src/HOL/Library/Binomial.thy
author chaieb
Fri Jan 30 12:48:56 2009 +0000 (2009-01-30)
changeset 29694 2f2558d7bc3e
parent 27487 c8a6ce181805
child 29906 80369da39838
permissions -rw-r--r--
Added a formalization of generalized raising Factorials (Pochhammer's symbol) and binomial coefficients
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(*  Title:      HOL/Binomial.thy
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    ID:         $Id$
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    Author:     Lawrence C Paulson, Amine Chaieb
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    Copyright   1997  University of Cambridge
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*)
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header {* Binomial Coefficients *}
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theory Binomial
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imports Fact Plain "~~/src/HOL/SetInterval" Presburger 
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begin
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text {* This development is based on the work of Andy Gordon and
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  Florian Kammueller. *}
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consts
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  binomial :: "nat \<Rightarrow> nat \<Rightarrow> nat"      (infixl "choose" 65)
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primrec
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  binomial_0: "(0 choose k) = (if k = 0 then 1 else 0)"
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  binomial_Suc: "(Suc n choose k) =
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                 (if k = 0 then 1 else (n choose (k - 1)) + (n choose k))"
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lemma binomial_n_0 [simp]: "(n choose 0) = 1"
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by (cases n) simp_all
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lemma binomial_0_Suc [simp]: "(0 choose Suc k) = 0"
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by simp
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lemma binomial_Suc_Suc [simp]:
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  "(Suc n choose Suc k) = (n choose k) + (n choose Suc k)"
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by simp
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lemma binomial_eq_0: "!!k. n < k ==> (n choose k) = 0"
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by (induct n) auto
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declare binomial_0 [simp del] binomial_Suc [simp del]
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lemma binomial_n_n [simp]: "(n choose n) = 1"
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by (induct n) (simp_all add: binomial_eq_0)
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lemma binomial_Suc_n [simp]: "(Suc n choose n) = Suc n"
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by (induct n) simp_all
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lemma binomial_1 [simp]: "(n choose Suc 0) = n"
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by (induct n) simp_all
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lemma zero_less_binomial: "k \<le> n ==> (n choose k) > 0"
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by (induct n k rule: diff_induct) simp_all
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lemma binomial_eq_0_iff: "(n choose k = 0) = (n<k)"
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apply (safe intro!: binomial_eq_0)
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apply (erule contrapos_pp)
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apply (simp add: zero_less_binomial)
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done
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lemma zero_less_binomial_iff: "(n choose k > 0) = (k\<le>n)"
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by(simp add: linorder_not_less binomial_eq_0_iff neq0_conv[symmetric]
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        del:neq0_conv)
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(*Might be more useful if re-oriented*)
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lemma Suc_times_binomial_eq:
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  "!!k. k \<le> n ==> Suc n * (n choose k) = (Suc n choose Suc k) * Suc k"
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apply (induct n)
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apply (simp add: binomial_0)
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apply (case_tac k)
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apply (auto simp add: add_mult_distrib add_mult_distrib2 le_Suc_eq
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    binomial_eq_0)
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done
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text{*This is the well-known version, but it's harder to use because of the
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  need to reason about division.*}
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lemma binomial_Suc_Suc_eq_times:
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    "k \<le> n ==> (Suc n choose Suc k) = (Suc n * (n choose k)) div Suc k"
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  by (simp add: Suc_times_binomial_eq div_mult_self_is_m zero_less_Suc
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    del: mult_Suc mult_Suc_right)
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text{*Another version, with -1 instead of Suc.*}
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lemma times_binomial_minus1_eq:
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    "[|k \<le> n;  0<k|] ==> (n choose k) * k = n * ((n - 1) choose (k - 1))"
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  apply (cut_tac n = "n - 1" and k = "k - 1" in Suc_times_binomial_eq)
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  apply (simp split add: nat_diff_split, auto)
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  done
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subsection {* Theorems about @{text "choose"} *}
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text {*
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  \medskip Basic theorem about @{text "choose"}.  By Florian
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  Kamm\"uller, tidied by LCP.
