src/HOL/Binomial.thy
author wenzelm
Sun Sep 18 20:33:48 2016 +0200 (2016-09-18)
changeset 63915 bab633745c7f
parent 63882 018998c00003
child 63918 6bf55e6e0b75
permissions -rw-r--r--
tuned proofs;
     1 (*  Title:      HOL/Binomial.thy
     2     Author:     Jacques D. Fleuriot
     3     Author:     Lawrence C Paulson
     4     Author:     Jeremy Avigad
     5     Author:     Chaitanya Mangla
     6     Author:     Manuel Eberl
     7 *)
     8 
     9 section \<open>Combinatorial Functions: Factorial Function, Rising Factorials, Binomial Coefficients and Binomial Theorem\<close>
    10 
    11 theory Binomial
    12   imports Main
    13 begin
    14 
    15 subsection \<open>Factorial\<close>
    16 
    17 context semiring_char_0
    18 begin
    19 
    20 definition fact :: "nat \<Rightarrow> 'a"
    21   where fact_setprod: "fact n = of_nat (\<Prod>{1..n})"
    22 
    23 lemma fact_setprod_Suc: "fact n = of_nat (setprod Suc {0..<n})"
    24   by (cases n)
    25     (simp_all add: fact_setprod setprod.atLeast_Suc_atMost_Suc_shift
    26       atLeastLessThanSuc_atLeastAtMost)
    27 
    28 lemma fact_setprod_rev: "fact n = of_nat (\<Prod>i = 0..<n. n - i)"
    29   using setprod.atLeast_atMost_rev [of "\<lambda>i. i" 1 n]
    30   by (cases n)
    31     (simp_all add: fact_setprod_Suc setprod.atLeast_Suc_atMost_Suc_shift
    32       atLeastLessThanSuc_atLeastAtMost)
    33 
    34 lemma fact_0 [simp]: "fact 0 = 1"
    35   by (simp add: fact_setprod)
    36 
    37 lemma fact_1 [simp]: "fact 1 = 1"
    38   by (simp add: fact_setprod)
    39 
    40 lemma fact_Suc_0 [simp]: "fact (Suc 0) = 1"
    41   by (simp add: fact_setprod)
    42 
    43 lemma fact_Suc [simp]: "fact (Suc n) = of_nat (Suc n) * fact n"
    44   by (simp add: fact_setprod atLeastAtMostSuc_conv algebra_simps)
    45 
    46 lemma fact_2 [simp]: "fact 2 = 2"
    47   by (simp add: numeral_2_eq_2)
    48 
    49 lemma fact_split: "k \<le> n \<Longrightarrow> fact n = of_nat (setprod Suc {n - k..<n}) * fact (n - k)"
    50   by (simp add: fact_setprod_Suc setprod.union_disjoint [symmetric]
    51     ivl_disj_un ac_simps of_nat_mult [symmetric])
    52 
    53 end
    54 
    55 lemma of_nat_fact [simp]: "of_nat (fact n) = fact n"
    56   by (simp add: fact_setprod)
    57 
    58 lemma of_int_fact [simp]: "of_int (fact n) = fact n"
    59   by (simp only: fact_setprod of_int_of_nat_eq)
    60 
    61 lemma fact_reduce: "n > 0 \<Longrightarrow> fact n = of_nat n * fact (n - 1)"
    62   by (cases n) auto
    63 
    64 lemma fact_nonzero [simp]: "fact n \<noteq> (0::'a::{semiring_char_0,semiring_no_zero_divisors})"
    65   apply (induct n)
    66   apply auto
    67   using of_nat_eq_0_iff
    68   apply fastforce
    69   done
    70 
    71 lemma fact_mono_nat: "m \<le> n \<Longrightarrow> fact m \<le> (fact n :: nat)"
    72   by (induct n) (auto simp: le_Suc_eq)
    73 
    74 lemma fact_in_Nats: "fact n \<in> \<nat>"
    75   by (induct n) auto
    76 
    77 lemma fact_in_Ints: "fact n \<in> \<int>"
    78   by (induct n) auto
    79 
    80 context
    81   assumes "SORT_CONSTRAINT('a::linordered_semidom)"
    82 begin
    83 
    84 lemma fact_mono: "m \<le> n \<Longrightarrow> fact m \<le> (fact n :: 'a)"
    85   by (metis of_nat_fact of_nat_le_iff fact_mono_nat)
    86 
    87 lemma fact_ge_1 [simp]: "fact n \<ge> (1 :: 'a)"
    88   by (metis le0 fact_0 fact_mono)
    89 
    90 lemma fact_gt_zero [simp]: "fact n > (0 :: 'a)"
    91   using fact_ge_1 less_le_trans zero_less_one by blast
    92 
    93 lemma fact_ge_zero [simp]: "fact n \<ge> (0 :: 'a)"
    94   by (simp add: less_imp_le)
    95 
    96 lemma fact_not_neg [simp]: "\<not> fact n < (0 :: 'a)"
    97   by (simp add: not_less_iff_gr_or_eq)
    98 
    99 lemma fact_le_power: "fact n \<le> (of_nat (n^n) :: 'a)"
   100 proof (induct n)
   101   case 0
   102   then show ?case by simp
   103 next
   104   case (Suc n)
   105   then have *: "fact n \<le> (of_nat (Suc n ^ n) ::'a)"
   106     by (rule order_trans) (simp add: power_mono del: of_nat_power)
   107   have "fact (Suc n) = (of_nat (Suc n) * fact n ::'a)"
   108     by (simp add: algebra_simps)
   109   also have "\<dots> \<le> of_nat (Suc n) * of_nat (Suc n ^ n)"
   110     by (simp add: * ordered_comm_semiring_class.comm_mult_left_mono del: of_nat_power)
   111   also have "\<dots> \<le> of_nat (Suc n ^ Suc n)"
   112     by (metis of_nat_mult order_refl power_Suc)
   113   finally show ?case .
   114 qed
   115 
   116 end
   117 
   118 text \<open>Note that @{term "fact 0 = fact 1"}\<close>
   119 lemma fact_less_mono_nat: "0 < m \<Longrightarrow> m < n \<Longrightarrow> fact m < (fact n :: nat)"
   120   by (induct n) (auto simp: less_Suc_eq)
   121 
   122 lemma fact_less_mono: "0 < m \<Longrightarrow> m < n \<Longrightarrow> fact m < (fact n :: 'a::linordered_semidom)"
   123   by (metis of_nat_fact of_nat_less_iff fact_less_mono_nat)
   124 
   125 lemma fact_ge_Suc_0_nat [simp]: "fact n \<ge> Suc 0"
   126   by (metis One_nat_def fact_ge_1)
   127 
   128 lemma dvd_fact: "1 \<le> m \<Longrightarrow> m \<le> n \<Longrightarrow> m dvd fact n"
   129   by (induct n) (auto simp: dvdI le_Suc_eq)
   130 
   131 lemma fact_ge_self: "fact n \<ge> n"
   132   by (cases "n = 0") (simp_all add: dvd_imp_le dvd_fact)
   133 
   134 lemma fact_dvd: "n \<le> m \<Longrightarrow> fact n dvd (fact m :: 'a::{semiring_div,linordered_semidom})"
   135   by (induct m) (auto simp: le_Suc_eq)
   136 
   137 lemma fact_mod: "m \<le> n \<Longrightarrow> fact n mod (fact m :: 'a::{semiring_div,linordered_semidom}) = 0"
   138   by (auto simp add: fact_dvd)
   139 
   140 lemma fact_div_fact:
   141   assumes "m \<ge> n"
   142   shows "fact m div fact n = \<Prod>{n + 1..m}"
   143 proof -
   144   obtain d where "d = m - n"
   145     by auto
   146   with assms have "m = n + d"
   147     by auto
   148   have "fact (n + d) div (fact n) = \<Prod>{n + 1..n + d}"
   149   proof (induct d)
   150     case 0
   151     show ?case by simp
   152   next
   153     case (Suc d')
   154     have "fact (n + Suc d') div fact n = Suc (n + d') * fact (n + d') div fact n"
   155       by simp
   156     also from Suc.hyps have "\<dots> = Suc (n + d') * \<Prod>{n + 1..n + d'}"
   157       unfolding div_mult1_eq[of _ "fact (n + d')"] by (simp add: fact_mod)
   158     also have "\<dots> = \<Prod>{n + 1..n + Suc d'}"
   159       by (simp add: atLeastAtMostSuc_conv)
   160     finally show ?case .
   161   qed
   162   with \<open>m = n + d\<close> show ?thesis by simp
   163 qed
   164 
   165 lemma fact_num_eq_if: "fact m = (if m = 0 then 1 else of_nat m * fact (m - 1))"
   166   by (cases m) auto
   167 
   168 lemma fact_div_fact_le_pow:
   169   assumes "r \<le> n"
   170   shows "fact n div fact (n - r) \<le> n ^ r"
   171 proof -
   172   have "r \<le> n \<Longrightarrow> \<Prod>{n - r..n} = (n - r) * \<Prod>{Suc (n - r)..n}" for r
   173     by (subst setprod.insert[symmetric]) (auto simp: atLeastAtMost_insertL)
   174   with assms show ?thesis
   175     by (induct r rule: nat.induct) (auto simp add: fact_div_fact Suc_diff_Suc mult_le_mono)
   176 qed
   177 
   178 lemma fact_numeral: "fact (numeral k) = numeral k * fact (pred_numeral k)"
   179   \<comment> \<open>Evaluation for specific numerals\<close>
   180   by (metis fact_Suc numeral_eq_Suc of_nat_numeral)
   181 
   182 
   183 subsection \<open>Binomial coefficients\<close>
   184 
   185 text \<open>This development is based on the work of Andy Gordon and Florian Kammueller.\<close>
   186 
   187 text \<open>Combinatorial definition\<close>
   188 
   189 definition binomial :: "nat \<Rightarrow> nat \<Rightarrow> nat"  (infixl "choose" 65)
   190   where "n choose k = card {K\<in>Pow {0..<n}. card K = k}"
   191 
   192 theorem n_subsets:
   193   assumes "finite A"
   194   shows "card {B. B \<subseteq> A \<and> card B = k} = card A choose k"
   195 proof -
   196   from assms obtain f where bij: "bij_betw f {0..<card A} A"
   197     by (blast dest: ex_bij_betw_nat_finite)
   198   then have [simp]: "card (f ` C) = card C" if "C \<subseteq> {0..<card A}" for C
   199     by (meson bij_betw_imp_inj_on bij_betw_subset card_image that)
   200   from bij have "bij_betw (image f) (Pow {0..<card A}) (Pow A)"
   201     by (rule bij_betw_Pow)
   202   then have "inj_on (image f) (Pow {0..<card A})"
   203     by (rule bij_betw_imp_inj_on)
   204   moreover have "{K. K \<subseteq> {0..<card A} \<and> card K = k} \<subseteq> Pow {0..<card A}"
   205     by auto
   206   ultimately have "inj_on (image f) {K. K \<subseteq> {0..<card A} \<and> card K = k}"
   207     by (rule inj_on_subset)
   208   then have "card {K. K \<subseteq> {0..<card A} \<and> card K = k} =
   209       card (image f ` {K. K \<subseteq> {0..<card A} \<and> card K = k})" (is "_ = card ?C")
   210     by (simp add: card_image)
   211   also have "?C = {K. K \<subseteq> f ` {0..<card A} \<and> card K = k}"
   212     by (auto elim!: subset_imageE)
   213   also have "f ` {0..<card A} = A"
   214     by (meson bij bij_betw_def)
   215   finally show ?thesis
   216     by (simp add: binomial_def)
   217 qed
   218 
   219 text \<open>Recursive characterization\<close>
   220 
   221 lemma binomial_n_0 [simp, code]: "n choose 0 = 1"
   222 proof -
   223   have "{K \<in> Pow {0..<n}. card K = 0} = {{}}"
   224     by (auto dest: finite_subset)
   225   then show ?thesis
   226     by (simp add: binomial_def)
   227 qed
   228 
   229 lemma binomial_0_Suc [simp, code]: "0 choose Suc k = 0"
   230   by (simp add: binomial_def)
   231 
   232 lemma binomial_Suc_Suc [simp, code]: "Suc n choose Suc k = (n choose k) + (n choose Suc k)"
   233 proof -
   234   let ?P = "\<lambda>n k. {K. K \<subseteq> {0..<n} \<and> card K = k}"
   235   let ?Q = "?P (Suc n) (Suc k)"
   236   have inj: "inj_on (insert n) (?P n k)"
   237     by rule (auto; metis atLeastLessThan_iff insert_iff less_irrefl subsetCE)
   238   have disjoint: "insert n ` ?P n k \<inter> ?P n (Suc k) = {}"
   239     by auto
   240   have "?Q = {K\<in>?Q. n \<in> K} \<union> {K\<in>?Q. n \<notin> K}"
   241     by auto
   242   also have "{K\<in>?Q. n \<in> K} = insert n ` ?P n k" (is "?A = ?B")
   243   proof (rule set_eqI)
   244     fix K
   245     have K_finite: "finite K" if "K \<subseteq> insert n {0..<n}"
   246       using that by (rule finite_subset) simp_all
   247     have Suc_card_K: "Suc (card K - Suc 0) = card K" if "n \<in> K"
   248       and "finite K"
   249     proof -
   250       from \<open>n \<in> K\<close> obtain L where "K = insert n L" and "n \<notin> L"
   251         by (blast elim: Set.set_insert)
   252       with that show ?thesis by (simp add: card_insert)
   253     qed
   254     show "K \<in> ?A \<longleftrightarrow> K \<in> ?B"
