eliminated hard tabulators, guessing at each author's individual tab-width;
tuned headers;
(* Title: HOL/Binomial.thy
Author: Lawrence C Paulson, Amine Chaieb
Copyright 1997 University of Cambridge
*)
header {* Binomial Coefficients *}
theory Binomial
imports Fact SetInterval Presburger Main Rational
begin
text {* This development is based on the work of Andy Gordon and
Florian Kammueller. *}
primrec binomial :: "nat \<Rightarrow> nat \<Rightarrow> nat" (infixl "choose" 65) where
binomial_0: "(0 choose k) = (if k = 0 then 1 else 0)"
| binomial_Suc: "(Suc n choose k) =
(if k = 0 then 1 else (n choose (k - 1)) + (n choose k))"
lemma binomial_n_0 [simp]: "(n choose 0) = 1"
by (cases n) simp_all
lemma binomial_0_Suc [simp]: "(0 choose Suc k) = 0"
by simp
lemma binomial_Suc_Suc [simp]:
"(Suc n choose Suc k) = (n choose k) + (n choose Suc k)"
by simp
lemma binomial_eq_0: "!!k. n < k ==> (n choose k) = 0"
by (induct n) auto
declare binomial_0 [simp del] binomial_Suc [simp del]
lemma binomial_n_n [simp]: "(n choose n) = 1"
by (induct n) (simp_all add: binomial_eq_0)
lemma binomial_Suc_n [simp]: "(Suc n choose n) = Suc n"
by (induct n) simp_all
lemma binomial_1 [simp]: "(n choose Suc 0) = n"
by (induct n) simp_all
lemma zero_less_binomial: "k \<le> n ==> (n choose k) > 0"
by (induct n k rule: diff_induct) simp_all
lemma binomial_eq_0_iff: "(n choose k = 0) = (n<k)"
apply (safe intro!: binomial_eq_0)
apply (erule contrapos_pp)
apply (simp add: zero_less_binomial)
done
lemma zero_less_binomial_iff: "(n choose k > 0) = (k\<le>n)"
by(simp add: linorder_not_less binomial_eq_0_iff neq0_conv[symmetric]
del:neq0_conv)
(*Might be more useful if re-oriented*)
lemma Suc_times_binomial_eq:
"!!k. k \<le> n ==> Suc n * (n choose k) = (Suc n choose Suc k) * Suc k"
apply (induct n)
apply (simp add: binomial_0)
apply (case_tac k)
apply (auto simp add: add_mult_distrib add_mult_distrib2 le_Suc_eq
binomial_eq_0)
done
text{*This is the well-known version, but it's harder to use because of the
need to reason about division.*}
lemma binomial_Suc_Suc_eq_times:
"k \<le> n ==> (Suc n choose Suc k) = (Suc n * (n choose k)) div Suc k"
by (simp add: Suc_times_binomial_eq div_mult_self_is_m zero_less_Suc
del: mult_Suc mult_Suc_right)
text{*Another version, with -1 instead of Suc.*}
lemma times_binomial_minus1_eq:
"[|k \<le> n; 0<k|] ==> (n choose k) * k = n * ((n - 1) choose (k - 1))"
apply (cut_tac n = "n - 1" and k = "k - 1" in Suc_times_binomial_eq)
apply (simp split add: nat_diff_split, auto)
done
subsection {* Theorems about @{text "choose"} *}
text {*
\medskip Basic theorem about @{text "choose"}. By Florian
Kamm\"uller, tidied by LCP.