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*}
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lemma card_s_0_eq_empty:
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    "finite A ==> card {B. B \<subseteq> A & card B = 0} = 1"
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  apply (simp cong add: conj_cong add: finite_subset [THEN card_0_eq])
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  apply (simp cong add: rev_conj_cong)
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  done
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lemma choose_deconstruct: "finite M ==> x \<notin> M
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  ==> {s. s <= insert x M & card(s) = Suc k}
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       = {s. s <= M & card(s) = Suc k} Un
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         {s. EX t. t <= M & card(t) = k & s = insert x t}"
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  apply safe
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   apply (auto intro: finite_subset [THEN card_insert_disjoint])
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  apply (drule_tac x = "xa - {x}" in spec)
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  apply (subgoal_tac "x \<notin> xa", auto)
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  apply (erule rev_mp, subst card_Diff_singleton)
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  apply (auto intro: finite_subset)
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  done
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text{*There are as many subsets of @{term A} having cardinality @{term k}
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 as there are sets obtained from the former by inserting a fixed element
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 @{term x} into each.*}
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lemma constr_bij:
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   "[|finite A; x \<notin> A|] ==>
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    card {B. EX C. C <= A & card(C) = k & B = insert x C} =
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    card {B. B <= A & card(B) = k}"
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  apply (rule_tac f = "%s. s - {x}" and g = "insert x" in card_bij_eq)
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       apply (auto elim!: equalityE simp add: inj_on_def)
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    apply (subst Diff_insert0, auto)
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   txt {* finiteness of the two sets *}
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   apply (rule_tac [2] B = "Pow (A)" in finite_subset)
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   apply (rule_tac B = "Pow (insert x A)" in finite_subset)
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   apply fast+
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  done
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text {*
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  Main theorem: combinatorial statement about number of subsets of a set.
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*}
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lemma n_sub_lemma:
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    "!!A. finite A ==> card {B. B <= A & card B = k} = (card A choose k)"
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  apply (induct k)
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   apply (simp add: card_s_0_eq_empty, atomize)
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  apply (rotate_tac -1, erule finite_induct)
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   apply (simp_all (no_asm_simp) cong add: conj_cong
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     add: card_s_0_eq_empty choose_deconstruct)
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  apply (subst card_Un_disjoint)
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     prefer 4 apply (force simp add: constr_bij)
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    prefer 3 apply force
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   prefer 2 apply (blast intro: finite_Pow_iff [THEN iffD2]
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     finite_subset [of _ "Pow (insert x F)", standard])
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  apply (blast intro: finite_Pow_iff [THEN iffD2, THEN [2] finite_subset])
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  done
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theorem n_subsets:
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    "finite A ==> card {B. B <= A & card B = k} = (card A choose k)"
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  by (simp add: n_sub_lemma)
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text{* The binomial theorem (courtesy of Tobias Nipkow): *}