   255       by (subst in_image_insert_iff)
   256         (auto simp add: card_insert subset_eq_atLeast0_lessThan_finite
   257           Diff_subset_conv K_finite Suc_card_K)
   258   qed
   259   also have "{K\<in>?Q. n \<notin> K} = ?P n (Suc k)"
   260     by (auto simp add: atLeast0_lessThan_Suc)
   261   finally show ?thesis using inj disjoint
   262     by (simp add: binomial_def card_Un_disjoint card_image)
   263 qed
   264 
   265 lemma binomial_eq_0: "n < k \<Longrightarrow> n choose k = 0"
   266   by (auto simp add: binomial_def dest: subset_eq_atLeast0_lessThan_card)
   267 
   268 lemma zero_less_binomial: "k \<le> n \<Longrightarrow> n choose k > 0"
   269   by (induct n k rule: diff_induct) simp_all
   270 
   271 lemma binomial_eq_0_iff [simp]: "n choose k = 0 \<longleftrightarrow> n < k"
   272   by (metis binomial_eq_0 less_numeral_extra(3) not_less zero_less_binomial)
   273 
   274 lemma zero_less_binomial_iff [simp]: "n choose k > 0 \<longleftrightarrow> k \<le> n"
   275   by (metis binomial_eq_0_iff not_less0 not_less zero_less_binomial)
   276 
   277 lemma binomial_n_n [simp]: "n choose n = 1"
   278   by (induct n) (simp_all add: binomial_eq_0)
   279 
   280 lemma binomial_Suc_n [simp]: "Suc n choose n = Suc n"
   281   by (induct n) simp_all
   282 
   283 lemma binomial_1 [simp]: "n choose Suc 0 = n"
   284   by (induct n) simp_all
   285 
   286 lemma choose_reduce_nat:
   287   "0 < n \<Longrightarrow> 0 < k \<Longrightarrow>
   288     n choose k = ((n - 1) choose (k - 1)) + ((n - 1) choose k)"
   289   using binomial_Suc_Suc [of "n - 1" "k - 1"] by simp
   290 
   291 lemma Suc_times_binomial_eq: "Suc n * (n choose k) = (Suc n choose Suc k) * Suc k"
   292   apply (induct n arbitrary: k)
   293    apply simp
   294    apply arith
   295   apply (case_tac k)
   296    apply (auto simp add: add_mult_distrib add_mult_distrib2 le_Suc_eq binomial_eq_0)
   297   done
   298 
   299 lemma binomial_le_pow2: "n choose k \<le> 2^n"
   300   apply (induct n arbitrary: k)
   301    apply (case_tac k)
   302     apply simp_all
   303   apply (case_tac k)
   304    apply auto
   305   apply (simp add: add_le_mono mult_2)
   306   done
   307 
   308 text \<open>The absorption property.\<close>
   309 lemma Suc_times_binomial: "Suc k * (Suc n choose Suc k) = Suc n * (n choose k)"
   310   using Suc_times_binomial_eq by auto
   311 
   312 text \<open>This is the well-known version of absorption, but it's harder to use
   313   because of the need to reason about division.\<close>
   314 lemma binomial_Suc_Suc_eq_times: "(Suc n choose Suc k) = (Suc n * (n choose k)) div Suc k"
   315   by (simp add: Suc_times_binomial_eq del: mult_Suc mult_Suc_right)
   316 
   317 text \<open>Another version of absorption, with \<open>-1\<close> instead of \<open>Suc\<close>.\<close>
   318 lemma times_binomial_minus1_eq: "0 < k \<Longrightarrow> k * (n choose k) = n * ((n - 1) choose (k - 1))"
   319   using Suc_times_binomial_eq [where n = "n - 1" and k = "k - 1"]
   320   by (auto split: nat_diff_split)
   321 
   322 
   323 subsection \<open>The binomial theorem (courtesy of Tobias Nipkow):\<close>
   324 
   325 text \<open>Avigad's version, generalized to any commutative ring\<close>
   326 theorem binomial_ring: "(a + b :: 'a::{comm_ring_1,power})^n =
   327   (\<Sum>k=0..n. (of_nat (n choose k)) * a^k * b^(n-k))"
   328 proof (induct n)
   329   case 0
   330   then show ?case by simp
   331 next
   332   case (Suc n)
   333   have decomp: "{0..n+1} = {0} \<union> {n + 1} \<union> {1..n}"
   334     by auto
   335   have decomp2: "{0..n} = {0} \<union> {1..n}"
   336     by auto
   337   have "(a + b)^(n+1) = (a + b) * (\<Sum>k=0..n. of_nat (n choose k) * a^k * b^(n - k))"
   338     using Suc.hyps by simp
   339   also have "\<dots> = a * (\<Sum>k=0..n. of_nat (n choose k) * a^k * b^(n-k)) +
   340       b * (\<Sum>k=0..n. of_nat (n choose k) * a^k * b^(n-k))"
   341     by (rule distrib_right)
   342   also have "\<dots> = (\<Sum>k=0..n. of_nat (n choose k) * a^(k+1) * b^(n-k)) +
   343       (\<Sum>k=0..n. of_nat (n choose k) * a^k * b^(n - k + 1))"
   344     by (auto simp add: setsum_right_distrib ac_simps)
   345   also have "\<dots> = (\<Sum>k=0..n. of_nat (n choose k) * a^k * b^(n + 1 - k)) +
   346       (\<Sum>k=1..n+1. of_nat (n choose (k - 1)) * a^k * b^(n + 1 - k))"
   347     by (simp add:setsum_shift_bounds_cl_Suc_ivl Suc_diff_le field_simps del: setsum_cl_ivl_Suc)
   348   also have "\<dots> = a^(n + 1) + b^(n + 1) +
   349       (\<Sum>k=1..n. of_nat (n choose (k - 1)) * a^k * b^(n + 1 - k)) +
   350       (\<Sum>k=1..n. of_nat (n choose k) * a^k * b^(n + 1 - k))"
   351     by (simp add: decomp2)
   352   also have "\<dots> = a^(n + 1) + b^(n + 1) +
   353       (\<Sum>k=1..n. of_nat (n + 1 choose k) * a^k * b^(n + 1 - k))"
   354     by (auto simp add: field_simps setsum.distrib [symmetric] choose_reduce_nat)
   355   also have "\<dots> = (\<Sum>k=0..n+1. of_nat (n + 1 choose k) * a^k * b^(n + 1 - k))"
   356     using decomp by (simp add: field_simps)
   357   finally show ?case
   358     by simp
   359 qed
   360 
   361 text \<open>Original version for the naturals.\<close>
   362 corollary binomial: "(a + b :: nat)^n = (\<Sum>k=0..n. (of_nat (n choose k)) * a^k * b^(n - k))"
   363   using binomial_ring [of "int a" "int b" n]
   364   by (simp only: of_nat_add [symmetric] of_nat_mult [symmetric] of_nat_power [symmetric]
   365       of_nat_setsum [symmetric] of_nat_eq_iff of_nat_id)
   366 
   367 lemma binomial_fact_lemma: "k \<le> n \<Longrightarrow> fact k * fact (n - k) * (n choose k) = fact n"
   368 proof (induct n arbitrary: k rule: nat_less_induct)
   369   fix n k
   370   assume H: "\<forall>m<n. \<forall>x\<le>m. fact x * fact (m - x) * (m choose x) = fact m"
   371   assume kn: "k \<le> n"
   372   let ?ths = "fact k * fact (n - k) * (n choose k) = fact n"
   373   consider "n = 0 \<or> k = 0 \<or> n = k" | m h where "n = Suc m" "k = Suc h" "h < m"
   374     using kn by atomize_elim presburger
   375   then show "fact k * fact (n - k) * (n choose k) = fact n"
   376   proof cases
   377     case 1
   378     with kn show ?thesis by auto
   379   next
   380     case 2
   381     note n = \<open>n = Suc m\<close>
   382     note k = \<open>k = Suc h\<close>
   383     note hm = \<open>h < m\<close>
   384     have mn: "m < n"
   385       using n by arith
   386     have hm': "h \<le> m"
   387       using hm by arith
   388     have km: "k \<le> m"
   389       using hm k n kn by arith
   390     have "m - h = Suc (m - Suc h)"
   391       using  k km hm by arith
   392     with km k have "fact (m - h) = (m - h) * fact (m - k)"
   393       by simp
   394     with n k have "fact k * fact (n - k) * (n choose k) =
   395         k * (fact h * fact (m - h) * (m choose h)) +
   396         (m - h) * (fact k * fact (m - k) * (m choose k))"
   397       by (simp add: field_simps)
   398     also have "\<dots> = (k + (m - h)) * fact m"
   399       using H[rule_format, OF mn hm'] H[rule_format, OF mn km]
   400       by (simp add: field_simps)
   401     finally show ?thesis
   402       using k n km by simp
   403   qed
   404 qed
   405 
   406 lemma binomial_fact':
   407   assumes "k \<le> n"
   408   shows "n choose k = fact n div (fact k * fact (n - k))"
   409   using binomial_fact_lemma [OF assms]
   410   by (metis fact_nonzero mult_eq_0_iff nonzero_mult_divide_cancel_left)
   411 
   412 lemma binomial_fact:
   413   assumes kn: "k \<le> n"
   414   shows "(of_nat (n choose k) :: 'a::field_char_0) = fact n / (fact k * fact (n - k))"
   415   using binomial_fact_lemma[OF kn]
   416   apply (simp add: field_simps)
   417   apply (metis mult.commute of_nat_fact of_nat_mult)
   418   done
   419 
   420 lemma fact_binomial:
   421   assumes "k \<le> n"
   422   shows "fact k * of_nat (n choose k) = (fact n / fact (n - k) :: 'a::field_char_0)"
   423   unfolding binomial_fact [OF assms] by (simp add: field_simps)
   424 
   425 lemma choose_two: "n choose 2 = n * (n - 1) div 2"
   426 proof (cases "n \<ge> 2")
   427   case False
   428   then have "n = 0 \<or> n = 1"
   429     by auto
   430   then show ?thesis by auto
   431 next
   432   case True
   433   define m where "m = n - 2"
   434   with True have "n = m + 2"
   435     by simp
   436   then have "fact n = n * (n - 1) * fact (n - 2)"
   437     by (simp add: fact_setprod_Suc atLeast0_lessThan_Suc algebra_simps)
   438   with True show ?thesis
   439     by (simp add: binomial_fact')
   440 qed
   441 
   442 lemma choose_row_sum: "(\<Sum>k=0..n. n choose k) = 2^n"
   443   using binomial [of 1 "1" n] by (simp add: numeral_2_eq_2)
   444 
   445 lemma sum_choose_lower: "(\<Sum>k=0..n. (r+k) choose k) = Suc (r+n) choose n"
   446   by (induct n) auto
   447 
   448 lemma sum_choose_upper: "(\<Sum>k=0..n. k choose m) = Suc n choose Suc m"
   449   by (induct n) auto
   450 
   451 lemma choose_alternating_sum:
   452   "n > 0 \<Longrightarrow> (\<Sum>i\<le>n. (-1)^i * of_nat (n choose i)) = (0 :: 'a::comm_ring_1)"
   453   using binomial_ring[of "-1 :: 'a" 1 n]
   454   by (simp add: atLeast0AtMost mult_of_nat_commute zero_power)
   455 
   456 lemma choose_even_sum:
   457   assumes "n > 0"
   458   shows "2 * (\<Sum>i\<le>n. if even i then of_nat (n choose i) else 0) = (2 ^ n :: 'a::comm_ring_1)"
   459 proof -
   460   have "2 ^ n = (\<Sum>i\<le>n. of_nat (n choose i)) + (\<Sum>i\<le>n. (-1) ^ i * of_nat (n choose i) :: 'a)"
   461     using choose_row_sum[of n]
   462     by (simp add: choose_alternating_sum assms atLeast0AtMost of_nat_setsum[symmetric])
   463   also have "\<dots> = (\<Sum>i\<le>n. of_nat (n choose i) + (-1) ^ i * of_nat (n choose i))"
   464     by (simp add: setsum.distrib)
   465   also have "\<dots> = 2 * (\<Sum>i\<le>n. if even i then of_nat (n choose i) else 0)"
   466     by (subst setsum_right_distrib, intro setsum.cong) simp_all
   467   finally show ?thesis ..
   468 qed
   469 
   470 lemma choose_odd_sum:
   471   assumes "n > 0"
   472   shows "2 * (\<Sum>i\<le>n. if odd i then of_nat (n choose i) else 0) = (2 ^ n :: 'a::comm_ring_1)"
   473 proof -
   474   have "2 ^ n = (\<Sum>i\<le>n. of_nat (n choose i)) - (\<Sum>i\<le>n. (-1) ^ i * of_nat (n choose i) :: 'a)"
   475     using choose_row_sum[of n]
   476     by (simp add: choose_alternating_sum assms atLeast0AtMost of_nat_setsum[symmetric])
   477   also have "\<dots> = (\<Sum>i\<le>n. of_nat (n choose i) - (-1) ^ i * of_nat (n choose i))"
   478     by (simp add: setsum_subtractf)
   479   also have "\<dots> = 2 * (\<Sum>i\<le>n. if odd i then of_nat (n choose i) else 0)"
   480     by (subst setsum_right_distrib, intro setsum.cong) simp_all
   481   finally show ?thesis ..
