftrightarrow> 2 dvd k + l"
```
```    49   using two_dvd_abs_add_iff [of l k] by (simp add: ac_simps)
```
```    50
```
```    51
```
```    52 subsection {* Ring structures with parity *}
```
```    53
```
```    54 class semiring_parity = semiring_dvd + semiring_numeral +
```
```    55   assumes two_not_dvd_one [simp]: "\<not> 2 dvd 1"
```
```    56   assumes not_dvd_not_dvd_dvd_add: "\<not> 2 dvd a \<Longrightarrow> \<not> 2 dvd b \<Longrightarrow> 2 dvd a + b"
```
```    57   assumes two_is_prime: "2 dvd a * b \<Longrightarrow> 2 dvd a \<or> 2 dvd b"
```
```    58   assumes not_dvd_ex_decrement: "\<not> 2 dvd a \<Longrightarrow> \<exists>b. a = b + 1"
```
```    59 begin
```
```    60
```
```    61 lemma two_dvd_plus_one_iff [simp]:
```
```    62   "2 dvd a + 1 \<longleftrightarrow> \<not> 2 dvd a"
```
```    63   by (auto simp add: dvd_add_right_iff intro: not_dvd_not_dvd_dvd_add)
```
```    64
```
```    65 lemma not_two_dvdE [elim?]:
```
```    66   assumes "\<not> 2 dvd a"
```
```    67   obtains b where "a = 2 * b + 1"
```
```    68 proof -
```
```    69   from assms obtain b where *: "a = b + 1"
```
```    70     by (blast dest: not_dvd_ex_decrement)
```
```    71   with assms have "2 dvd b + 2" by simp
```
```    72   then have "2 dvd b" by simp
```
```    73   then obtain c where "b = 2 * c" ..
```
```    74   with * have "a = 2 * c + 1" by simp
```
```    75   with that show thesis .
```
```    76 qed
```
```    77
```
```    78 end
```
```    79
```
```    80 instance nat :: semiring_parity
```
```    81 proof
```
```    82   show "\<not> (2 :: nat) dvd 1"
```
```    83     by (rule notI, erule dvdE) simp
```
```    84 next
```
```    85   fix m n :: nat
```
```    86   assume "\<not> 2 dvd m"
```
```    87   moreover assume "\<not> 2 dvd n"
```
```    88   ultimately have *: "2 dvd Suc m \<and> 2 dvd Suc n"
```
```    89     by (simp add: two_dvd_Suc_iff)
```
```    90   then have "2 dvd Suc m + Suc n"
```
```    91     by (blast intro: dvd_add)
```
```    92   also have "Suc m + Suc n = m + n + 2"
```
```    93     by simp
```
```    94   finally show "2 dvd m + n"
```
```    95     using dvd_add_triv_right_iff [of 2 "m + n"] by simp
```
```    96 next
```
```    97   fix m n :: nat
```
```    98   assume *: "2 dvd m * n"
```
```    99   show "2 dvd m \<or> 2 dvd n"
```
```   100   proof (rule disjCI)
```
```   101     assume "\<not> 2 dvd n"
```
```   102     then have "2 dvd Suc n" by (simp add: two_dvd_Suc_iff)
```
```   103     then obtain r where "Suc n = 2 * r" ..
```
```   104     moreover from * obtain s where "m * n = 2 * s" ..
```
```   105     then have "2 * s + m = m * Suc n" by simp
```
```   106     ultimately have " 2 * s + m = 2 * (m * r)" by simp
```
```   107     then have "m = 2 * (m * r - s)" by simp
```
```   108     then show "2 dvd m" ..