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theorem binomial: "(a+b::nat)^n = (\<Sum>k=0..n. (n choose k) * a^k * b^(n-k))"
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proof (induct n)
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  case 0 thus ?case by simp
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next
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  case (Suc n)
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  have decomp: "{0..n+1} = {0} \<union> {n+1} \<union> {1..n}"
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    by (auto simp add:atLeastAtMost_def atLeast_def atMost_def)
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  have decomp2: "{0..n} = {0} \<union> {1..n}"
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    by (auto simp add:atLeastAtMost_def atLeast_def atMost_def)
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  have "(a+b::nat)^(n+1) = (a+b) * (\<Sum>k=0..n. (n choose k) * a^k * b^(n-k))"
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    using Suc by simp
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  also have "\<dots> =  a*(\<Sum>k=0..n. (n choose k) * a^k * b^(n-k)) +
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                   b*(\<Sum>k=0..n. (n choose k) * a^k * b^(n-k))"
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    by (rule nat_distrib)
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  also have "\<dots> = (\<Sum>k=0..n. (n choose k) * a^(k+1) * b^(n-k)) +
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                  (\<Sum>k=0..n. (n choose k) * a^k * b^(n-k+1))"
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    by (simp add: setsum_right_distrib mult_ac)
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  also have "\<dots> = (\<Sum>k=0..n. (n choose k) * a^k * b^(n+1-k)) +
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                  (\<Sum>k=1..n+1. (n choose (k - 1)) * a^k * b^(n+1-k))"
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    by (simp add:setsum_shift_bounds_cl_Suc_ivl Suc_diff_le
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             del:setsum_cl_ivl_Suc)
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  also have "\<dots> = a^(n+1) + b^(n+1) +
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                  (\<Sum>k=1..n. (n choose (k - 1)) * a^k * b^(n+1-k)) +
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                  (\<Sum>k=1..n. (n choose k) * a^k * b^(n+1-k))"
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    by (simp add: decomp2)
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  also have
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      "\<dots> = a^(n+1) + b^(n+1) + (\<Sum>k=1..n. (n+1 choose k) * a^k * b^(n+1-k))"
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    by (simp add: nat_distrib setsum_addf binomial.simps)
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  also have "\<dots> = (\<Sum>k=0..n+1. (n+1 choose k) * a^k * b^(n+1-k))"
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    using decomp by simp
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  finally show ?case by simp
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qed
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section{* Pochhammer's symbol : generalized raising factorial*}
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definition "pochhammer (a::'a::comm_semiring_1) n = (if n = 0 then 1 else setprod (\<lambda>n. a + of_nat n) {0 .. n - 1})"
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lemma pochhammer_0[simp]: "pochhammer a 0 = 1" 
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  by (simp add: pochhammer_def)
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lemma pochhammer_1[simp]: "pochhammer a 1 = a" by (simp add: pochhammer_def)
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lemma pochhammer_Suc0[simp]: "pochhammer a (Suc 0) = a" 
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  by (simp add: pochhammer_def)
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lemma pochhammer_Suc_setprod: "pochhammer a (Suc n) = setprod (\<lambda>n. a + of_nat n) {0 .. n}"
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  by (simp add: pochhammer_def)
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lemma setprod_nat_ivl_Suc: "setprod f {0 .. Suc n} = setprod f {0..n} * f (Suc n)"
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proof-
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  have th: "finite {0..n}" "finite {Suc n}" "{0..n} \<inter> {Suc n} = {}" by auto
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  have eq: "{0..Suc n} = {0..n} \<union> {Suc n}" by auto
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  show ?thesis unfolding eq setprod_Un_disjoint[OF th] by simp
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qed