   482 qed
   483 
   484 lemma choose_row_sum': "(\<Sum>k\<le>n. (n choose k)) = 2 ^ n"
   485   using choose_row_sum[of n] by (simp add: atLeast0AtMost)
   486 
   487 text\<open>NW diagonal sum property\<close>
   488 lemma sum_choose_diagonal:
   489   assumes "m \<le> n"
   490   shows "(\<Sum>k=0..m. (n - k) choose (m - k)) = Suc n choose m"
   491 proof -
   492   have "(\<Sum>k=0..m. (n-k) choose (m - k)) = (\<Sum>k=0..m. (n - m + k) choose k)"
   493     using setsum.atLeast_atMost_rev [of "\<lambda>k. (n - k) choose (m - k)" 0 m] assms
   494       by simp
   495   also have "\<dots> = Suc (n - m + m) choose m"
   496     by (rule sum_choose_lower)
   497   also have "\<dots> = Suc n choose m"
   498     using assms by simp
   499   finally show ?thesis .
   500 qed
   501 
   502 
   503 subsection \<open>Pochhammer's symbol: generalized rising factorial\<close>
   504 
   505 text \<open>See \<^url>\<open>http://en.wikipedia.org/wiki/Pochhammer_symbol\<close>.\<close>
   506 
   507 context comm_semiring_1
   508 begin
   509 
   510 definition pochhammer :: "'a \<Rightarrow> nat \<Rightarrow> 'a"
   511   where pochhammer_setprod: "pochhammer a n = setprod (\<lambda>i. a + of_nat i) {0..<n}"
   512 
   513 lemma pochhammer_setprod_rev: "pochhammer a n = setprod (\<lambda>i. a + of_nat (n - i)) {1..n}"
   514   using setprod.atLeast_lessThan_rev_at_least_Suc_atMost [of "\<lambda>i. a + of_nat i" 0 n]
   515   by (simp add: pochhammer_setprod)
   516 
   517 lemma pochhammer_Suc_setprod: "pochhammer a (Suc n) = setprod (\<lambda>i. a + of_nat i) {0..n}"
   518   by (simp add: pochhammer_setprod atLeastLessThanSuc_atLeastAtMost)
   519 
   520 lemma pochhammer_Suc_setprod_rev: "pochhammer a (Suc n) = setprod (\<lambda>i. a + of_nat (n - i)) {0..n}"
   521   by (simp add: pochhammer_setprod_rev setprod.atLeast_Suc_atMost_Suc_shift)
   522 
   523 lemma pochhammer_0 [simp]: "pochhammer a 0 = 1"
   524   by (simp add: pochhammer_setprod)
   525 
   526 lemma pochhammer_1 [simp]: "pochhammer a 1 = a"
   527   by (simp add: pochhammer_setprod lessThan_Suc)
   528 
   529 lemma pochhammer_Suc0 [simp]: "pochhammer a (Suc 0) = a"
   530   by (simp add: pochhammer_setprod lessThan_Suc)
   531 
   532 lemma pochhammer_Suc: "pochhammer a (Suc n) = pochhammer a n * (a + of_nat n)"
   533   by (simp add: pochhammer_setprod atLeast0_lessThan_Suc ac_simps)
   534 
   535 end
   536 
   537 lemma pochhammer_nonneg: 
   538   fixes x :: "'a :: linordered_semidom"
   539   shows "x > 0 \<Longrightarrow> pochhammer x n \<ge> 0"
   540   by (induction n) (auto simp: pochhammer_Suc intro!: mult_nonneg_nonneg add_nonneg_nonneg)
   541 
   542 lemma pochhammer_pos: 
   543   fixes x :: "'a :: linordered_semidom"
   544   shows "x > 0 \<Longrightarrow> pochhammer x n > 0"
   545   by (induction n) (auto simp: pochhammer_Suc intro!: mult_pos_pos add_pos_nonneg)
   546 
   547 lemma pochhammer_of_nat: "pochhammer (of_nat x) n = of_nat (pochhammer x n)"
   548   by (simp add: pochhammer_setprod)
   549 
   550 lemma pochhammer_of_int: "pochhammer (of_int x) n = of_int (pochhammer x n)"
   551   by (simp add: pochhammer_setprod)
   552 
   553 lemma pochhammer_rec: "pochhammer a (Suc n) = a * pochhammer (a + 1) n"
   554   by (simp add: pochhammer_setprod setprod.atLeast0_lessThan_Suc_shift ac_simps)
   555 
   556 lemma pochhammer_rec': "pochhammer z (Suc n) = (z + of_nat n) * pochhammer z n"
   557   by (simp add: pochhammer_setprod setprod.atLeast0_lessThan_Suc ac_simps)
   558 
   559 lemma pochhammer_fact: "fact n = pochhammer 1 n"
   560   by (simp add: pochhammer_setprod fact_setprod_Suc)
   561 
   562 lemma pochhammer_of_nat_eq_0_lemma: "k > n \<Longrightarrow> pochhammer (- (of_nat n :: 'a:: idom)) k = 0"
   563   by (auto simp add: pochhammer_setprod)
   564 
   565 lemma pochhammer_of_nat_eq_0_lemma':
   566   assumes kn: "k \<le> n"
   567   shows "pochhammer (- (of_nat n :: 'a::{idom,ring_char_0})) k \<noteq> 0"
   568 proof (cases k)
   569   case 0
   570   then show ?thesis by simp
   571 next
   572   case (Suc h)
   573   then show ?thesis
   574     apply (simp add: pochhammer_Suc_setprod)
   575     using Suc kn
   576     apply (auto simp add: algebra_simps)
   577     done
   578 qed
   579 
   580 lemma pochhammer_of_nat_eq_0_iff:
   581   "pochhammer (- (of_nat n :: 'a::{idom,ring_char_0})) k = 0 \<longleftrightarrow> k > n"
   582   (is "?l = ?r")
   583   using pochhammer_of_nat_eq_0_lemma[of n k, where ?'a='a]
   584     pochhammer_of_nat_eq_0_lemma'[of k n, where ?'a = 'a]
   585   by (auto simp add: not_le[symmetric])
   586 
   587 lemma pochhammer_eq_0_iff: "pochhammer a n = (0::'a::field_char_0) \<longleftrightarrow> (\<exists>k < n. a = - of_nat k)"
   588   by (auto simp add: pochhammer_setprod eq_neg_iff_add_eq_0)
   589 
   590 lemma pochhammer_eq_0_mono:
   591   "pochhammer a n = (0::'a::field_char_0) \<Longrightarrow> m \<ge> n \<Longrightarrow> pochhammer a m = 0"
   592   unfolding pochhammer_eq_0_iff by auto
   593 
   594 lemma pochhammer_neq_0_mono:
   595   "pochhammer a m \<noteq> (0::'a::field_char_0) \<Longrightarrow> m \<ge> n \<Longrightarrow> pochhammer a n \<noteq> 0"
   596   unfolding pochhammer_eq_0_iff by auto
   597 
   598 lemma pochhammer_minus:
   599   "pochhammer (- b) k = ((- 1) ^ k :: 'a::comm_ring_1) * pochhammer (b - of_nat k + 1) k"
   600 proof (cases k)
   601   case 0
   602   then show ?thesis by simp
   603 next
   604   case (Suc h)
   605   have eq: "((- 1) ^ Suc h :: 'a) = (\<Prod>i = 0..h. - 1)"
   606     using setprod_constant [where A="{0.. h}" and y="- 1 :: 'a"]
   607     by auto
   608   with Suc show ?thesis
   609     using pochhammer_Suc_setprod_rev [of "b - of_nat k + 1"]
   610     by (auto simp add: pochhammer_Suc_setprod setprod.distrib [symmetric] eq of_nat_diff)
   611 qed
   612 
   613 lemma pochhammer_minus':
   614   "pochhammer (b - of_nat k + 1) k = ((- 1) ^ k :: 'a::comm_ring_1) * pochhammer (- b) k"
   615   apply (simp only: pochhammer_minus [where b = b])
   616   apply (simp only: mult.assoc [symmetric])
   617   apply (simp only: power_add [symmetric])
   618   apply simp
   619   done
   620 
   621 lemma pochhammer_same: "pochhammer (- of_nat n) n =
   622     ((- 1) ^ n :: 'a::{semiring_char_0,comm_ring_1,semiring_no_zero_divisors}) * fact n"
   623   unfolding pochhammer_minus
   624   by (simp add: of_nat_diff pochhammer_fact)
   625 
   626 lemma pochhammer_product': "pochhammer z (n + m) = pochhammer z n * pochhammer (z + of_nat n) m"
   627 proof (induct n arbitrary: z)
   628   case 0
   629   then show ?case by simp
   630 next
   631   case (Suc n z)
   632   have "pochhammer z (Suc n) * pochhammer (z + of_nat (Suc n)) m =
   633       z * (pochhammer (z + 1) n * pochhammer (z + 1 + of_nat n) m)"
   634     by (simp add: pochhammer_rec ac_simps)
   635   also note Suc[symmetric]
   636   also have "z * pochhammer (z + 1) (n + m) = pochhammer z (Suc (n + m))"
   637     by (subst pochhammer_rec) simp
   638   finally show ?case
   639     by simp
   640 qed
   641 
   642 lemma pochhammer_product:
   643   "m \<le> n \<Longrightarrow> pochhammer z n = pochhammer z m * pochhammer (z + of_nat m) (n - m)"
   644   using pochhammer_product'[of z m "n - m"] by simp
   645 
   646 lemma pochhammer_times_pochhammer_half:
   647   fixes z :: "'a::field_char_0"
   648   shows "pochhammer z (Suc n) * pochhammer (z + 1/2) (Suc n) = (\<Prod>k=0..2*n+1. z + of_nat k / 2)"
   649 proof (induct n)
   650   case 0
   651   then show ?case
   652     by (simp add: atLeast0_atMost_Suc)
   653 next
   654   case (Suc n)
   655   define n' where "n' = Suc n"
   656   have "pochhammer z (Suc n') * pochhammer (z + 1 / 2) (Suc n') =
   657       (pochhammer z n' * pochhammer (z + 1 / 2) n') * ((z + of_nat n') * (z + 1/2 + of_nat n'))"
   658     (is "_ = _ * ?A")
   659     by (simp_all add: pochhammer_rec' mult_ac)
   660   also have "?A = (z + of_nat (Suc (2 * n + 1)) / 2) * (z + of_nat (Suc (Suc (2 * n + 1))) / 2)"
   661     (is "_ = ?B")
   662     by (simp add: field_simps n'_def)
   663   also note Suc[folded n'_def]
   664   also have "(\<Prod>k=0..2 * n + 1. z + of_nat k / 2) * ?B = (\<Prod>k=0..2 * Suc n + 1. z + of_nat k / 2)"
   665     by (simp add: atLeast0_atMost_Suc)
   666   finally show ?case
   667     by (simp add: n'_def)
   668 qed
   669 
   670 lemma pochhammer_double:
   671   fixes z :: "'a::field_char_0"
   672   shows "pochhammer (2 * z) (2 * n) = of_nat (2^(2*n)) * pochhammer z n * pochhammer (z+1/2) n"
   673 proof (induct n)
   674   case 0
   675   then show ?case by simp
   676 next
   677   case (Suc n)
   678   have "pochhammer (2 * z) (2 * (Suc n)) = pochhammer (2 * z) (2 * n) *
   679       (2 * (z + of_nat n)) * (2 * (z + of_nat n) + 1)"
   680     by (simp add: pochhammer_rec' ac_simps)
   681   also note Suc
   682   also have "of_nat (2 ^ (2 * n)) * pochhammer z n * pochhammer (z + 1/2) n *
   683         (2 * (z + of_nat n)) * (2 * (z + of_nat n) + 1) =
   684       of_nat (2 ^ (2 * (Suc n))) * pochhammer z (Suc n) * pochhammer (z + 1/2) (Suc n)"
   685     by (simp add: field_simps pochhammer_rec')
   686   finally show ?case .
   687 qed
   688 
   689 lemma fact_double:
   690   "fact (2 * n) = (2 ^ (2 * n) * pochhammer (1 / 2) n * fact n :: 'a::field_char_0)"
   691   using pochhammer_double[of "1/2::'a" n] by (simp add: pochhammer_fact)
   692 
   693 lemma pochhammer_absorb_comp: "(r - of_nat k) * pochhammer (- r) k = r * pochhammer (-r + 1) k"
   694   (is "?lhs = ?rhs")
   695   for r :: "'a::comm_ring_1"
   696 proof -
   697   have "?lhs = - pochhammer (- r) (Suc k)"
   698     by (subst pochhammer_rec') (simp add: algebra_simps)
   699   also have "\<dots> = ?rhs"
   700     by (subst pochhammer_rec) simp
   701   finally show ?thesis .