*}
lemma card_s_0_eq_empty:
"finite A ==> card {B. B \<subseteq> A & card B = 0} = 1"
by (simp cong add: conj_cong add: finite_subset [THEN card_0_eq])
lemma choose_deconstruct: "finite M ==> x \<notin> M
==> {s. s <= insert x M & card(s) = Suc k}
= {s. s <= M & card(s) = Suc k} Un
{s. EX t. t <= M & card(t) = k & s = insert x t}"
apply safe
apply (auto intro: finite_subset [THEN card_insert_disjoint])
apply (drule_tac x = "xa - {x}" in spec)
apply (subgoal_tac "x \<notin> xa", auto)
apply (erule rev_mp, subst card_Diff_singleton)
apply (auto intro: finite_subset)
done
(*
lemma "finite(UN y. {x. P x y})"
apply simp
lemma Collect_ex_eq
lemma "{x. EX y. P x y} = (UN y. {x. P x y})"
apply blast
*)
lemma finite_bex_subset[simp]:
"finite B \<Longrightarrow> (!!A. A<=B \<Longrightarrow> finite{x. P x A}) \<Longrightarrow> finite{x. EX A<=B. P x A}"
apply(subgoal_tac "{x. EX A<=B. P x A} = (UN A:Pow B. {x. P x A})")
apply simp
apply blast
done
text{*There are as many subsets of @{term A} having cardinality @{term k}
as there are sets obtained from the former by inserting a fixed element
@{term x} into each.*}
lemma constr_bij:
"[|finite A; x \<notin> A|] ==>
card {B. EX C. C <= A & card(C) = k & B = insert x C} =
card {B. B <= A & card(B) = k}"
apply (rule_tac f = "%s. s - {x}" and g = "insert x" in card_bij_eq)
apply (auto elim!: equalityE simp add: inj_on_def)
apply (subst Diff_insert0, auto)
done
text {*
Main theorem: combinatorial statement about number of subsets of a set.
*}
lemma n_sub_lemma:
"!!A. finite A ==> card {B. B <= A & card B = k} = (card A choose k)"
apply (induct k)
apply (simp add: card_s_0_eq_empty, atomize)
apply (rotate_tac -1, erule finite_induct)
apply (simp_all (no_asm_simp) cong add: conj_cong
add: card_s_0_eq_empty choose_deconstruct)
apply (subst card_Un_disjoint)
prefer 4 apply (force simp add: constr_bij)
prefer 3 apply force
prefer 2 apply (blast intro: finite_Pow_iff [THEN iffD2]
finite_subset [of _ "Pow (insert x F)", standard])
apply (blast intro: finite_Pow_iff [THEN iffD2, THEN [2] finite_subset])
done
theorem n_subsets:
"finite A ==> card {B. B <= A & card B = k} = (card A choose k)"
by (simp add: n_sub_lemma)
text{* The binomial theorem (courtesy of Tobias Nipkow): *}
theorem binomial: "(a+b::nat)^n = (\<Sum>k=0..n. (n choose k) * a^k * b^(n-k))"
proof (induct n)
case 0 thus ?case by simp
next
case (Suc n)
have decomp: "{0..n+1} = {0} \<union> {n+1} \<union> {1..n}"
by (auto simp add:atLeastAtMost_def atLeast_def atMost_def)
have decomp2: "{0..n} = {0} \<union> {1..n}"
by (auto simp add:atLeastAtMost_def atLeast_def atMost_def)
have "(a+b::nat)^(n+1) = (a+b) * (\<Sum>k=0..n. (n choose k) * a^k * b^(n-k))"
using Suc by simp
also have "\<dots> = a*(\<Sum>k=0..n. (n choose k) * a^k * b^(n-k)) +
b*(\<Sum>k=0..n. (n choose k) * a^k * b^(n-k))"
by (rule nat_distrib)
also have "\<dots> = (\<Sum>k=0..n. (n choose k) * a^(k+1) * b^(n-k)) +
(\<Sum>k=0..n. (n choose k) * a^k * b^(n-k+1))"