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lemma setprod_nat_ivl_1_Suc: "setprod f {0 .. Suc n} = f 0 * setprod f {1.. Suc n}"
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proof-
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  have th: "finite {0}" "finite {1..Suc n}" "{0} \<inter> {1.. Suc n} = {}" by auto
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  have eq: "{0..Suc n} = {0} \<union> {1 .. Suc n}" by auto
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  show ?thesis unfolding eq setprod_Un_disjoint[OF th] by simp
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qed
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lemma pochhammer_Suc: "pochhammer a (Suc n) = pochhammer a n * (a + of_nat n)"
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proof-
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  {assume "n=0" then have ?thesis by simp}
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  moreover
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  {fix m assume m: "n = Suc m"
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    have ?thesis  unfolding m pochhammer_Suc_setprod setprod_nat_ivl_Suc ..}
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  ultimately show ?thesis by (cases n, auto)
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qed 
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lemma pochhammer_rec: "pochhammer a (Suc n) = a * pochhammer (a + 1) n"
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proof-
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  {assume "n=0" then have ?thesis by (simp add: pochhammer_Suc_setprod)}
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  moreover
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  {assume n0: "n \<noteq> 0"
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    have th0: "finite {1 .. n}" "0 \<notin> {1 .. n}" by auto
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    have eq: "insert 0 {1 .. n} = {0..n}" by auto
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    have th1: "(\<Prod>n\<in>{1\<Colon>nat..n}. a + of_nat n) =
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      (\<Prod>n\<in>{0\<Colon>nat..n - 1}. a + 1 + of_nat n)"
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      apply (rule setprod_reindex_cong[where f = "Suc"])
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      using n0 by (auto simp add: expand_fun_eq ring_simps)
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    have ?thesis apply (simp add: pochhammer_def)
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    unfolding setprod_insert[OF th0, unfolded eq]
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    using th1 by (simp add: ring_simps)}
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ultimately show ?thesis by blast
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qed
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lemma fact_setprod: "fact n = setprod id {1 .. n}"
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  apply (induct n, simp)
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  apply (simp only: fact_Suc atLeastAtMostSuc_conv)
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  apply (subst setprod_insert)
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  by simp_all
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lemma pochhammer_fact: "of_nat (fact n) = pochhammer 1 n"
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  unfolding fact_setprod
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  apply (cases n, simp_all add: of_nat_setprod pochhammer_Suc_setprod)
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  apply (rule setprod_reindex_cong[where f=Suc])
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  by (auto simp add: expand_fun_eq)
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lemma pochhammer_of_nat_eq_0_lemma: assumes kn: "k > n"
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  shows "pochhammer (- (of_nat n :: 'a:: idom)) k = 0"
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proof-
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  from kn obtain h where h: "k = Suc h" by (cases k, auto)
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  {assume n0: "n=0" then have ?thesis using kn 
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      by (cases k, simp_all add: pochhammer_rec del: pochhammer_Suc)}
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  moreover
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  {assume n0: "n \<noteq> 0"
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    then have ?thesis apply (simp add: h pochhammer_Suc_setprod)
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  apply (rule iffD2[OF setprod_zero_eq])
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  apply auto