   702 qed
   703 
   704 
   705 subsection \<open>Generalized binomial coefficients\<close>
   706 
   707 definition gbinomial :: "'a::{semidom_divide,semiring_char_0} \<Rightarrow> nat \<Rightarrow> 'a"  (infixl "gchoose" 65)
   708   where gbinomial_setprod_rev: "a gchoose n = setprod (\<lambda>i. a - of_nat i) {0..<n} div fact n"
   709 
   710 lemma gbinomial_0 [simp]:
   711   "a gchoose 0 = 1"
   712   "0 gchoose (Suc n) = 0"
   713   by (simp_all add: gbinomial_setprod_rev setprod.atLeast0_lessThan_Suc_shift)
   714 
   715 lemma gbinomial_Suc: "a gchoose (Suc k) = setprod (\<lambda>i. a - of_nat i) {0..k} div fact (Suc k)"
   716   by (simp add: gbinomial_setprod_rev atLeastLessThanSuc_atLeastAtMost)
   717 
   718 lemma gbinomial_mult_fact: "fact n * (a gchoose n) = (\<Prod>i = 0..<n. a - of_nat i)"
   719   for a :: "'a::field_char_0"
   720   by (simp_all add: gbinomial_setprod_rev field_simps)
   721 
   722 lemma gbinomial_mult_fact': "(a gchoose n) * fact n = (\<Prod>i = 0..<n. a - of_nat i)"
   723   for a :: "'a::field_char_0"
   724   using gbinomial_mult_fact [of n a] by (simp add: ac_simps)
   725 
   726 lemma gbinomial_pochhammer: "a gchoose n = (- 1) ^ n * pochhammer (- a) n / fact n"
   727   for a :: "'a::field_char_0"
   728   by (cases n)
   729     (simp_all add: pochhammer_minus,
   730      simp_all add: gbinomial_setprod_rev pochhammer_setprod_rev
   731        power_mult_distrib [symmetric] atLeastLessThanSuc_atLeastAtMost
   732        setprod.atLeast_Suc_atMost_Suc_shift of_nat_diff)
   733 
   734 lemma gbinomial_pochhammer': "s gchoose n = pochhammer (s - of_nat n + 1) n / fact n"
   735   for s :: "'a::field_char_0"
   736 proof -
   737   have "s gchoose n = ((-1)^n * (-1)^n) * pochhammer (s - of_nat n + 1) n / fact n"
   738     by (simp add: gbinomial_pochhammer pochhammer_minus mult_ac)
   739   also have "(-1 :: 'a)^n * (-1)^n = 1"
   740     by (subst power_add [symmetric]) simp
   741   finally show ?thesis
   742     by simp
   743 qed
   744 
   745 lemma gbinomial_binomial: "n gchoose k = n choose k"
   746 proof (cases "k \<le> n")
   747   case False
   748   then have "n < k"
   749     by (simp add: not_le)
   750   then have "0 \<in> (op - n) ` {0..<k}"
   751     by auto
   752   then have "setprod (op - n) {0..<k} = 0"
   753     by (auto intro: setprod_zero)
   754   with \<open>n < k\<close> show ?thesis
   755     by (simp add: binomial_eq_0 gbinomial_setprod_rev setprod_zero)
   756 next
   757   case True
   758   then have "inj_on (op - n) {0..<k}"
   759     by (auto intro: inj_onI)
   760   then have "\<Prod>(op - n ` {0..<k}) = setprod (op - n) {0..<k}"
   761     by (auto dest: setprod.reindex)
   762   also have "op - n ` {0..<k} = {Suc (n - k)..n}"
   763     using True by (auto simp add: image_def Bex_def) presburger  (* FIXME slow *)
   764   finally have *: "setprod (\<lambda>q. n - q) {0..<k} = \<Prod>{Suc (n - k)..n}" ..
   765   from True have "n choose k = fact n div (fact k * fact (n - k))"
   766     by (rule binomial_fact')
   767   with * show ?thesis
   768     by (simp add: gbinomial_setprod_rev mult.commute [of "fact k"] div_mult2_eq fact_div_fact)
   769 qed
   770 
   771 lemma of_nat_gbinomial: "of_nat (n gchoose k) = (of_nat n gchoose k :: 'a::field_char_0)"
   772 proof (cases "k \<le> n")
   773   case False
   774   then show ?thesis
   775     by (simp add: not_le gbinomial_binomial binomial_eq_0 gbinomial_setprod_rev)
   776 next
   777   case True
   778   define m where "m = n - k"
   779   with True have n: "n = m + k"
   780     by arith
   781   from n have "fact n = ((\<Prod>i = 0..<m + k. of_nat (m + k - i) ):: 'a)"
   782     by (simp add: fact_setprod_rev)
   783   also have "\<dots> = ((\<Prod>i\<in>{0..<k} \<union> {k..<m + k}. of_nat (m + k - i)) :: 'a)"
   784     by (simp add: ivl_disj_un)
   785   finally have "fact n = (fact m * (\<Prod>i = 0..<k. of_nat m + of_nat k - of_nat i) :: 'a)"
   786     using setprod_shift_bounds_nat_ivl [of "\<lambda>i. of_nat (m + k - i) :: 'a" 0 k m]
   787     by (simp add: fact_setprod_rev [of m] setprod.union_disjoint of_nat_diff)
   788   then have "fact n / fact (n - k) = ((\<Prod>i = 0..<k. of_nat n - of_nat i) :: 'a)"
   789     by (simp add: n)
   790   with True have "fact k * of_nat (n gchoose k) = (fact k * (of_nat n gchoose k) :: 'a)"
   791     by (simp only: gbinomial_mult_fact [of k "of_nat n"] gbinomial_binomial [of n k] fact_binomial)
   792   then show ?thesis
   793     by simp
   794 qed
   795 
   796 lemma binomial_gbinomial: "of_nat (n choose k) = (of_nat n gchoose k :: 'a::field_char_0)"
   797   by (simp add: gbinomial_binomial [symmetric] of_nat_gbinomial)
   798 
   799 setup
   800   \<open>Sign.add_const_constraint (@{const_name gbinomial}, SOME @{typ "'a::field_char_0 \<Rightarrow> nat \<Rightarrow> 'a"})\<close>
   801 
   802 lemma gbinomial_1[simp]: "a gchoose 1 = a"
   803   by (simp add: gbinomial_setprod_rev lessThan_Suc)
   804 
   805 lemma gbinomial_Suc0[simp]: "a gchoose (Suc 0) = a"
   806   by (simp add: gbinomial_setprod_rev lessThan_Suc)
   807 
   808 lemma gbinomial_mult_1:
   809   fixes a :: "'a::field_char_0"
   810   shows "a * (a gchoose n) = of_nat n * (a gchoose n) + of_nat (Suc n) * (a gchoose (Suc n))"
   811   (is "?l = ?r")
   812 proof -
   813   have "?r = ((- 1) ^n * pochhammer (- a) n / (fact n)) * (of_nat n - (- a + of_nat n))"
   814     apply (simp only: gbinomial_pochhammer pochhammer_Suc right_diff_distrib power_Suc)
   815     apply (simp del: of_nat_Suc fact_Suc)
   816     apply (auto simp add: field_simps simp del: of_nat_Suc)
   817     done
   818   also have "\<dots> = ?l"
   819     by (simp add: field_simps gbinomial_pochhammer)
   820   finally show ?thesis ..
   821 qed
   822 
   823 lemma gbinomial_mult_1':
   824   "(a gchoose n) * a = of_nat n * (a gchoose n) + of_nat (Suc n) * (a gchoose (Suc n))"
   825   for a :: "'a::field_char_0"
   826   by (simp add: mult.commute gbinomial_mult_1)
   827 
   828 lemma gbinomial_Suc_Suc: "(a + 1) gchoose (Suc k) = a gchoose k + (a gchoose (Suc k))"
   829   for a :: "'a::field_char_0"
   830 proof (cases k)
   831   case 0
   832   then show ?thesis by simp
   833 next
   834   case (Suc h)
   835   have eq0: "(\<Prod>i\<in>{1..k}. (a + 1) - of_nat i) = (\<Prod>i\<in>{0..h}. a - of_nat i)"
   836     apply (rule setprod.reindex_cong [where l = Suc])
   837       using Suc
   838       apply (auto simp add: image_Suc_atMost)
   839     done
   840   have "fact (Suc k) * (a gchoose k + (a gchoose (Suc k))) =
   841       (a gchoose Suc h) * (fact (Suc (Suc h))) +
   842       (a gchoose Suc (Suc h)) * (fact (Suc (Suc h)))"
   843     by (simp add: Suc field_simps del: fact_Suc)
   844   also have "\<dots> =
   845     (a gchoose Suc h) * of_nat (Suc (Suc h) * fact (Suc h)) + (\<Prod>i=0..Suc h. a - of_nat i)"
   846     apply (simp del: fact_Suc add: gbinomial_mult_fact field_simps mult.left_commute [of _ "2"])
   847     apply (simp del: fact_Suc add: fact_Suc [of "Suc h"] field_simps gbinomial_mult_fact
   848       mult.left_commute [of _ "2"] atLeastLessThanSuc_atLeastAtMost)
   849     done
   850   also have "\<dots> =
   851     (fact (Suc h) * (a gchoose Suc h)) * of_nat (Suc (Suc h)) + (\<Prod>i=0..Suc h. a - of_nat i)"
   852     by (simp only: fact_Suc mult.commute mult.left_commute of_nat_fact of_nat_id of_nat_mult)
   853   also have "\<dots> =
   854     of_nat (Suc (Suc h)) * (\<Prod>i=0..h. a - of_nat i) + (\<Prod>i=0..Suc h. a - of_nat i)"
   855     unfolding gbinomial_mult_fact atLeastLessThanSuc_atLeastAtMost by auto
   856   also have "\<dots> =
   857     (\<Prod>i=0..Suc h. a - of_nat i) + (of_nat h * (\<Prod>i=0..h. a - of_nat i) + 2 * (\<Prod>i=0..h. a - of_nat i))"
   858     by (simp add: field_simps)
   859   also have "\<dots> =
   860     ((a gchoose Suc h) * (fact (Suc h)) * of_nat (Suc k)) + (\<Prod>i\<in>{0..Suc h}. a - of_nat i)"
   861     unfolding gbinomial_mult_fact'
   862     by (simp add: comm_semiring_class.distrib field_simps Suc atLeastLessThanSuc_atLeastAtMost)
   863   also have "\<dots> = (\<Prod>i\<in>{0..h}. a - of_nat i) * (a + 1)"
   864     unfolding gbinomial_mult_fact' atLeast0_atMost_Suc
   865     by (simp add: field_simps Suc atLeastLessThanSuc_atLeastAtMost)
   866   also have "\<dots> = (\<Prod>i\<in>{0..k}. (a + 1) - of_nat i)"
   867     using eq0
   868     by (simp add: Suc setprod.atLeast0_atMost_Suc_shift)
   869   also have "\<dots> = (fact (Suc k)) * ((a + 1) gchoose (Suc k))"
   870     by (simp only: gbinomial_mult_fact atLeastLessThanSuc_atLeastAtMost)
   871   finally show ?thesis
   872     using fact_nonzero [of "Suc k"] by auto
   873 qed
   874 
   875 lemma gbinomial_reduce_nat: "0 < k \<Longrightarrow> a gchoose k = (a - 1) gchoose (k - 1) + ((a - 1) gchoose k)"
   876   for a :: "'a::field_char_0"
   877   by (metis Suc_pred' diff_add_cancel gbinomial_Suc_Suc)
   878 
   879 lemma gchoose_row_sum_weighted:
   880   "(\<Sum>k = 0..m. (r gchoose k) * (r/2 - of_nat k)) = of_nat(Suc m) / 2 * (r gchoose (Suc m))"
   881   for r :: "'a::field_char_0"
   882   by (induct m) (simp_all add: field_simps distrib gbinomial_mult_1)
   883 
   884 lemma binomial_symmetric:
   885   assumes kn: "k \<le> n"
   886   shows "n choose k = n choose (n - k)"
   887 proof -
   888   have kn': "n - k \<le> n"
   889     using kn by arith
   890   from binomial_fact_lemma[OF kn] binomial_fact_lemma[OF kn']
   891   have "fact k * fact (n - k) * (n choose k) = fact (n - k) * fact (n - (n - k)) * (n choose (n - k))"
   892     by simp
   893   then show ?thesis
   894     using kn by simp
   895 qed
   896 
   897 lemma choose_rising_sum:
   898   "(\<Sum>j\<le>m. ((n + j) choose n)) = ((n + m + 1) choose (n + 1))"
   899   "(\<Sum>j\<le>m. ((n + j) choose n)) = ((n + m + 1) choose m)"
   900 proof -
   901   show "(\<Sum>j\<le>m. ((n + j) choose n)) = ((n + m + 1) choose (n + 1))"
   902     by (induct m) simp_all
   903   also have "\<dots> = (n + m + 1) choose m"
   904     by (subst binomial_symmetric) simp_all
   905   finally show "(\<Sum>j\<le>m. ((n + j) choose n)) = (n + m + 1) choose m" .