by (simp add: setsum_right_distrib mult_ac)
also have "\<dots> = (\<Sum>k=0..n. (n choose k) * a^k * b^(n+1-k)) +
(\<Sum>k=1..n+1. (n choose (k - 1)) * a^k * b^(n+1-k))"
by (simp add:setsum_shift_bounds_cl_Suc_ivl Suc_diff_le
del:setsum_cl_ivl_Suc)
also have "\<dots> = a^(n+1) + b^(n+1) +
(\<Sum>k=1..n. (n choose (k - 1)) * a^k * b^(n+1-k)) +
(\<Sum>k=1..n. (n choose k) * a^k * b^(n+1-k))"
by (simp add: decomp2)
also have
"\<dots> = a^(n+1) + b^(n+1) + (\<Sum>k=1..n. (n+1 choose k) * a^k * b^(n+1-k))"
by (simp add: nat_distrib setsum_addf binomial.simps)
also have "\<dots> = (\<Sum>k=0..n+1. (n+1 choose k) * a^k * b^(n+1-k))"
using decomp by simp
finally show ?case by simp
qed
subsection{* Pochhammer's symbol : generalized raising factorial*}
definition "pochhammer (a::'a::comm_semiring_1) n = (if n = 0 then 1 else setprod (\<lambda>n. a + of_nat n) {0 .. n - 1})"
lemma pochhammer_0[simp]: "pochhammer a 0 = 1"
by (simp add: pochhammer_def)
lemma pochhammer_1[simp]: "pochhammer a 1 = a" by (simp add: pochhammer_def)
lemma pochhammer_Suc0[simp]: "pochhammer a (Suc 0) = a"
by (simp add: pochhammer_def)
lemma pochhammer_Suc_setprod: "pochhammer a (Suc n) = setprod (\<lambda>n. a + of_nat n) {0 .. n}"
by (simp add: pochhammer_def)
lemma setprod_nat_ivl_Suc: "setprod f {0 .. Suc n} = setprod f {0..n} * f (Suc n)"
proof-
have th: "finite {0..n}" "finite {Suc n}" "{0..n} \<inter> {Suc n} = {}" by auto
have eq: "{0..Suc n} = {0..n} \<union> {Suc n}" by auto
show ?thesis unfolding eq setprod_Un_disjoint[OF th] by simp
qed
lemma setprod_nat_ivl_1_Suc: "setprod f {0 .. Suc n} = f 0 * setprod f {1.. Suc n}"
proof-
have th: "finite {0}" "finite {1..Suc n}" "{0} \<inter> {1.. Suc n} = {}" by auto
have eq: "{0..Suc n} = {0} \<union> {1 .. Suc n}" by auto
show ?thesis unfolding eq setprod_Un_disjoint[OF th] by simp
qed
lemma pochhammer_Suc: "pochhammer a (Suc n) = pochhammer a n * (a + of_nat n)"
proof-
{assume "n=0" then have ?thesis by simp}
moreover
{fix m assume m: "n = Suc m"
have ?thesis unfolding m pochhammer_Suc_setprod setprod_nat_ivl_Suc ..}
ultimately show ?thesis by (cases n, auto)
qed
lemma pochhammer_rec: "pochhammer a (Suc n) = a * pochhammer (a + 1) n"
proof-
{assume "n=0" then have ?thesis by (simp add: pochhammer_Suc_setprod)}
moreover
{assume n0: "n \<noteq> 0"
have th0: "finite {1 .. n}" "0 \<notin> {1 .. n}" by auto
have eq: "insert 0 {1 .. n} = {0..n}" by auto
have th1: "(\<Prod>n\<in>{1\<Colon>nat..n}. a + of_nat n) =
(\<Prod>n\<in>{0\<Colon>nat..n - 1}. a + 1 + of_nat n)"
apply (rule setprod_reindex_cong[where f = "Suc"])
using n0 by (auto simp add: expand_fun_eq ring_simps)
have ?thesis apply (simp add: pochhammer_def)
unfolding setprod_insert[OF th0, unfolded eq]
using th1 by (simp add: ring_simps)}
ultimately show ?thesis by blast
qed
lemma pochhammer_fact: "of_nat (fact n) = pochhammer 1 n"
unfolding fact_altdef_nat
apply (cases n, simp_all add: of_nat_setprod pochhammer_Suc_setprod)
apply (rule setprod_reindex_cong[where f=Suc])