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  apply (rule_tac x="n" in bexI)
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  using h kn by auto}
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ultimately show ?thesis by blast
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qed
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lemma pochhammer_of_nat_eq_0_lemma': assumes kn: "k \<le> n"
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  shows "pochhammer (- (of_nat n :: 'a:: {idom, ring_char_0})) k \<noteq> 0"
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proof-
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  {assume "k=0" then have ?thesis by simp}
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  moreover
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  {fix h assume h: "k = Suc h"
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    then have ?thesis apply (simp add: pochhammer_Suc_setprod)
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      apply (subst setprod_zero_eq_field)
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      using h kn by (auto simp add: ring_simps)}
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  ultimately show ?thesis by (cases k, auto)
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qed
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lemma pochhammer_of_nat_eq_0_iff: 
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  shows "pochhammer (- (of_nat n :: 'a:: {idom, ring_char_0})) k = 0 \<longleftrightarrow> k > n"
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  (is "?l = ?r")
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  using pochhammer_of_nat_eq_0_lemma[of n k, where ?'a='a] 
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    pochhammer_of_nat_eq_0_lemma'[of k n, where ?'a = 'a]
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  by (auto simp add: not_le[symmetric])
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section{* Generalized binomial coefficients *}
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definition gbinomial :: "'a::{field, recpower,ring_char_0} \<Rightarrow> nat \<Rightarrow> 'a" (infixl "gchoose" 65)
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  where "a gchoose n = (if n = 0 then 1 else (setprod (\<lambda>i. a - of_nat i) {0 .. n - 1}) / of_nat (fact n))"
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   292
chaieb@29694
   293
lemma gbinomial_0[simp]: "a gchoose 0 = 1" "0 gchoose (Suc n) = 0"
chaieb@29694
   294
  apply (simp_all add: gbinomial_def)
chaieb@29694
   295
  apply (subgoal_tac "(\<Prod>i\<Colon>nat\<in>{0\<Colon>nat..n}. - of_nat i) = (0::'b)")
chaieb@29694
   296
  apply simp
chaieb@29694
   297
  apply (rule iffD2[OF setprod_zero_eq])
chaieb@29694
   298
  by auto
chaieb@29694
   299
chaieb@29694
   300
lemma gbinomial_pochhammer: "a gchoose n = (- 1) ^ n * pochhammer (- a) n / of_nat (fact n)"
chaieb@29694
   301
proof-
chaieb@29694
   302
chaieb@29694
   303
  {assume "n=0" then have ?thesis by simp}
chaieb@29694
   304
  moreover
chaieb@29694
   305
  {assume n0: "n\<noteq>0"
chaieb@29694
   306
    from n0 setprod_constant[of "{0 .. n - 1}" "- (1:: 'a)"]
chaieb@29694
   307
    have eq: "(- (1\<Colon>'a)) ^ n = setprod (\<lambda>i. - 1) {0 .. n - 1}"
chaieb@29694
   308
      by auto
chaieb@29694
   309
    from n0 have ?thesis 
chaieb@29694
   310
      by (simp add: pochhammer_def gbinomial_def field_simps eq setprod_timesf[symmetric])}
chaieb@29694
   311
  ultimately show ?thesis by blast
chaieb@29694
   312
qed
chaieb@29694
   313
chaieb@29694
   314
lemma binomial_fact_lemma:
chaieb@29694
   315
  "k \<le> n \<Longrightarrow> fact k * fact (n - k) * (n choose k) = fact n"
chaieb@29694
   316
proof(induct n arbitrary: k rule: nat_less_induct)
chaieb@29694
   317
  fix n k assume H: "\<forall>m<n. \<forall>x\<le>m. fact x * fact (m - x) * (m choose x) =
chaieb@29694
   318
                      fact m" and kn: "k \<le> n"
chaieb@29694
   319
    let ?ths = "fact k * fact (n - k) * (n choose k) = fact n"
chaieb@29694
   320
  {assume "n=0" then have ?ths using kn by simp}
chaieb@29694
   321
  moreover
chaieb@29694
   322
  {assume "k=0" then have ?ths using kn by simp}
chaieb@29694
   323
  moreover
chaieb@29694
   324
  {assume nk: "n=k" then have ?ths by simp}
chaieb@29694
   325
  moreover
chaieb@29694
   326
  {fix m h assume n: "n = Suc m" and h: "k = Suc h" and hm: "h < m"
chaieb@29694
   327
    from n have mn: "m < n" by arith
chaieb@29694
   328
    from hm have hm': "h \<le> m" by arith
chaieb@29694
   329
    from hm h n kn have km: "k \<le> m" by arith
chaieb@29694
   330
    have "m - h = Suc (m - Suc h)" using  h km hm by arith 
chaieb@29694
   331
    with km h have th0: "fact (m - h) = (m - h) * fact (m - k)"
chaieb@29694
   332
      by simp
chaieb@29694
   333
    from n h th0 
chaieb@29694
   334
    have "fact k * fact (n - k) * (n choose k) = k * (fact h * fact (m - h) * (m choose h)) +  (m - h) * (fact k * fact (m - k) * (m choose k))"