   906 qed
   907 
   908 lemma choose_linear_sum: "(\<Sum>i\<le>n. i * (n choose i)) = n * 2 ^ (n - 1)"
   909 proof (cases n)
   910   case 0
   911   then show ?thesis by simp
   912 next
   913   case (Suc m)
   914   have "(\<Sum>i\<le>n. i * (n choose i)) = (\<Sum>i\<le>Suc m. i * (Suc m choose i))"
   915     by (simp add: Suc)
   916   also have "\<dots> = Suc m * 2 ^ m"
   917     by (simp only: setsum_atMost_Suc_shift Suc_times_binomial setsum_right_distrib[symmetric])
   918        (simp add: choose_row_sum')
   919   finally show ?thesis
   920     using Suc by simp
   921 qed
   922 
   923 lemma choose_alternating_linear_sum:
   924   assumes "n \<noteq> 1"
   925   shows "(\<Sum>i\<le>n. (-1)^i * of_nat i * of_nat (n choose i) :: 'a::comm_ring_1) = 0"
   926 proof (cases n)
   927   case 0
   928   then show ?thesis by simp
   929 next
   930   case (Suc m)
   931   with assms have "m > 0"
   932     by simp
   933   have "(\<Sum>i\<le>n. (-1) ^ i * of_nat i * of_nat (n choose i) :: 'a) =
   934       (\<Sum>i\<le>Suc m. (-1) ^ i * of_nat i * of_nat (Suc m choose i))"
   935     by (simp add: Suc)
   936   also have "\<dots> = (\<Sum>i\<le>m. (-1) ^ (Suc i) * of_nat (Suc i * (Suc m choose Suc i)))"
   937     by (simp only: setsum_atMost_Suc_shift setsum_right_distrib[symmetric] mult_ac of_nat_mult) simp
   938   also have "\<dots> = - of_nat (Suc m) * (\<Sum>i\<le>m. (-1) ^ i * of_nat (m choose i))"
   939     by (subst setsum_right_distrib, rule setsum.cong[OF refl], subst Suc_times_binomial)
   940        (simp add: algebra_simps)
   941   also have "(\<Sum>i\<le>m. (-1 :: 'a) ^ i * of_nat ((m choose i))) = 0"
   942     using choose_alternating_sum[OF \<open>m > 0\<close>] by simp
   943   finally show ?thesis
   944     by simp
   945 qed
   946 
   947 lemma vandermonde: "(\<Sum>k\<le>r. (m choose k) * (n choose (r - k))) = (m + n) choose r"
   948 proof (induct n arbitrary: r)
   949   case 0
   950   have "(\<Sum>k\<le>r. (m choose k) * (0 choose (r - k))) = (\<Sum>k\<le>r. if k = r then (m choose k) else 0)"
   951     by (intro setsum.cong) simp_all
   952   also have "\<dots> = m choose r"
   953     by (simp add: setsum.delta)
   954   finally show ?case
   955     by simp
   956 next
   957   case (Suc n r)
   958   show ?case
   959     by (cases r) (simp_all add: Suc [symmetric] algebra_simps setsum.distrib Suc_diff_le)
   960 qed
   961 
   962 lemma choose_square_sum: "(\<Sum>k\<le>n. (n choose k)^2) = ((2*n) choose n)"
   963   using vandermonde[of n n n]
   964   by (simp add: power2_eq_square mult_2 binomial_symmetric [symmetric])
   965 
   966 lemma pochhammer_binomial_sum:
   967   fixes a b :: "'a::comm_ring_1"
   968   shows "pochhammer (a + b) n = (\<Sum>k\<le>n. of_nat (n choose k) * pochhammer a k * pochhammer b (n - k))"
   969 proof (induction n arbitrary: a b)
   970   case 0
   971   then show ?case by simp
   972 next
   973   case (Suc n a b)
   974   have "(\<Sum>k\<le>Suc n. of_nat (Suc n choose k) * pochhammer a k * pochhammer b (Suc n - k)) =
   975       (\<Sum>i\<le>n. of_nat (n choose i) * pochhammer a (Suc i) * pochhammer b (n - i)) +
   976       ((\<Sum>i\<le>n. of_nat (n choose Suc i) * pochhammer a (Suc i) * pochhammer b (n - i)) +
   977       pochhammer b (Suc n))"
   978     by (subst setsum_atMost_Suc_shift) (simp add: ring_distribs setsum.distrib)
   979   also have "(\<Sum>i\<le>n. of_nat (n choose i) * pochhammer a (Suc i) * pochhammer b (n - i)) =
   980       a * pochhammer ((a + 1) + b) n"
   981     by (subst Suc) (simp add: setsum_right_distrib pochhammer_rec mult_ac)
   982   also have "(\<Sum>i\<le>n. of_nat (n choose Suc i) * pochhammer a (Suc i) * pochhammer b (n - i)) +
   983         pochhammer b (Suc n) =
   984       (\<Sum>i=0..Suc n. of_nat (n choose i) * pochhammer a i * pochhammer b (Suc n - i))"
   985     apply (subst setsum_head_Suc)
   986     apply simp
   987     apply (subst setsum_shift_bounds_cl_Suc_ivl)
   988     apply (simp add: atLeast0AtMost)
   989     done
   990   also have "\<dots> = (\<Sum>i\<le>n. of_nat (n choose i) * pochhammer a i * pochhammer b (Suc n - i))"
   991     using Suc by (intro setsum.mono_neutral_right) (auto simp: not_le binomial_eq_0)
   992   also have "\<dots> = (\<Sum>i\<le>n. of_nat (n choose i) * pochhammer a i * pochhammer b (Suc (n - i)))"
   993     by (intro setsum.cong) (simp_all add: Suc_diff_le)
   994   also have "\<dots> = b * pochhammer (a + (b + 1)) n"
   995     by (subst Suc) (simp add: setsum_right_distrib mult_ac pochhammer_rec)
   996   also have "a * pochhammer ((a + 1) + b) n + b * pochhammer (a + (b + 1)) n =
   997       pochhammer (a + b) (Suc n)"
   998     by (simp add: pochhammer_rec algebra_simps)
   999   finally show ?case ..
  1000 qed
  1001 
  1002 text \<open>Contributed by Manuel Eberl, generalised by LCP.
  1003   Alternative definition of the binomial coefficient as @{term "\<Prod>i<k. (n - i) / (k - i)"}.\<close>
  1004 lemma gbinomial_altdef_of_nat: "x gchoose k = (\<Prod>i = 0..<k. (x - of_nat i) / of_nat (k - i) :: 'a)"
  1005   for k :: nat and x :: "'a::field_char_0"
  1006   by (simp add: setprod_dividef gbinomial_setprod_rev fact_setprod_rev)
  1007 
  1008 lemma gbinomial_ge_n_over_k_pow_k:
  1009   fixes k :: nat
  1010     and x :: "'a::linordered_field"
  1011   assumes "of_nat k \<le> x"
  1012   shows "(x / of_nat k :: 'a) ^ k \<le> x gchoose k"
  1013 proof -
  1014   have x: "0 \<le> x"
  1015     using assms of_nat_0_le_iff order_trans by blast
  1016   have "(x / of_nat k :: 'a) ^ k = (\<Prod>i = 0..<k. x / of_nat k :: 'a)"
  1017     by (simp add: setprod_constant)
  1018   also have "\<dots> \<le> x gchoose k" (* FIXME *)
  1019     unfolding gbinomial_altdef_of_nat
  1020     apply (safe intro!: setprod_mono)
  1021     apply simp_all
  1022     prefer 2
  1023     subgoal premises for i
  1024     proof -
  1025       from assms have "x * of_nat i \<ge> of_nat (i * k)"
  1026         by (metis mult.commute mult_le_cancel_right of_nat_less_0_iff of_nat_mult)
  1027       then have "x * of_nat k - x * of_nat i \<le> x * of_nat k - of_nat (i * k)"
  1028         by arith
  1029       then have "x * of_nat (k - i) \<le> (x - of_nat i) * of_nat k"
  1030         using \<open>i < k\<close> by (simp add: algebra_simps zero_less_mult_iff of_nat_diff)
  1031       then have "x * of_nat (k - i) \<le> (x - of_nat i) * (of_nat k :: 'a)"
  1032         by (simp only: of_nat_mult[symmetric] of_nat_le_iff)
  1033       with assms show ?thesis
  1034         using \<open>i < k\<close> by (simp add: field_simps)
  1035     qed
  1036     apply (simp add: x zero_le_divide_iff)
  1037     done
  1038   finally show ?thesis .
  1039 qed
  1040 
  1041 lemma gbinomial_negated_upper: "(a gchoose b) = (-1) ^ b * ((of_nat b - a - 1) gchoose b)"
  1042   by (simp add: gbinomial_pochhammer pochhammer_minus algebra_simps)
  1043 
  1044 lemma gbinomial_minus: "((-a) gchoose b) = (-1) ^ b * ((a + of_nat b - 1) gchoose b)"
  1045   by (subst gbinomial_negated_upper) (simp add: add_ac)
  1046 
  1047 lemma Suc_times_gbinomial: "of_nat (Suc b) * ((a + 1) gchoose (Suc b)) = (a + 1) * (a gchoose b)"
  1048 proof (cases b)
  1049   case 0
  1050   then show ?thesis by simp
  1051 next
  1052   case (Suc b)
  1053   then have "((a + 1) gchoose (Suc (Suc b))) = (\<Prod>i = 0..Suc b. a + (1 - of_nat i)) / fact (b + 2)"
  1054     by (simp add: field_simps gbinomial_setprod_rev atLeastLessThanSuc_atLeastAtMost)
  1055   also have "(\<Prod>i = 0..Suc b. a + (1 - of_nat i)) = (a + 1) * (\<Prod>i = 0..b. a - of_nat i)"
  1056     by (simp add: setprod.atLeast0_atMost_Suc_shift)
  1057   also have "\<dots> / fact (b + 2) = (a + 1) / of_nat (Suc (Suc b)) * (a gchoose Suc b)"
  1058     by (simp_all add: gbinomial_setprod_rev atLeastLessThanSuc_atLeastAtMost)
  1059   finally show ?thesis by (simp add: Suc field_simps del: of_nat_Suc)
  1060 qed
  1061 
  1062 lemma gbinomial_factors: "((a + 1) gchoose (Suc b)) = (a + 1) / of_nat (Suc b) * (a gchoose b)"
  1063 proof (cases b)
  1064   case 0
  1065   then show ?thesis by simp
  1066 next
  1067   case (Suc b)
  1068   then have "((a + 1) gchoose (Suc (Suc b))) = (\<Prod>i = 0 .. Suc b. a + (1 - of_nat i)) / fact (b + 2)"
  1069     by (simp add: field_simps gbinomial_setprod_rev atLeastLessThanSuc_atLeastAtMost)
  1070   also have "(\<Prod>i = 0 .. Suc b. a + (1 - of_nat i)) = (a + 1) * (\<Prod>i = 0..b. a - of_nat i)"
  1071     by (simp add: setprod.atLeast0_atMost_Suc_shift)
  1072   also have "\<dots> / fact (b + 2) = (a + 1) / of_nat (Suc (Suc b)) * (a gchoose Suc b)"
  1073     by (simp_all add: gbinomial_setprod_rev atLeastLessThanSuc_atLeastAtMost atLeast0AtMost)
  1074   finally show ?thesis
  1075     by (simp add: Suc)
  1076 qed
  1077 
  1078 lemma gbinomial_rec: "((r + 1) gchoose (Suc k)) = (r gchoose k) * ((r + 1) / of_nat (Suc k))"
  1079   using gbinomial_mult_1[of r k]
  1080   by (subst gbinomial_Suc_Suc) (simp add: field_simps del: of_nat_Suc, simp add: algebra_simps)
  1081 
  1082 lemma gbinomial_of_nat_symmetric: "k \<le> n \<Longrightarrow> (of_nat n) gchoose k = (of_nat n) gchoose (n - k)"
  1083   using binomial_symmetric[of k n] by (simp add: binomial_gbinomial [symmetric])
  1084 
  1085 
  1086 text \<open>The absorption identity (equation 5.5 \cite[p.~157]{GKP}):
  1087 \[
  1088 {r \choose k} = \frac{r}{k}{r - 1 \choose k - 1},\quad \textnormal{integer } k \neq 0.
  1089 \]\<close>
  1090 lemma gbinomial_absorption': "k > 0 \<Longrightarrow> r gchoose k = (r / of_nat k) * (r - 1 gchoose (k - 1))"
  1091   using gbinomial_rec[of "r - 1" "k - 1"]
  1092   by (simp_all add: gbinomial_rec field_simps del: of_nat_Suc)
  1093 
  1094 text \<open>The absorption identity is written in the following form to avoid
  1095 division by $k$ (the lower index) and therefore remove the $k \neq 0$
  1096 restriction\cite[p.~157]{GKP}:
  1097 \[
  1098 k{r \choose k} = r{r - 1 \choose k - 1}, \quad \textnormal{integer } k.