```
```   109   qed
```
```   110 next
```
```   111   fix n :: nat
```
```   112   assume "\<not> 2 dvd n"
```
```   113   then show "\<exists>m. n = m + 1"
```
```   114     by (cases n) simp_all
```
```   115 qed
```
```   116
```
```   117 class ring_parity = comm_ring_1 + semiring_parity
```
```   118
```
```   119 instance int :: ring_parity
```
```   120 proof
```
```   121   show "\<not> (2 :: int) dvd 1" by (simp add: dvd_int_unfold_dvd_nat)
```
```   122   fix k l :: int
```
```   123   assume "\<not> 2 dvd k"
```
```   124   moreover assume "\<not> 2 dvd l"
```
```   125   ultimately have "2 dvd nat \<bar>k\<bar> + nat \<bar>l\<bar>"
```
```   126     by (auto simp add: dvd_int_unfold_dvd_nat intro: not_dvd_not_dvd_dvd_add)
```
```   127   then have "2 dvd \<bar>k\<bar> + \<bar>l\<bar>"
```
```   128     by (simp add: dvd_int_unfold_dvd_nat nat_add_distrib)
```
```   129   then show "2 dvd k + l"
```
```   130     by (simp add: two_dvd_abs_add_iff two_dvd_add_abs_iff)
```
```   131 next
```
```   132   fix k l :: int
```
```   133   assume "2 dvd k * l"
```
```   134   then show "2 dvd k \<or> 2 dvd l"
```
```   135     by (simp add: dvd_int_unfold_dvd_nat two_is_prime nat_abs_mult_distrib)
```
```   136 next
```
```   137   fix k :: int
```
```   138   have "k = (k - 1) + 1" by simp
```
```   139   then show "\<exists>l. k = l + 1" ..
```
```   140 qed
```
```   141
```
```   142 context semiring_div_parity
```
```   143 begin
```
```   144
```
```   145 subclass semiring_parity
```
```   146 proof (unfold_locales, unfold dvd_eq_mod_eq_0 not_mod_2_eq_0_eq_1)
```
```   147   fix a b c
```
```   148   show "(c * a + b) mod a = 0 \<longleftrightarrow> b mod a = 0"
```
```   149     by simp
```
```   150 next
```
```   151   fix a b c
```
```   152   assume "(b + c) mod a = 0"
```
```   153   with mod_add_eq [of b c a]
```
```   154   have "(b mod a + c mod a) mod a = 0"
```
```   155     by simp
```
```   156   moreover assume "b mod a = 0"
```
```   157   ultimately show "c mod a = 0"
```
```   158     by simp
```
```   159 next
```
```   160   show "1 mod 2 = 1"
```
```   161     by (fact one_mod_two_eq_one)
```
```   162 next
```
```   163   fix a b
```
```   164   assume "a mod 2 = 1"
```
```   165   moreover assume "b mod 2 = 1"
```
```   166   ultimately show "(a + b) mod 2 = 0"
```
```   167     using mod_add_eq [of a b 2] by simp
```
```   168 next
```
```   169   fix a b
```
```   170   assume "(a * b) mod 2 = 0"
```
```   171   then have "(a mod 2) * (b mod 2) = 0"
```
```   172     by (cases "a mod 2 = 0") (simp_all add: mod_mult_eq [of a b 2])
```
```   173   then show "a mod 2 = 0 \<or> b mod 2 = 0"
```
```   174     by (rule divisors_zero)
```
```   175 next
```
```   176   fix a
```
```   177   assume "a mod 2 = 1"
```
```   178   then have "a = a div 2 * 2 + 1" using mod_div_equality [of a 2] by simp
```
```   179   then show "\<exists>b. a = b + 1" ..
```
```   180 qed
```
```   181
```
```   182 end
```
```   183
```
```   184
```
```   185 subsection {* Dedicated @{text even}/@{text odd} predicate *}
```
```   186
```
```   187 subsubsection {* Properties *}
```
```   188
```
```   189 context semiring_parity
```
```   190 begin
```
```   191
```
```   192 definition even :: "'a \<Rightarrow> bool"
```
```   193 where
```
```   194   [algebra]: "even a \<longleftrightarrow> 2 dvd a"
```
```   195
```
```   196 abbreviation odd :: "'a \<Rightarrow> bool"
```
```   197 where
```
```   198   "odd a \<equiv> \<not> even a"
```
```   199
```
```   200 lemma oddE [elim?]:
```
```   201   assumes "odd a"
```
```   202   obtains b where "a = 2 * b + 1"
```
```   203 proof -
```
```   204   from assms have "\<not> 2 dvd a" by (simp add: even_def)
```
```   205   then show thesis using that by (rule not_two_dvdE)
```
```   206 qed
```
```   207
```
```   208 lemma even_times_iff [simp, presburger, algebra]:
```
```   209   "even (a * b) \<longleftrightarrow> even a \<or> even b"
```
```   210   by (auto simp add: even_def dest: two_is_prime)
```
```   211
```
```   212 lemma even_zero [simp]:
```
```   213   "even 0"
```
```   214   by (simp add: even_def)
```
```   215
```
```   216 lemma odd_one [simp]:
```
```   217   "odd 1"
```
```   218   by (simp add: even_def)
```
```   219
```
```   220 lemma even_numeral [simp]:
```
```   221   "even (numeral (Num.Bit0 n))"
```
```   222 proof -
```
```   223   have "even (2 * numeral n)"
```
```   224     unfolding even_times_iff by (simp add: even_def)
```
```   225   then have "even (numeral n + numeral n)"
```
```   226     unfolding mult_2 .