chaieb@29694
   335
      by (simp add: ring_simps)
chaieb@29694
   336
    also have "\<dots> = (k + (m - h)) * fact m"
chaieb@29694
   337
      using H[rule_format, OF mn hm'] H[rule_format, OF mn km]
chaieb@29694
   338
      by (simp add: ring_simps)
chaieb@29694
   339
    finally have ?ths using h n km by simp}
chaieb@29694
   340
  moreover have "n=0 \<or> k = 0 \<or> k = n \<or> (EX m h. n=Suc m \<and> k = Suc h \<and> h < m)" using kn by presburger
chaieb@29694
   341
  ultimately show ?ths by blast
chaieb@29694
   342
qed
chaieb@29694
   343
  
chaieb@29694
   344
lemma binomial_fact: 
chaieb@29694
   345
  assumes kn: "k \<le> n" 
chaieb@29694
   346
  shows "(of_nat (n choose k) :: 'a::{field, ring_char_0}) = of_nat (fact n) / (of_nat (fact k) * of_nat (fact (n - k)))"
chaieb@29694
   347
  using binomial_fact_lemma[OF kn]
chaieb@29694
   348
  by (simp add: field_simps fact_not_eq_zero of_nat_mult[symmetric])
chaieb@29694
   349
chaieb@29694
   350
chaieb@29694
   351
lemma binomial_gbinomial: "of_nat (n choose k) = of_nat n gchoose k"
chaieb@29694
   352
proof-
chaieb@29694
   353
  {assume kn: "k > n" 
chaieb@29694
   354
    from kn binomial_eq_0[OF kn] have ?thesis 
chaieb@29694
   355
      by (simp add: gbinomial_pochhammer field_simps
chaieb@29694
   356
	pochhammer_of_nat_eq_0_iff)}
chaieb@29694
   357
  moreover
chaieb@29694
   358
  {assume "k=0" then have ?thesis by simp}
chaieb@29694
   359
  moreover
chaieb@29694
   360
  {assume kn: "k \<le> n" and k0: "k\<noteq> 0"
chaieb@29694
   361
    from k0 obtain h where h: "k = Suc h" by (cases k, auto)
chaieb@29694
   362
    from h
chaieb@29694
   363
    have eq:"(- 1 :: 'a) ^ k = setprod (\<lambda>i. - 1) {0..h}"
chaieb@29694
   364
      by (subst setprod_constant, auto)
chaieb@29694
   365
    have eq': "(\<Prod>i\<in>{0..h}. of_nat n + - (of_nat i :: 'a)) = (\<Prod>i\<in>{n - h..n}. of_nat i)"
chaieb@29694
   366
      apply (rule strong_setprod_reindex_cong[where f="op - n"])
chaieb@29694
   367
      using h kn 
chaieb@29694
   368
      apply (simp_all add: inj_on_def image_iff Bex_def expand_set_eq)
chaieb@29694
   369
      apply clarsimp
chaieb@29694
   370
      apply (presburger)
chaieb@29694
   371
      apply presburger
chaieb@29694
   372
      by (simp add: expand_fun_eq ring_simps of_nat_add[symmetric] del: of_nat_add)
chaieb@29694
   373
    have th0: "finite {1..n - Suc h}" "finite {n - h .. n}" 
chaieb@29694
   374
"{1..n - Suc h} \<inter> {n - h .. n} = {}" and eq3: "{1..n - Suc h} \<union> {n - h .. n} = {1..n}" using h kn by auto
chaieb@29694
   375
    from eq[symmetric]
chaieb@29694
   376
    have ?thesis using kn
chaieb@29694
   377
      apply (simp add: binomial_fact[OF kn, where ?'a = 'a] 
chaieb@29694
   378
	gbinomial_pochhammer field_simps pochhammer_Suc_setprod)
chaieb@29694
   379
      apply (simp add: pochhammer_Suc_setprod fact_setprod h of_nat_setprod setprod_timesf[symmetric] eq' del: One_nat_def)
chaieb@29694
   380
      unfolding setprod_Un_disjoint[OF th0, unfolded eq3, of "of_nat:: nat \<Rightarrow> 'a"] eq[unfolded h]
chaieb@29694
   381
      unfolding mult_assoc[symmetric] 
chaieb@29694
   382
      unfolding setprod_timesf[symmetric]
chaieb@29694
   383
      apply simp
chaieb@29694
   384
      apply (rule disjI2)
chaieb@29694
   385
      apply (rule strong_setprod_reindex_cong[where f= "op - n"])
chaieb@29694
   386
      apply (auto simp add: inj_on_def image_iff Bex_def)
chaieb@29694
   387
      apply presburger
chaieb@29694
   388
      apply (subgoal_tac "(of_nat (n - x) :: 'a) = of_nat n - of_nat x")
chaieb@29694
   389
      apply simp
chaieb@29694
   390
      by (rule of_nat_diff, simp)
chaieb@29694
   391
  }
chaieb@29694
   392
  moreover
chaieb@29694
   393
  have "k > n \<or> k = 0 \<or> (k \<le> n \<and> k \<noteq> 0)" by arith
chaieb@29694
   394
  ultimately show ?thesis by blast
chaieb@29694
   395
qed
chaieb@29694
   396
chaieb@29694
   397
lemma gbinomial_1[simp]: "a gchoose 1 = a"
chaieb@29694
   398
  by (simp add: gbinomial_def)
chaieb@29694
   399
chaieb@29694
   400
lemma gbinomial_Suc0[simp]: "a gchoose (Suc 0) = a"
chaieb@29694
   401
  by (simp add: gbinomial_def)
chaieb@29694
   402
chaieb@29694
   403
lemma gbinomial_mult_1: "a * (a gchoose n) = of_nat n * (a gchoose n) + of_nat (Suc n) * (a gchoose (Suc n))" (is "?l = ?r")
chaieb@29694
   404
proof-
chaieb@29694
   405
  have "?r = ((- 1) ^n * pochhammer (- a) n / of_nat (fact n)) * (of_nat n - (- a + of_nat n))"
chaieb@29694
   406
    unfolding gbinomial_pochhammer
chaieb@29694
   407
    pochhammer_Suc fact_Suc of_nat_mult right_diff_distrib power_Suc
chaieb@29694
   408
    by (simp add:  field_simps del: of_nat_Suc)
chaieb@29694
   409
  also have "\<dots> = ?l" unfolding gbinomial_pochhammer
chaieb@29694
   410
    by (simp add: ring_simps)
chaieb@29694
   411
  finally show ?thesis ..