by (auto simp add: expand_fun_eq)
lemma pochhammer_of_nat_eq_0_lemma: assumes kn: "k > n"
shows "pochhammer (- (of_nat n :: 'a:: idom)) k = 0"
proof-
from kn obtain h where h: "k = Suc h" by (cases k, auto)
{assume n0: "n=0" then have ?thesis using kn
by (cases k, simp_all add: pochhammer_rec del: pochhammer_Suc)}
moreover
{assume n0: "n \<noteq> 0"
then have ?thesis apply (simp add: h pochhammer_Suc_setprod)
apply (rule_tac x="n" in bexI)
using h kn by auto}
ultimately show ?thesis by blast
qed
lemma pochhammer_of_nat_eq_0_lemma': assumes kn: "k \<le> n"
shows "pochhammer (- (of_nat n :: 'a:: {idom, ring_char_0})) k \<noteq> 0"
proof-
{assume "k=0" then have ?thesis by simp}
moreover
{fix h assume h: "k = Suc h"
then have ?thesis apply (simp add: pochhammer_Suc_setprod)
using h kn by (auto simp add: algebra_simps)}
ultimately show ?thesis by (cases k, auto)
qed
lemma pochhammer_of_nat_eq_0_iff:
shows "pochhammer (- (of_nat n :: 'a:: {idom, ring_char_0})) k = 0 \<longleftrightarrow> k > n"
(is "?l = ?r")
using pochhammer_of_nat_eq_0_lemma[of n k, where ?'a='a]
pochhammer_of_nat_eq_0_lemma'[of k n, where ?'a = 'a]
by (auto simp add: not_le[symmetric])
lemma pochhammer_eq_0_iff:
"pochhammer a n = (0::'a::field_char_0) \<longleftrightarrow> (EX k < n . a = - of_nat k) "
apply (auto simp add: pochhammer_of_nat_eq_0_iff)
apply (cases n, auto simp add: pochhammer_def algebra_simps group_add_class.eq_neg_iff_add_eq_0)
apply (rule_tac x=x in exI)
apply auto
done
lemma pochhammer_eq_0_mono:
"pochhammer a n = (0::'a::field_char_0) \<Longrightarrow> m \<ge> n \<Longrightarrow> pochhammer a m = 0"
unfolding pochhammer_eq_0_iff by auto
lemma pochhammer_neq_0_mono:
"pochhammer a m \<noteq> (0::'a::field_char_0) \<Longrightarrow> m \<ge> n \<Longrightarrow> pochhammer a n \<noteq> 0"
unfolding pochhammer_eq_0_iff by auto
lemma pochhammer_minus:
assumes kn: "k \<le> n"
shows "pochhammer (- b) k = ((- 1) ^ k :: 'a::comm_ring_1) * pochhammer (b - of_nat k + 1) k"
proof-
{assume k0: "k = 0" then have ?thesis by simp}
moreover
{fix h assume h: "k = Suc h"
have eq: "((- 1) ^ Suc h :: 'a) = setprod (%i. - 1) {0 .. h}"
using setprod_constant[where A="{0 .. h}" and y="- 1 :: 'a"]
by auto
have ?thesis
unfolding h h pochhammer_Suc_setprod eq setprod_timesf[symmetric]
apply (rule strong_setprod_reindex_cong[where f = "%i. h - i"])
apply (auto simp add: inj_on_def image_def h )
apply (rule_tac x="h - x" in bexI)
by (auto simp add: expand_fun_eq h of_nat_diff)}
ultimately show ?thesis by (cases k, auto)
qed
lemma pochhammer_minus':
assumes kn: "k \<le> n"
shows "pochhammer (b - of_nat k + 1) k = ((- 1) ^ k :: 'a::comm_ring_1) * pochhammer (- b) k"
unfolding pochhammer_minus[OF kn, where b=b]
unfolding mult_assoc[symmetric]
unfolding power_add[symmetric]
apply simp
done
lemma pochhammer_same: "pochhammer (- of_nat n) n = ((- 1) ^ n :: 'a::comm_ring_1) * of_nat (fact n)"
unfolding pochhammer_minus[OF le_refl[of n]]
by (simp add: of_nat_diff pochhammer_fact)