```
```   227   then show ?thesis
```
```   228     unfolding numeral.simps .
```
```   229 qed
```
```   230
```
```   231 lemma odd_numeral [simp]:
```
```   232   "odd (numeral (Num.Bit1 n))"
```
```   233 proof
```
```   234   assume "even (numeral (num.Bit1 n))"
```
```   235   then have "even (numeral n + numeral n + 1)"
```
```   236     unfolding numeral.simps .
```
```   237   then have "even (2 * numeral n + 1)"
```
```   238     unfolding mult_2 .
```
```   239   then have "2 dvd numeral n * 2 + 1"
```
```   240     unfolding even_def by (simp add: ac_simps)
```
```   241   with dvd_add_times_triv_left_iff [of 2 "numeral n" 1]
```
```   242     have "2 dvd 1"
```
```   243     by simp
```
```   244   then show False by simp
```
```   245 qed
```
```   246
```
```   247 lemma even_add [simp]:
```
```   248   "even (a + b) \<longleftrightarrow> (even a \<longleftrightarrow> even b)"
```
```   249   by (auto simp add: even_def dvd_add_right_iff dvd_add_left_iff not_dvd_not_dvd_dvd_add)
```
```   250
```
```   251 lemma odd_add [simp]:
```
```   252   "odd (a + b) \<longleftrightarrow> (\<not> (odd a \<longleftrightarrow> odd b))"
```
```   253   by simp
```
```   254
```
```   255 lemma even_power [simp, presburger]:
```
```   256   "even (a ^ n) \<longleftrightarrow> even a \<and> n \<noteq> 0"
```
```   257   by (induct n) auto
```
```   258
```
```   259 end
```
```   260
```
```   261 context ring_parity
```
```   262 begin
```
```   263
```
```   264 lemma even_minus [simp, presburger, algebra]:
```
```   265   "even (- a) \<longleftrightarrow> even a"
```
```   266   by (simp add: even_def)
```
```   267
```
```   268 lemma even_diff [simp]:
```
```   269   "even (a - b) \<longleftrightarrow> even (a + b)"
```
```   270   using even_add [of a "- b"] by simp
```
```   271
```
```   272 end
```
```   273
```
```   274 context semiring_div_parity
```
```   275 begin
```
```   276
```
```   277 lemma even_iff_mod_2_eq_zero [presburger]:
```
```   278   "even a \<longleftrightarrow> a mod 2 = 0"
```
```   279   by (simp add: even_def dvd_eq_mod_eq_0)
```
```   280
```
```   281 end
```
```   282
```
```   283
```
```   284 subsubsection {* Particularities for @{typ nat} and @{typ int} *}
```
```   285
```
```   286 lemma even_Suc [simp, presburger, algebra]:
```
```   287   "even (Suc n) = odd n"
```
```   288   by (simp add: even_def two_dvd_Suc_iff)
```
```   289
```
```   290 lemma even_diff_nat [simp]:
```
```   291   fixes m n :: nat
```
```   292   shows "even (m - n) \<longleftrightarrow> m < n \<or> even (m + n)"
```
```   293   by (simp add: even_def two_dvd_diff_nat_iff)
```
```   294
```
```   295 lemma even_int_iff:
```
```   296   "even (int n) \<longleftrightarrow> even n"
```
```   297   by (simp add: even_def dvd_int_iff)
```
```   298
```
```   299 lemma even_nat_iff:
```
```   300   "0 \<le> k \<Longrightarrow> even (nat k) \<longleftrightarrow> even k"
```
```   301   by (simp add: even_int_iff [symmetric])
```
```   302
```
```   303
```
```   304 subsubsection {* Tools setup *}
```
```   305
```
```   306 declare transfer_morphism_int_nat [transfer add return:
```
```   307   even_int_iff
```
```   308 ]
```
```   309
```
```   310 lemma [presburger]:
```
```   311   "even n \<longleftrightarrow> even (int n)"
```
```   312   using even_int_iff [of n] by simp
```
```   313
```
```   314 lemma (in semiring_parity) [presburger]:
```