chaieb@29694
   412
qed
chaieb@29694
   413
chaieb@29694
   414
lemma gbinomial_mult_1': "(a gchoose n) * a = of_nat n * (a gchoose n) + of_nat (Suc n) * (a gchoose (Suc n))"
chaieb@29694
   415
  by (simp add: mult_commute gbinomial_mult_1)
chaieb@29694
   416
chaieb@29694
   417
lemma gbinomial_Suc: "a gchoose (Suc k) = (setprod (\<lambda>i. a - of_nat i) {0 .. k}) / of_nat (fact (Suc k))"
chaieb@29694
   418
  by (simp add: gbinomial_def)
chaieb@29694
   419
 
chaieb@29694
   420
lemma gbinomial_mult_fact:
chaieb@29694
   421
  "(of_nat (fact (Suc k)) :: 'a) * ((a::'a::{field, ring_char_0,recpower}) gchoose (Suc k)) = (setprod (\<lambda>i. a - of_nat i) {0 .. k})"
chaieb@29694
   422
  unfolding gbinomial_Suc
chaieb@29694
   423
  by (simp_all add: field_simps del: fact_Suc)
chaieb@29694
   424
chaieb@29694
   425
lemma gbinomial_mult_fact':
chaieb@29694
   426
  "((a::'a::{field, ring_char_0,recpower}) gchoose (Suc k)) * (of_nat (fact (Suc k)) :: 'a) = (setprod (\<lambda>i. a - of_nat i) {0 .. k})"
chaieb@29694
   427
  using gbinomial_mult_fact[of k a]
chaieb@29694
   428
  apply (subst mult_commute) .
chaieb@29694
   429
chaieb@29694
   430
lemma gbinomial_Suc_Suc: "((a::'a::{field,recpower, ring_char_0}) + 1) gchoose (Suc k) = a gchoose k + (a gchoose (Suc k))"
chaieb@29694
   431
proof-
chaieb@29694
   432
  {assume "k = 0" then have ?thesis by simp}
chaieb@29694
   433
  moreover
chaieb@29694
   434
  {fix h assume h: "k = Suc h"
chaieb@29694
   435
   have eq0: "(\<Prod>i\<in>{1..k}. (a + 1) - of_nat i) = (\<Prod>i\<in>{0..h}. a - of_nat i)"
chaieb@29694
   436
     apply (rule strong_setprod_reindex_cong[where f = Suc])
chaieb@29694
   437
     using h by auto
chaieb@29694
   438
chaieb@29694
   439
    have "of_nat (fact (Suc k)) * (a gchoose k + (a gchoose (Suc k))) = ((a gchoose Suc h) * of_nat (fact (Suc h)) * of_nat (Suc k)) + (\<Prod>i\<in>{0\<Colon>nat..Suc h}. a - of_nat i)" 
chaieb@29694
   440
      unfolding h
chaieb@29694
   441
      apply (simp add: ring_simps del: fact_Suc)
chaieb@29694
   442
      unfolding gbinomial_mult_fact'
chaieb@29694
   443
      apply (subst fact_Suc)
chaieb@29694
   444
      unfolding of_nat_mult 
chaieb@29694
   445
      apply (subst mult_commute)
chaieb@29694
   446
      unfolding mult_assoc
chaieb@29694
   447
      unfolding gbinomial_mult_fact
chaieb@29694
   448
      by (simp add: ring_simps)
chaieb@29694
   449
    also have "\<dots> = (\<Prod>i\<in>{0..h}. a - of_nat i) * (a + 1)"
chaieb@29694
   450
      unfolding gbinomial_mult_fact' setprod_nat_ivl_Suc
chaieb@29694
   451
      by (simp add: ring_simps h)
chaieb@29694
   452
    also have "\<dots> = (\<Prod>i\<in>{0..k}. (a + 1) - of_nat i)"
chaieb@29694
   453
      using eq0
chaieb@29694
   454
      unfolding h  setprod_nat_ivl_1_Suc
chaieb@29694
   455
      by simp
chaieb@29694
   456
    also have "\<dots> = of_nat (fact (Suc k)) * ((a + 1) gchoose (Suc k))"
chaieb@29694
   457
      unfolding gbinomial_mult_fact ..
chaieb@29694
   458
    finally have ?thesis by (simp del: fact_Suc) }
chaieb@29694
   459
  ultimately show ?thesis by (cases k, auto)
chaieb@29694
   460
qed
chaieb@29694
   461
wenzelm@21256
   462
end