  1099 \]\<close>
  1100 lemma gbinomial_absorption: "of_nat (Suc k) * (r gchoose Suc k) = r * ((r - 1) gchoose k)"
  1101   using gbinomial_absorption'[of "Suc k" r] by (simp add: field_simps del: of_nat_Suc)
  1102 
  1103 text \<open>The absorption identity for natural number binomial coefficients:\<close>
  1104 lemma binomial_absorption: "Suc k * (n choose Suc k) = n * ((n - 1) choose k)"
  1105   by (cases n) (simp_all add: binomial_eq_0 Suc_times_binomial del: binomial_Suc_Suc mult_Suc)
  1106 
  1107 text \<open>The absorption companion identity for natural number coefficients,
  1108   following the proof by GKP \cite[p.~157]{GKP}:\<close>
  1109 lemma binomial_absorb_comp: "(n - k) * (n choose k) = n * ((n - 1) choose k)"
  1110   (is "?lhs = ?rhs")
  1111 proof (cases "n \<le> k")
  1112   case True
  1113   then show ?thesis by auto
  1114 next
  1115   case False
  1116   then have "?rhs = Suc ((n - 1) - k) * (n choose Suc ((n - 1) - k))"
  1117     using binomial_symmetric[of k "n - 1"] binomial_absorption[of "(n - 1) - k" n]
  1118     by simp
  1119   also have "Suc ((n - 1) - k) = n - k"
  1120     using False by simp
  1121   also have "n choose \<dots> = n choose k"
  1122     using False by (intro binomial_symmetric [symmetric]) simp_all
  1123   finally show ?thesis ..
  1124 qed
  1125 
  1126 text \<open>The generalised absorption companion identity:\<close>
  1127 lemma gbinomial_absorb_comp: "(r - of_nat k) * (r gchoose k) = r * ((r - 1) gchoose k)"
  1128   using pochhammer_absorb_comp[of r k] by (simp add: gbinomial_pochhammer)
  1129 
  1130 lemma gbinomial_addition_formula:
  1131   "r gchoose (Suc k) = ((r - 1) gchoose (Suc k)) + ((r - 1) gchoose k)"
  1132   using gbinomial_Suc_Suc[of "r - 1" k] by (simp add: algebra_simps)
  1133 
  1134 lemma binomial_addition_formula:
  1135   "0 < n \<Longrightarrow> n choose (Suc k) = ((n - 1) choose (Suc k)) + ((n - 1) choose k)"
  1136   by (subst choose_reduce_nat) simp_all
  1137 
  1138 text \<open>
  1139   Equation 5.9 of the reference material \cite[p.~159]{GKP} is a useful
  1140   summation formula, operating on both indices:
  1141   \[
  1142    \sum\limits_{k \leq n}{r + k \choose k} = {r + n + 1 \choose n},
  1143    \quad \textnormal{integer } n.
  1144   \]
  1145 \<close>
  1146 lemma gbinomial_parallel_sum: "(\<Sum>k\<le>n. (r + of_nat k) gchoose k) = (r + of_nat n + 1) gchoose n"
  1147 proof (induct n)
  1148   case 0
  1149   then show ?case by simp
  1150 next
  1151   case (Suc m)
  1152   then show ?case
  1153     using gbinomial_Suc_Suc[of "(r + of_nat m + 1)" m]
  1154     by (simp add: add_ac)
  1155 qed
  1156 
  1157 
  1158 subsubsection \<open>Summation on the upper index\<close>
  1159 
  1160 text \<open>
  1161   Another summation formula is equation 5.10 of the reference material \cite[p.~160]{GKP},
  1162   aptly named \emph{summation on the upper index}:\[\sum_{0 \leq k \leq n} {k \choose m} =
  1163   {n + 1 \choose m + 1}, \quad \textnormal{integers } m, n \geq 0.\]
  1164 \<close>
  1165 lemma gbinomial_sum_up_index:
  1166   "(\<Sum>k = 0..n. (of_nat k gchoose m) :: 'a::field_char_0) = (of_nat n + 1) gchoose (m + 1)"
  1167 proof (induct n)
  1168   case 0
  1169   show ?case
  1170     using gbinomial_Suc_Suc[of 0 m]
  1171     by (cases m) auto
  1172 next
  1173   case (Suc n)
  1174   then show ?case
  1175     using gbinomial_Suc_Suc[of "of_nat (Suc n) :: 'a" m]
  1176     by (simp add: add_ac)
  1177 qed
  1178 
  1179 lemma gbinomial_index_swap:
  1180   "((-1) ^ m) * ((- (of_nat n) - 1) gchoose m) = ((-1) ^ n) * ((- (of_nat m) - 1) gchoose n)"
  1181   (is "?lhs = ?rhs")
  1182 proof -
  1183   have "?lhs = (of_nat (m + n) gchoose m)"
  1184     by (subst gbinomial_negated_upper) (simp add: power_mult_distrib [symmetric])
  1185   also have "\<dots> = (of_nat (m + n) gchoose n)"
  1186     by (subst gbinomial_of_nat_symmetric) simp_all
  1187   also have "\<dots> = ?rhs"
  1188     by (subst gbinomial_negated_upper) simp
  1189   finally show ?thesis .
  1190 qed
  1191 
  1192 lemma gbinomial_sum_lower_neg: "(\<Sum>k\<le>m. (r gchoose k) * (- 1) ^ k) = (- 1) ^ m * (r - 1 gchoose m)"
  1193   (is "?lhs = ?rhs")
  1194 proof -
  1195   have "?lhs = (\<Sum>k\<le>m. -(r + 1) + of_nat k gchoose k)"
  1196     by (intro setsum.cong[OF refl]) (subst gbinomial_negated_upper, simp add: power_mult_distrib)
  1197   also have "\<dots>  = - r + of_nat m gchoose m"
  1198     by (subst gbinomial_parallel_sum) simp
  1199   also have "\<dots> = ?rhs"
  1200     by (subst gbinomial_negated_upper) (simp add: power_mult_distrib)
  1201   finally show ?thesis .
  1202 qed
  1203 
  1204 lemma gbinomial_partial_row_sum:
  1205   "(\<Sum>k\<le>m. (r gchoose k) * ((r / 2) - of_nat k)) = ((of_nat m + 1)/2) * (r gchoose (m + 1))"
  1206 proof (induct m)
  1207   case 0
  1208   then show ?case by simp
  1209 next
  1210   case (Suc mm)
  1211   then have "(\<Sum>k\<le>Suc mm. (r gchoose k) * (r / 2 - of_nat k)) =
  1212       (r - of_nat (Suc mm)) * (r gchoose Suc mm) / 2"
  1213     by (simp add: field_simps)
  1214   also have "\<dots> = r * (r - 1 gchoose Suc mm) / 2"
  1215     by (subst gbinomial_absorb_comp) (rule refl)
  1216   also have "\<dots> = (of_nat (Suc mm) + 1) / 2 * (r gchoose (Suc mm + 1))"
  1217     by (subst gbinomial_absorption [symmetric]) simp
  1218   finally show ?case .
  1219 qed
  1220 
  1221 lemma setsum_bounds_lt_plus1: "(\<Sum>k<mm. f (Suc k)) = (\<Sum>k=1..mm. f k)"
  1222   by (induct mm) simp_all
  1223 
  1224 lemma gbinomial_partial_sum_poly:
  1225   "(\<Sum>k\<le>m. (of_nat m + r gchoose k) * x^k * y^(m-k)) =
  1226     (\<Sum>k\<le>m. (-r gchoose k) * (-x)^k * (x + y)^(m-k))"
  1227   (is "?lhs m = ?rhs m")
  1228 proof (induction m)
  1229   case 0
  1230   then show ?case by simp
  1231 next
  1232   case (Suc mm)
  1233   define G where "G i k = (of_nat i + r gchoose k) * x^k * y^(i - k)" for i k
  1234   define S where "S = ?lhs"
  1235   have SG_def: "S = (\<lambda>i. (\<Sum>k\<le>i. (G i k)))"
  1236     unfolding S_def G_def ..
  1237 
  1238   have "S (Suc mm) = G (Suc mm) 0 + (\<Sum>k=Suc 0..Suc mm. G (Suc mm) k)"
  1239     using SG_def by (simp add: setsum_head_Suc atLeast0AtMost [symmetric])
  1240   also have "(\<Sum>k=Suc 0..Suc mm. G (Suc mm) k) = (\<Sum>k=0..mm. G (Suc mm) (Suc k))"
  1241     by (subst setsum_shift_bounds_cl_Suc_ivl) simp
  1242   also have "\<dots> = (\<Sum>k=0..mm. ((of_nat mm + r gchoose (Suc k)) +
  1243       (of_nat mm + r gchoose k)) * x^(Suc k) * y^(mm - k))"
  1244     unfolding G_def by (subst gbinomial_addition_formula) simp
  1245   also have "\<dots> = (\<Sum>k=0..mm. (of_nat mm + r gchoose (Suc k)) * x^(Suc k) * y^(mm - k)) +
  1246       (\<Sum>k=0..mm. (of_nat mm + r gchoose k) * x^(Suc k) * y^(mm - k))"
  1247     by (subst setsum.distrib [symmetric]) (simp add: algebra_simps)
  1248   also have "(\<Sum>k=0..mm. (of_nat mm + r gchoose (Suc k)) * x^(Suc k) * y^(mm - k)) =
  1249       (\<Sum>k<Suc mm. (of_nat mm + r gchoose (Suc k)) * x^(Suc k) * y^(mm - k))"
  1250     by (simp only: atLeast0AtMost lessThan_Suc_atMost)
  1251   also have "\<dots> = (\<Sum>k<mm. (of_nat mm + r gchoose Suc k) * x^(Suc k) * y^(mm-k)) +
  1252       (of_nat mm + r gchoose (Suc mm)) * x^(Suc mm)"
  1253     (is "_ = ?A + ?B")
  1254     by (subst setsum_lessThan_Suc) simp
  1255   also have "?A = (\<Sum>k=1..mm. (of_nat mm + r gchoose k) * x^k * y^(mm - k + 1))"
  1256   proof (subst setsum_bounds_lt_plus1 [symmetric], intro setsum.cong[OF refl], clarify)
  1257     fix k
  1258     assume "k < mm"
  1259     then have "mm - k = mm - Suc k + 1"
  1260       by linarith
  1261     then show "(of_nat mm + r gchoose Suc k) * x ^ Suc k * y ^ (mm - k) =
  1262         (of_nat mm + r gchoose Suc k) * x ^ Suc k * y ^ (mm - Suc k + 1)"
  1263       by (simp only:)
  1264   qed
  1265   also have "\<dots> + ?B = y * (\<Sum>k=1..mm. (G mm k)) + (of_nat mm + r gchoose (Suc mm)) * x^(Suc mm)"
  1266     unfolding G_def by (subst setsum_right_distrib) (simp add: algebra_simps)
  1267   also have "(\<Sum>k=0..mm. (of_nat mm + r gchoose k) * x^(Suc k) * y^(mm - k)) = x * (S mm)"
  1268     unfolding S_def by (subst setsum_right_distrib) (simp add: atLeast0AtMost algebra_simps)
  1269   also have "(G (Suc mm) 0) = y * (G mm 0)"
  1270     by (simp add: G_def)
  1271   finally have "S (Suc mm) =
  1272       y * (G mm 0 + (\<Sum>k=1..mm. (G mm k))) + (of_nat mm + r gchoose (Suc mm)) * x^(Suc mm) + x * (S mm)"
  1273     by (simp add: ring_distribs)
  1274   also have "G mm 0 + (\<Sum>k=1..mm. (G mm k)) = S mm"
  1275     by (simp add: setsum_head_Suc[symmetric] SG_def atLeast0AtMost)
  1276   finally have "S (Suc mm) = (x + y) * (S mm) + (of_nat mm + r gchoose (Suc mm)) * x^(Suc mm)"
  1277     by (simp add: algebra_simps)
  1278   also have "(of_nat mm + r gchoose (Suc mm)) = (-1) ^ (Suc mm) * (- r gchoose (Suc mm))"
  1279     by (subst gbinomial_negated_upper) simp
  1280   also have "(-1) ^ Suc mm * (- r gchoose Suc mm) * x ^ Suc mm =
  1281       (- r gchoose (Suc mm)) * (-x) ^ Suc mm"
  1282     by (simp add: power_minus[of x])
  1283   also have "(x + y) * S mm + \<dots> = (x + y) * ?rhs mm + (- r gchoose (Suc mm)) * (- x)^Suc mm"
  1284     unfolding S_def by (subst Suc.IH) simp
  1285   also have "(x + y) * ?rhs mm = (\<Sum>n\<le>mm. ((- r gchoose n) * (- x) ^ n * (x + y) ^ (Suc mm - n)))"
  1286     by (subst setsum_right_distrib, rule setsum.cong) (simp_all add: Suc_diff_le)
  1287   also have "\<dots> + (-r gchoose (Suc mm)) * (-x)^Suc mm =
  1288       (\<Sum>n\<le>Suc mm. (- r gchoose n) * (- x) ^ n * (x + y) ^ (Suc mm - n))"
  1289     by simp
  1290   finally show ?case
  1291     by (simp only: S_def)
  1292 qed
  1293 
  1294 lemma gbinomial_partial_sum_poly_xpos:
  1295   "(\<Sum>k\<le>m. (of_nat m + r gchoose k) * x^k * y^(m-k)) =
  1296      (\<Sum>k\<le>m. (of_nat k + r - 1 gchoose k) * x^k * (x + y)^(m-k))"
  1297   apply (subst gbinomial_partial_sum_poly)
  1298   apply (subst gbinomial_negated_upper)
  1299   apply (intro setsum.cong, rule refl)
  1300   apply (simp add: power_mult_distrib [symmetric])
  1301   done
  1302 
  1303 lemma binomial_r_part_sum: "(\<Sum>k\<le>m. (2 * m + 1 choose k)) = 2 ^ (2 * m)"
  1304 proof -
  1305   have "2 * 2^(2*m) = (\<Sum>k = 0..(2 * m + 1). (2 * m + 1 choose k))"
  1306     using choose_row_sum[where n="2 * m + 1"] by simp
  1307   also have "(\<Sum>k = 0..(2 * m + 1). (2 * m + 1 choose k)) =
  1308       (\<Sum>k = 0..m. (2 * m + 1 choose k)) +
  1309       (\<Sum>k = m+1..2*m+1. (2 * m + 1 choose k))"
  1310     using setsum_ub_add_nat[of 0 m "\<lambda>k. 2 * m + 1 choose k" "m+1"]
  1311     by (simp add: mult_2)
  1312   also have "(\<Sum>k = m+1..2*m+1. (2 * m + 1 choose k)) =
  1313       (\<Sum>k = 0..m. (2 * m + 1 choose (k + (m + 1))))"
  1314     by (subst setsum_shift_bounds_cl_nat_ivl [symmetric]) (simp add: mult_2)
  1315   also have "\<dots> = (\<Sum>k = 0..m. (2 * m + 1 choose (m - k)))"
  1316     by (intro setsum.cong[OF refl], subst binomial_symmetric) simp_all
  1317   also have "\<dots> = (\<Sum>k = 0..m. (2 * m + 1 choose k))"
  1318     using setsum.atLeast_atMost_rev [of "\<lambda>k. 2 * m + 1 choose (m - k)" 0 m]
  1319     by simp
  1320   also have "\<dots> + \<dots> = 2 * \<dots>"
  1321     by simp
  1322   finally show ?thesis
  1323     by (subst (asm) mult_cancel1) (simp add: atLeast0AtMost)
  1324 qed
  1325 
  1326 lemma gbinomial_r_part_sum: "(\<Sum>k\<le>m. (2 * (of_nat m) + 1 gchoose k)) = 2 ^ (2 * m)"
  1327   (is "?lhs = ?rhs")
  1328 proof -
  1329   have "?lhs = of_nat (\<Sum>k\<le>m. (2 * m + 1) choose k)"
  1330     by (simp add: binomial_gbinomial add_ac)
  1331   also have "\<dots> = of_nat (2 ^ (2 * m))"
  1332     by (subst binomial_r_part_sum) (rule refl)
  1333   finally show ?thesis by simp
  1334 qed
  1335 
  1336 lemma gbinomial_sum_nat_pow2:
  1337   "(\<Sum>k\<le>m. (of_nat (m + k) gchoose k :: 'a::field_char_0) / 2 ^ k) = 2 ^ m"
  1338   (is "?lhs = ?rhs")
  1339 proof -
  1340   have "2 ^ m * 2 ^ m = (2 ^ (2*m) :: 'a)"
  1341     by (induct m) simp_all
  1342   also have "\<dots> = (\<Sum>k\<le>m. (2 * (of_nat m) + 1 gchoose k))"
  1343     using gbinomial_r_part_sum ..