```   315   "even (a + b) \<longleftrightarrow> even a \<and> even b \<or> odd a \<and> odd b"
```
```   316   by auto
```
```   317
```
```   318 lemma [presburger, algebra]:
```
```   319   fixes m n :: nat
```
```   320   shows "even (m - n) \<longleftrightarrow> m < n \<or> even m \<and> even n \<or> odd m \<and> odd n"
```
```   321   by auto
```
```   322
```
```   323 lemma [presburger, algebra]:
```
```   324   fixes m n :: nat
```
```   325   shows "even (m ^ n) \<longleftrightarrow> even m \<and> 0 < n"
```
```   326   by simp
```
```   327
```
```   328 lemma [presburger]:
```
```   329   fixes k :: int
```
```   330   shows "(k + 1) div 2 = k div 2 \<longleftrightarrow> even k"
```
```   331   by presburger
```
```   332
```
```   333 lemma [presburger]:
```
```   334   fixes k :: int
```
```   335   shows "(k + 1) div 2 = k div 2 + 1 \<longleftrightarrow> odd k"
```
```   336   by presburger
```
```   337
```
```   338 lemma [presburger]:
```
```   339   "Suc n div Suc (Suc 0) = n div Suc (Suc 0) \<longleftrightarrow> even n"
```
```   340   by presburger
```
```   341
```
```   342
```
```   343 subsubsection {* Miscellaneous *}
```
```   344
```
```   345 lemma even_nat_plus_one_div_two:
```
```   346   "even (x::nat) ==> (Suc x) div Suc (Suc 0) = x div Suc (Suc 0)"
```
```   347   by presburger
```
```   348
```
```   349 lemma odd_nat_plus_one_div_two:
```
```   350   "odd (x::nat) ==> (Suc x) div Suc (Suc 0) = Suc (x div Suc (Suc 0))"
```
```   351   by presburger
```
```   352
```
```   353 lemma even_nat_mod_two_eq_zero:
```
```   354   "even (x::nat) ==> x mod Suc (Suc 0) = 0"
```
```   355   by presburger
```
```   356
```
```   357 lemma odd_nat_mod_two_eq_one:
```
```   358   "odd (x::nat) ==> x mod Suc (Suc 0) = Suc 0"
```
```   359   by presburger
```
```   360
```
```   361 lemma even_nat_equiv_def:
```
```   362   "even (x::nat) = (x mod Suc (Suc 0) = 0)"
```
```   363   by presburger
```
```   364
```
```   365 lemma odd_nat_equiv_def:
```
```   366   "odd (x::nat) = (x mod Suc (Suc 0) = Suc 0)"
```
```   367   by presburger
```
```   368
```
```   369 lemma even_nat_div_two_times_two:
```
```   370   "even (x::nat) ==> Suc (Suc 0) * (x div Suc (Suc 0)) = x"
```
```   371   by presburger
```
```   372
```
```   373 lemma odd_nat_div_two_times_two_plus_one:
```
```   374   "odd (x::nat) ==> Suc (Suc (Suc 0) * (x div Suc (Suc 0))) = x"
```
```   375   by presburger
```
```   376
```
```   377 lemma even_mult_two_ex: "even(n) = (\<exists>m::nat. n = 2*m)" by presburger
```
```   378 lemma odd_Suc_mult_two_ex: "odd(n) = (\<exists>m. n = Suc (2*m))" by presburger
```
```   379
```
```   380 lemma lemma_even_div2 [simp]: "even (n::nat) ==> (n + 1) div 2 = n div 2"
```
```   381   by presburger
```
```   382
```
```   383 lemma lemma_odd_div2 [simp]: "odd n ==> (n + 1) div 2 = Suc (n div 2)"
```
```   384   by presburger
```
```   385
```
```   386 lemma even_num_iff: "0 < n ==> even n = (odd (n - 1 :: nat))" by presburger
```
```   387 lemma even_even_mod_4_iff: "even (n::nat) = even (n mod 4)" by presburger
```
```   388
```
```   389 lemma lemma_odd_mod_4_div_2: "n mod 4 = (3::nat) ==> odd((n - 1) div 2)" by presburger
```
```   390
```
```   391 lemma lemma_even_mod_4_div_2: "n mod 4 = (1::nat) ==> even ((n - 1) div 2)"
```
```   392   by presburger
```
```   393
```
```   394
```
```   395 subsubsection {* Parity and powers *}
```
```   396
```
```   397 lemma (in comm_ring_1) neg_power_if:
```
```   398   "(- a) ^ n = (if even n then (a ^ n) else - (a ^ n))"
```
```   399   by (induct n) simp_all
```
```   400
```
```   401 lemma (in comm_ring_1)
```
```   402   shows neg_one_even_power [simp]: "even n \<Longrightarrow> (- 1) ^ n = 1"
```
```   403   and neg_one_odd_power [simp]: "odd n \<Longrightarrow> (- 1) ^ n = - 1"
```
```   404   by (simp_all add: neg_power_if)
```
```   405
```
```   406 lemma zero_le_even_power: "even n ==>
```
```   407     0 <= (x::'a::{linordered_ring,monoid_mult}) ^ n"
```
```   408   apply (simp add: even_def)
```
```   409   apply (erule dvdE)
```
```   410   apply (erule ssubst)
```
```   411   unfolding mult.commute [of 2]
```
```   412   unfolding power_mult power2_eq_square
```
```   413   apply (rule zero_le_square)
```
```   414   done
```
```   415
```
```   416 lemma zero_le_odd_power:
```
```   417   "odd n \<Longrightarrow> 0 \<le> (x::'a::{linordered_idom}) ^ n \<longleftrightarrow> 0 \<le> x"
```
```   418   by (erule oddE) (auto simp: power_add zero_le_mult_iff mult_2 order_antisym_conv)
```
```   419
```
```   420 lemma zero_le_power_eq [presburger]: "(0 <= (x::'a::{linordered_idom}) ^ n) =
```
```   421     (even n | (odd n & 0 <= x))"
```
```   422   apply auto
```
```   423   apply (subst zero_le_odd_power [symmetric])
```
```   424   apply assumption+
```
```   425   apply (erule zero_le_even_power)
```
```   426   done
```
```   427
```
```   428 lemma zero_less_power_eq[presburger]: "(0 < (x::'a::{linordered_idom}) ^ n) =
```
```   429     (n = 0 | (even n & x ~= 0) | (odd n & 0 < x))"
```
```   430   unfolding order_less_le zero_le_power_eq by auto
```
```   431
```
```   432 lemma power_less_zero_eq[presburger]: "((x::'a::{linordered_idom}) ^ n < 0) =
```
```   433     (odd n & x < 0)"
```
```   434   apply (subst linorder_not_le [symmetric])+
```
```   435   apply (subst zero_le_power_eq)
```
```   436   apply auto
```
```   437   done
```
```   438
```
```   439 lemma power_le_zero_eq[presburger]: "((x::'a::{linordered_idom}) ^ n <= 0) =
```
```   440     (n ~= 0 & ((odd n & x <= 0) | (even n & x = 0)))"
```
```   441   apply (subst linorder_not_less [symmetric])+
```
```   442   apply (subst zero_less_power_eq)
```
```   443   apply auto
```
```   444   done
```
```   445
```
```   446 lemma power_even_abs: "even n ==>
```
```   447     (abs (x::'a::{linordered_idom}))^n = x^n"
```
```   448   apply (subst power_abs [symmetric])
```
```   449   apply (simp add: zero_le_even_power)
```
```   450   done
```
```   451
```
```   452 lemma power_minus_even [simp]: "even n ==>
```
```   453     (- x)^n = (x^n::'a::{comm_ring_1})"
```
```   454   apply (subst power_minus)
```
```   455   apply simp
```
```   456   done
```
```   457
```
```   458 lemma power_minus_odd [simp]: "odd n ==>
```
```   459     (- x)^n = - (x^n::'a::{comm_ring_1})"
```
```   460   apply (subst power_minus)
```
```   461   apply simp
```
```   462   done
```
```   463
```
```   464 lemma power_mono_even: fixes x y :: "'a :: {linordered_idom}"
```
```   465   assumes "even n" and "\<bar>x\<bar> \<le> \<bar>y\<bar>"
```
```   466   shows "x^n \<le> y^n"
```
```   467 proof -
```
```   468   have "0 \<le> \<bar>x\<bar>" by auto
```
```   469   with `\<bar>x\<bar> \<le> \<bar>y\<bar>`
```
```   470   have "\<bar>x\<bar>^n \<le> \<bar>y\<bar>^n" by (rule power_mono)
```
```   471   thus ?thesis unfolding power_even_abs[OF `even n`] .