  1344   also have "\<dots> = (\<Sum>k\<le>m. (of_nat (m + k) gchoose k) * 2 ^ (m - k))"
  1345     using gbinomial_partial_sum_poly_xpos[where x="1" and y="1" and r="of_nat m + 1" and m="m"]
  1346     by (simp add: add_ac)
  1347   also have "\<dots> = 2 ^ m * (\<Sum>k\<le>m. (of_nat (m + k) gchoose k) / 2 ^ k)"
  1348     by (subst setsum_right_distrib) (simp add: algebra_simps power_diff)
  1349   finally show ?thesis
  1350     by (subst (asm) mult_left_cancel) simp_all
  1351 qed
  1352 
  1353 lemma gbinomial_trinomial_revision:
  1354   assumes "k \<le> m"
  1355   shows "(r gchoose m) * (of_nat m gchoose k) = (r gchoose k) * (r - of_nat k gchoose (m - k))"
  1356 proof -
  1357   have "(r gchoose m) * (of_nat m gchoose k) = (r gchoose m) * fact m / (fact k * fact (m - k))"
  1358     using assms by (simp add: binomial_gbinomial [symmetric] binomial_fact)
  1359   also have "\<dots> = (r gchoose k) * (r - of_nat k gchoose (m - k))"
  1360     using assms by (simp add: gbinomial_pochhammer power_diff pochhammer_product)
  1361   finally show ?thesis .
  1362 qed
  1363 
  1364 text \<open>Versions of the theorems above for the natural-number version of "choose"\<close>
  1365 lemma binomial_altdef_of_nat:
  1366   "k \<le> n \<Longrightarrow> of_nat (n choose k) = (\<Prod>i = 0..<k. of_nat (n - i) / of_nat (k - i) :: 'a)"
  1367   for n k :: nat and x :: "'a::field_char_0"
  1368   by (simp add: gbinomial_altdef_of_nat binomial_gbinomial of_nat_diff)
  1369 
  1370 lemma binomial_ge_n_over_k_pow_k: "k \<le> n \<Longrightarrow> (of_nat n / of_nat k :: 'a) ^ k \<le> of_nat (n choose k)"
  1371   for k n :: nat and x :: "'a::linordered_field"
  1372   by (simp add: gbinomial_ge_n_over_k_pow_k binomial_gbinomial of_nat_diff)
  1373 
  1374 lemma binomial_le_pow:
  1375   assumes "r \<le> n"
  1376   shows "n choose r \<le> n ^ r"
  1377 proof -
  1378   have "n choose r \<le> fact n div fact (n - r)"
  1379     using assms by (subst binomial_fact_lemma[symmetric]) auto
  1380   with fact_div_fact_le_pow [OF assms] show ?thesis
  1381     by auto
  1382 qed
  1383 
  1384 lemma binomial_altdef_nat: "k \<le> n \<Longrightarrow> n choose k = fact n div (fact k * fact (n - k))"
  1385   for k n :: nat
  1386   by (subst binomial_fact_lemma [symmetric]) auto
  1387 
  1388 lemma choose_dvd:
  1389   "k \<le> n \<Longrightarrow> fact k * fact (n - k) dvd (fact n :: 'a::{semiring_div,linordered_semidom})"
  1390   unfolding dvd_def
  1391   apply (rule exI [where x="of_nat (n choose k)"])
  1392   using binomial_fact_lemma [of k n, THEN arg_cong [where f = of_nat and 'b='a]]
  1393   apply auto
  1394   done
  1395 
  1396 lemma fact_fact_dvd_fact:
  1397   "fact k * fact n dvd (fact (k + n) :: 'a::{semiring_div,linordered_semidom})"
  1398   by (metis add.commute add_diff_cancel_left' choose_dvd le_add2)
  1399 
  1400 lemma choose_mult_lemma:
  1401   "((m + r + k) choose (m + k)) * ((m + k) choose k) = ((m + r + k) choose k) * ((m + r) choose m)"
  1402   (is "?lhs = _")
  1403 proof -
  1404   have "?lhs =
  1405       fact (m + r + k) div (fact (m + k) * fact (m + r - m)) * (fact (m + k) div (fact k * fact m))"
  1406     by (simp add: binomial_altdef_nat)
  1407   also have "\<dots> = fact (m + r + k) div (fact r * (fact k * fact m))"
  1408     apply (subst div_mult_div_if_dvd)
  1409     apply (auto simp: algebra_simps fact_fact_dvd_fact)
  1410     apply (metis add.assoc add.commute fact_fact_dvd_fact)
  1411     done
  1412   also have "\<dots> = (fact (m + r + k) * fact (m + r)) div (fact r * (fact k * fact m) * fact (m + r))"
  1413     apply (subst div_mult_div_if_dvd [symmetric])
  1414     apply (auto simp add: algebra_simps)
  1415     apply (metis fact_fact_dvd_fact dvd_trans nat_mult_dvd_cancel_disj)
  1416     done
  1417   also have "\<dots> =
  1418       (fact (m + r + k) div (fact k * fact (m + r)) * (fact (m + r) div (fact r * fact m)))"
  1419     apply (subst div_mult_div_if_dvd)
  1420     apply (auto simp: fact_fact_dvd_fact algebra_simps)
  1421     done
  1422   finally show ?thesis
  1423     by (simp add: binomial_altdef_nat mult.commute)
  1424 qed
  1425 
  1426 text \<open>The "Subset of a Subset" identity.\<close>
  1427 lemma choose_mult:
  1428   "k \<le> m \<Longrightarrow> m \<le> n \<Longrightarrow> (n choose m) * (m choose k) = (n choose k) * ((n - k) choose (m - k))"
  1429   using choose_mult_lemma [of "m-k" "n-m" k] by simp
  1430 
  1431 
  1432 subsection \<open>More on Binomial Coefficients\<close>
  1433 
  1434 lemma choose_one: "n choose 1 = n" for n :: nat
  1435   by simp
  1436 
  1437 lemma card_UNION:
  1438   assumes "finite A"
  1439     and "\<forall>k \<in> A. finite k"
  1440   shows "card (\<Union>A) = nat (\<Sum>I | I \<subseteq> A \<and> I \<noteq> {}. (- 1) ^ (card I + 1) * int (card (\<Inter>I)))"
  1441   (is "?lhs = ?rhs")
  1442 proof -
  1443   have "?rhs = nat (\<Sum>I | I \<subseteq> A \<and> I \<noteq> {}. (- 1) ^ (card I + 1) * (\<Sum>_\<in>\<Inter>I. 1))"
  1444     by simp
  1445   also have "\<dots> = nat (\<Sum>I | I \<subseteq> A \<and> I \<noteq> {}. (\<Sum>_\<in>\<Inter>I. (- 1) ^ (card I + 1)))"
  1446     (is "_ = nat ?rhs")
  1447     by (subst setsum_right_distrib) simp
  1448   also have "?rhs = (\<Sum>(I, _)\<in>Sigma {I. I \<subseteq> A \<and> I \<noteq> {}} Inter. (- 1) ^ (card I + 1))"
  1449     using assms by (subst setsum.Sigma) auto
  1450   also have "\<dots> = (\<Sum>(x, I)\<in>(SIGMA x:UNIV. {I. I \<subseteq> A \<and> I \<noteq> {} \<and> x \<in> \<Inter>I}). (- 1) ^ (card I + 1))"
  1451     by (rule setsum.reindex_cong [where l = "\<lambda>(x, y). (y, x)"]) (auto intro: inj_onI simp add: split_beta)
  1452   also have "\<dots> = (\<Sum>(x, I)\<in>(SIGMA x:\<Union>A. {I. I \<subseteq> A \<and> I \<noteq> {} \<and> x \<in> \<Inter>I}). (- 1) ^ (card I + 1))"
  1453     using assms
  1454     by (auto intro!: setsum.mono_neutral_cong_right finite_SigmaI2 intro: finite_subset[where B="\<Union>A"])
  1455   also have "\<dots> = (\<Sum>x\<in>\<Union>A. (\<Sum>I|I \<subseteq> A \<and> I \<noteq> {} \<and> x \<in> \<Inter>I. (- 1) ^ (card I + 1)))"
  1456     using assms by (subst setsum.Sigma) auto
  1457   also have "\<dots> = (\<Sum>_\<in>\<Union>A. 1)" (is "setsum ?lhs _ = _")
  1458   proof (rule setsum.cong[OF refl])
  1459     fix x
  1460     assume x: "x \<in> \<Union>A"
  1461     define K where "K = {X \<in> A. x \<in> X}"
  1462     with \<open>finite A\<close> have K: "finite K"
  1463       by auto
  1464     let ?I = "\<lambda>i. {I. I \<subseteq> A \<and> card I = i \<and> x \<in> \<Inter>I}"
  1465     have "inj_on snd (SIGMA i:{1..card A}. ?I i)"
  1466       using assms by (auto intro!: inj_onI)
  1467     moreover have [symmetric]: "snd ` (SIGMA i:{1..card A}. ?I i) = {I. I \<subseteq> A \<and> I \<noteq> {} \<and> x \<in> \<Inter>I}"
  1468       using assms
  1469       by (auto intro!: rev_image_eqI[where x="(card a, a)" for a]
  1470         simp add: card_gt_0_iff[folded Suc_le_eq]
  1471         dest: finite_subset intro: card_mono)
  1472     ultimately have "?lhs x = (\<Sum>(i, I)\<in>(SIGMA i:{1..card A}. ?I i). (- 1) ^ (i + 1))"
  1473       by (rule setsum.reindex_cong [where l = snd]) fastforce
  1474     also have "\<dots> = (\<Sum>i=1..card A. (\<Sum>I|I \<subseteq> A \<and> card I = i \<and> x \<in> \<Inter>I. (- 1) ^ (i + 1)))"
  1475       using assms by (subst setsum.Sigma) auto
  1476     also have "\<dots> = (\<Sum>i=1..card A. (- 1) ^ (i + 1) * (\<Sum>I|I \<subseteq> A \<and> card I = i \<and> x \<in> \<Inter>I. 1))"
  1477       by (subst setsum_right_distrib) simp
  1478     also have "\<dots> = (\<Sum>i=1..card K. (- 1) ^ (i + 1) * (\<Sum>I|I \<subseteq> K \<and> card I = i. 1))"
  1479       (is "_ = ?rhs")
  1480     proof (rule setsum.mono_neutral_cong_right[rule_format])
  1481       show "finite {1..card A}"
  1482         by simp
  1483       show "{1..card K} \<subseteq> {1..card A}"
  1484         using \<open>finite A\<close> by (auto simp add: K_def intro: card_mono)
  1485     next
  1486       fix i
  1487       assume "i \<in> {1..card A} - {1..card K}"
  1488       then have i: "i \<le> card A" "card K < i"
  1489         by auto
  1490       have "{I. I \<subseteq> A \<and> card I = i \<and> x \<in> \<Inter>I} = {I. I \<subseteq> K \<and> card I = i}"
  1491         by (auto simp add: K_def)
  1492       also have "\<dots> = {}"
  1493         using \<open>finite A\<close> i by (auto simp add: K_def dest: card_mono[rotated 1])
  1494       finally show "(- 1) ^ (i + 1) * (\<Sum>I | I \<subseteq> A \<and> card I = i \<and> x \<in> \<Inter>I. 1 :: int) = 0"
  1495         by (simp only:) simp
  1496     next
  1497       fix i
  1498       have "(\<Sum>I | I \<subseteq> A \<and> card I = i \<and> x \<in> \<Inter>I. 1) = (\<Sum>I | I \<subseteq> K \<and> card I = i. 1 :: int)"
  1499         (is "?lhs = ?rhs")
  1500         by (rule setsum.cong) (auto simp add: K_def)
  1501       then show "(- 1) ^ (i + 1) * ?lhs = (- 1) ^ (i + 1) * ?rhs"
  1502         by simp
  1503     qed
  1504     also have "{I. I \<subseteq> K \<and> card I = 0} = {{}}"
  1505       using assms by (auto simp add: card_eq_0_iff K_def dest: finite_subset)
  1506     then have "?rhs = (\<Sum>i = 0..card K. (- 1) ^ (i + 1) * (\<Sum>I | I \<subseteq> K \<and> card I = i. 1 :: int)) + 1"
  1507       by (subst (2) setsum_head_Suc) simp_all
  1508     also have "\<dots> = (\<Sum>i = 0..card K. (- 1) * ((- 1) ^ i * int (card K choose i))) + 1"
  1509       using K by (subst n_subsets[symmetric]) simp_all
  1510     also have "\<dots> = - (\<Sum>i = 0..card K. (- 1) ^ i * int (card K choose i)) + 1"
  1511       by (subst setsum_right_distrib[symmetric]) simp
  1512     also have "\<dots> =  - ((-1 + 1) ^ card K) + 1"
  1513       by (subst binomial_ring) (simp add: ac_simps)
  1514     also have "\<dots> = 1"
  1515       using x K by (auto simp add: K_def card_gt_0_iff)
  1516     finally show "?lhs x = 1" .