```
```   472 qed
```
```   473
```
```   474 lemma odd_pos: "odd (n::nat) \<Longrightarrow> 0 < n" by presburger
```
```   475
```
```   476 lemma power_mono_odd: fixes x y :: "'a :: {linordered_idom}"
```
```   477   assumes "odd n" and "x \<le> y"
```
```   478   shows "x^n \<le> y^n"
```
```   479 proof (cases "y < 0")
```
```   480   case True with `x \<le> y` have "-y \<le> -x" and "0 \<le> -y" by auto
```
```   481   hence "(-y)^n \<le> (-x)^n" by (rule power_mono)
```
```   482   thus ?thesis unfolding power_minus_odd[OF `odd n`] by auto
```
```   483 next
```
```   484   case False
```
```   485   show ?thesis
```
```   486   proof (cases "x < 0")
```
```   487     case True hence "n \<noteq> 0" and "x \<le> 0" using `odd n`[THEN odd_pos] by auto
```
```   488     hence "x^n \<le> 0" unfolding power_le_zero_eq using `odd n` by auto
```
```   489     moreover
```
```   490     from `\<not> y < 0` have "0 \<le> y" by auto
```
```   491     hence "0 \<le> y^n" by auto
```
```   492     ultimately show ?thesis by auto
```
```   493   next
```
```   494     case False hence "0 \<le> x" by auto
```
```   495     with `x \<le> y` show ?thesis using power_mono by auto
```
```   496   qed
```
```   497 qed
```
```   498
```
```   499 lemma (in linordered_idom) zero_le_power_iff [presburger]:
```
```   500   "0 \<le> a ^ n \<longleftrightarrow> 0 \<le> a \<or> even n"
```
```   501 proof (cases "even n")
```
```   502   case True
```
```   503   then have "2 dvd n" by (simp add: even_def)
```
```   504   then obtain k where "n = 2 * k" ..
```
```   505   thus ?thesis by (simp add: zero_le_even_power True)
```
```   506 next
```
```   507   case False
```
```   508   then obtain k where "n = 2 * k + 1" ..
```
```   509   moreover have "a ^ (2 * k) \<le> 0 \<Longrightarrow> a = 0"
```
```   510     by (induct k) (auto simp add: zero_le_mult_iff mult_le_0_iff)
```
```   511   ultimately show ?thesis
```
```   512     by (auto simp add: zero_le_mult_iff zero_le_even_power)
```
```   513 qed
```
```   514
```
```   515 text {* Simplify, when the exponent is a numeral *}
```
```   516
```
```   517 lemmas zero_le_power_eq_numeral [simp] =
```
```   518   zero_le_power_eq [of _ "numeral w"] for w
```
```   519
```
```   520 lemmas zero_less_power_eq_numeral [simp] =
```
```   521   zero_less_power_eq [of _ "numeral w"] for w
```
```   522
```
```   523 lemmas power_le_zero_eq_numeral [simp] =
```
```   524   power_le_zero_eq [of _ "numeral w"] for w
```
```   525
```
```   526 lemmas power_less_zero_eq_numeral [simp] =
```
```   527   power_less_zero_eq [of _ "numeral w"] for w
```
```   528
```
```   529 lemmas zero_less_power_nat_eq_numeral [simp] =
```
```   530   nat_zero_less_power_iff [of _ "numeral w"] for w
```
```   531
```
```   532 lemmas power_eq_0_iff_numeral [simp] =
```
```   533   power_eq_0_iff [of _ "numeral w"] for w
```
```   534
```
```   535 lemmas power_even_abs_numeral [simp] =
```
```   536   power_even_abs [of "numeral w" _] for w
```
```   537
```
```   538 end
```
```   539
```