  1517   qed
  1518   also have "nat \<dots> = card (\<Union>A)"
  1519     by simp
  1520   finally show ?thesis ..
  1521 qed
  1522 
  1523 text \<open>The number of nat lists of length \<open>m\<close> summing to \<open>N\<close> is @{term "(N + m - 1) choose N"}:\<close>
  1524 lemma card_length_sum_list_rec:
  1525   assumes "m \<ge> 1"
  1526   shows "card {l::nat list. length l = m \<and> sum_list l = N} =
  1527       card {l. length l = (m - 1) \<and> sum_list l = N} +
  1528       card {l. length l = m \<and> sum_list l + 1 = N}"
  1529     (is "card ?C = card ?A + card ?B")
  1530 proof -
  1531   let ?A' = "{l. length l = m \<and> sum_list l = N \<and> hd l = 0}"
  1532   let ?B' = "{l. length l = m \<and> sum_list l = N \<and> hd l \<noteq> 0}"
  1533   let ?f = "\<lambda>l. 0 # l"
  1534   let ?g = "\<lambda>l. (hd l + 1) # tl l"
  1535   have 1: "xs \<noteq> [] \<Longrightarrow> x = hd xs \<Longrightarrow> x # tl xs = xs" for x xs
  1536     by simp
  1537   have 2: "xs \<noteq> [] \<Longrightarrow> sum_list(tl xs) = sum_list xs - hd xs" for xs :: "nat list"
  1538     by (auto simp add: neq_Nil_conv)
  1539   have f: "bij_betw ?f ?A ?A'"
  1540     apply (rule bij_betw_byWitness[where f' = tl])
  1541     using assms
  1542     apply (auto simp: 2 length_0_conv[symmetric] 1 simp del: length_0_conv)
  1543     done
  1544   have 3: "xs \<noteq> [] \<Longrightarrow> hd xs + (sum_list xs - hd xs) = sum_list xs" for xs :: "nat list"
  1545     by (metis 1 sum_list_simps(2) 2)
  1546   have g: "bij_betw ?g ?B ?B'"
  1547     apply (rule bij_betw_byWitness[where f' = "\<lambda>l. (hd l - 1) # tl l"])
  1548     using assms
  1549     by (auto simp: 2 length_0_conv[symmetric] intro!: 3
  1550         simp del: length_greater_0_conv length_0_conv)
  1551   have fin: "finite {xs. size xs = M \<and> set xs \<subseteq> {0..<N}}" for M N :: nat
  1552     using finite_lists_length_eq[OF finite_atLeastLessThan] conj_commute by auto
  1553   have fin_A: "finite ?A" using fin[of _ "N+1"]
  1554     by (intro finite_subset[where ?A = "?A" and ?B = "{xs. size xs = m - 1 \<and> set xs \<subseteq> {0..<N+1}}"])
  1555       (auto simp: member_le_sum_list_nat less_Suc_eq_le)
  1556   have fin_B: "finite ?B"
  1557     by (intro finite_subset[where ?A = "?B" and ?B = "{xs. size xs = m \<and> set xs \<subseteq> {0..<N}}"])
  1558       (auto simp: member_le_sum_list_nat less_Suc_eq_le fin)
  1559   have uni: "?C = ?A' \<union> ?B'"
  1560     by auto
  1561   have disj: "?A' \<inter> ?B' = {}"
  1562     by auto
  1563   have "card ?C = card(?A' \<union> ?B')"
  1564     using uni by simp
  1565   also have "\<dots> = card ?A + card ?B"
  1566     using card_Un_disjoint[OF _ _ disj] bij_betw_finite[OF f] bij_betw_finite[OF g]
  1567       bij_betw_same_card[OF f] bij_betw_same_card[OF g] fin_A fin_B
  1568     by presburger
  1569   finally show ?thesis .
  1570 qed
  1571 
  1572 lemma card_length_sum_list: "card {l::nat list. size l = m \<and> sum_list l = N} = (N + m - 1) choose N"
  1573   \<comment> "by Holden Lee, tidied by Tobias Nipkow"
  1574 proof (cases m)
  1575   case 0
  1576   then show ?thesis
  1577     by (cases N) (auto cong: conj_cong)
  1578 next
  1579   case (Suc m')
  1580   have m: "m \<ge> 1"
  1581     by (simp add: Suc)
  1582   then show ?thesis
  1583   proof (induct "N + m - 1" arbitrary: N m)
  1584     case 0  \<comment> "In the base case, the only solution is [0]."
  1585     have [simp]: "{l::nat list. length l = Suc 0 \<and> (\<forall>n\<in>set l. n = 0)} = {[0]}"
  1586       by (auto simp: length_Suc_conv)
  1587     have "m = 1 \<and> N = 0"
  1588       using 0 by linarith
  1589     then show ?case
  1590       by simp
  1591   next
  1592     case (Suc k)
  1593     have c1: "card {l::nat list. size l = (m - 1) \<and> sum_list l =  N} = (N + (m - 1) - 1) choose N"
  1594     proof (cases "m = 1")
  1595       case True
  1596       with Suc.hyps have "N \<ge> 1"
  1597         by auto
  1598       with True show ?thesis
  1599         by (simp add: binomial_eq_0)
  1600     next
  1601       case False
  1602       then show ?thesis
  1603         using Suc by fastforce
  1604     qed
  1605     from Suc have c2: "card {l::nat list. size l = m \<and> sum_list l + 1 = N} =
  1606       (if N > 0 then ((N - 1) + m - 1) choose (N - 1) else 0)"
  1607     proof -
  1608       have *: "n > 0 \<Longrightarrow> Suc m = n \<longleftrightarrow> m = n - 1" for m n
  1609         by arith
  1610       from Suc have "N > 0 \<Longrightarrow>
  1611         card {l::nat list. size l = m \<and> sum_list l + 1 = N} =
  1612           ((N - 1) + m - 1) choose (N - 1)"
  1613         by (simp add: *)
  1614       then show ?thesis
  1615         by auto
  1616     qed
  1617     from Suc.prems have "(card {l::nat list. size l = (m - 1) \<and> sum_list l = N} +
  1618           card {l::nat list. size l = m \<and> sum_list l + 1 = N}) = (N + m - 1) choose N"
  1619       by (auto simp: c1 c2 choose_reduce_nat[of "N + m - 1" N] simp del: One_nat_def)
  1620     then show ?case
  1621       using card_length_sum_list_rec[OF Suc.prems] by auto
  1622   qed
  1623 qed
  1624 
  1625 lemma Suc_times_binomial_add: "Suc a * (Suc (a + b) choose Suc a) = Suc b * (Suc (a + b) choose a)"
  1626   \<comment> \<open>by Lukas Bulwahn\<close>
  1627 proof -
  1628   have dvd: "Suc a * (fact a * fact b) dvd fact (Suc (a + b))" for a b
  1629     using fact_fact_dvd_fact[of "Suc a" "b", where 'a=nat]
  1630     by (simp only: fact_Suc add_Suc[symmetric] of_nat_id mult.assoc)
  1631   have "Suc a * (fact (Suc (a + b)) div (Suc a * fact a * fact b)) =
  1632       Suc a * fact (Suc (a + b)) div (Suc a * (fact a * fact b))"
  1633     by (subst div_mult_swap[symmetric]; simp only: mult.assoc dvd)
  1634   also have "\<dots> = Suc b * fact (Suc (a + b)) div (Suc b * (fact a * fact b))"
  1635     by (simp only: div_mult_mult1)
  1636   also have "\<dots> = Suc b * (fact (Suc (a + b)) div (Suc b * (fact a * fact b)))"
  1637     using dvd[of b a] by (subst div_mult_swap[symmetric]; simp only: ac_simps dvd)
  1638   finally show ?thesis
  1639     by (subst (1 2) binomial_altdef_nat)
  1640       (simp_all only: ac_simps diff_Suc_Suc Suc_diff_le diff_add_inverse fact_Suc of_nat_id)
  1641 qed
  1642 
  1643 
  1644 subsection \<open>Misc\<close>
  1645 
  1646 lemma fact_code [code]:
  1647   "fact n = (of_nat (fold_atLeastAtMost_nat (op *) 2 n 1) :: 'a::semiring_char_0)"
  1648 proof -
  1649   have "fact n = (of_nat (\<Prod>{1..n}) :: 'a)"
  1650     by (simp add: fact_setprod)
  1651   also have "\<Prod>{1..n} = \<Prod>{2..n}"
  1652     by (intro setprod.mono_neutral_right) auto
  1653   also have "\<dots> = fold_atLeastAtMost_nat (op *) 2 n 1"
  1654     by (simp add: setprod_atLeastAtMost_code)
  1655   finally show ?thesis .
  1656 qed
  1657 
  1658 lemma pochhammer_code [code]:
  1659   "pochhammer a n =
  1660     (if n = 0 then 1
  1661      else fold_atLeastAtMost_nat (\<lambda>n acc. (a + of_nat n) * acc) 0 (n - 1) 1)"
  1662   by (cases n)
  1663     (simp_all add: pochhammer_setprod setprod_atLeastAtMost_code [symmetric]
  1664       atLeastLessThanSuc_atLeastAtMost)
  1665 
  1666 lemma gbinomial_code [code]:
  1667   "a gchoose n =
  1668     (if n = 0 then 1
  1669      else fold_atLeastAtMost_nat (\<lambda>n acc. (a - of_nat n) * acc) 0 (n - 1) 1 / fact n)"
  1670   by (cases n)
  1671     (simp_all add: gbinomial_setprod_rev setprod_atLeastAtMost_code [symmetric]
  1672       atLeastLessThanSuc_atLeastAtMost)
  1673 
  1674 (* FIXME *)
  1675 (*TODO: This code equation breaks Scala code generation in HOL-Codegenerator_Test. We have to figure out why and how to prevent that. *)
  1676 
  1677 (*
  1678 lemma binomial_code [code]:
  1679   "(n choose k) =
  1680       (if k > n then 0
  1681        else if 2 * k > n then (n choose (n - k))
  1682        else (fold_atLeastAtMost_nat (op * ) (n-k+1) n 1 div fact k))"
  1683 proof -
  1684   {
  1685     assume "k \<le> n"
  1686     then have "{1..n} = {1..n-k} \<union> {n-k+1..n}" by auto
  1687     then have "(fact n :: nat) = fact (n-k) * \<Prod>{n-k+1..n}"
  1688       by (simp add: setprod.union_disjoint fact_altdef_nat)
  1689   }
  1690   then show ?thesis by (auto simp: binomial_altdef_nat mult_ac setprod_atLeastAtMost_code)
  1691 qed
  1692 *)
  1693 
  1694 end