subsection{* Generalized binomial coefficients *}
definition gbinomial :: "'a::field_char_0 \<Rightarrow> nat \<Rightarrow> 'a" (infixl "gchoose" 65)
where "a gchoose n = (if n = 0 then 1 else (setprod (\<lambda>i. a - of_nat i) {0 .. n - 1}) / of_nat (fact n))"
lemma gbinomial_0[simp]: "a gchoose 0 = 1" "0 gchoose (Suc n) = 0"
apply (simp_all add: gbinomial_def)
apply (subgoal_tac "(\<Prod>i\<Colon>nat\<in>{0\<Colon>nat..n}. - of_nat i) = (0::'b)")
apply (simp del:setprod_zero_iff)
apply simp
done
lemma gbinomial_pochhammer: "a gchoose n = (- 1) ^ n * pochhammer (- a) n / of_nat (fact n)"
proof-
{assume "n=0" then have ?thesis by simp}
moreover
{assume n0: "n\<noteq>0"
from n0 setprod_constant[of "{0 .. n - 1}" "- (1:: 'a)"]
have eq: "(- (1\<Colon>'a)) ^ n = setprod (\<lambda>i. - 1) {0 .. n - 1}"
by auto
from n0 have ?thesis
by (simp add: pochhammer_def gbinomial_def field_simps eq setprod_timesf[symmetric])}
ultimately show ?thesis by blast
qed
lemma binomial_fact_lemma:
"k \<le> n \<Longrightarrow> fact k * fact (n - k) * (n choose k) = fact n"
proof(induct n arbitrary: k rule: nat_less_induct)
fix n k assume H: "\<forall>m<n. \<forall>x\<le>m. fact x * fact (m - x) * (m choose x) =
fact m" and kn: "k \<le> n"
let ?ths = "fact k * fact (n - k) * (n choose k) = fact n"
{assume "n=0" then have ?ths using kn by simp}
moreover
{assume "k=0" then have ?ths using kn by simp}
moreover
{assume nk: "n=k" then have ?ths by simp}
moreover
{fix m h assume n: "n = Suc m" and h: "k = Suc h" and hm: "h < m"
from n have mn: "m < n" by arith
from hm have hm': "h \<le> m" by arith
from hm h n kn have km: "k \<le> m" by arith
have "m - h = Suc (m - Suc h)" using h km hm by arith
with km h have th0: "fact (m - h) = (m - h) * fact (m - k)"
by simp
from n h th0
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))"
by (simp add: ring_simps)
also have "\<dots> = (k + (m - h)) * fact m"
using H[rule_format, OF mn hm'] H[rule_format, OF mn km]
by (simp add: ring_simps)
finally have ?ths using h n km by simp}
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
ultimately show ?ths by blast
qed
lemma binomial_fact:
assumes kn: "k \<le> n"
shows "(of_nat (n choose k) :: 'a::field_char_0) = of_nat (fact n) / (of_nat (fact k) * of_nat (fact (n - k)))"
using binomial_fact_lemma[OF kn]
by (simp add: field_simps of_nat_mult[symmetric])
lemma binomial_gbinomial: "of_nat (n choose k) = of_nat n gchoose k"
proof-
{assume kn: "k > n"
from kn binomial_eq_0[OF kn] have ?thesis
by (simp add: gbinomial_pochhammer field_simps
pochhammer_of_nat_eq_0_iff)}
moreover
{assume "k=0" then have ?thesis by simp}
moreover
{assume kn: "k \<le> n" and k0: "k\<noteq> 0"
from k0 obtain h where h: "k = Suc h" by (cases k, auto)
from h
have eq:"(- 1 :: 'a) ^ k = setprod (\<lambda>i. - 1) {0..h}"
by (subst setprod_constant, auto)
have eq': "(\<Prod>i\<in>{0..h}. of_nat n + - (of_nat i :: 'a)) = (\<Prod>i\<in>{n - h..n}. of_nat i)"
apply (rule strong_setprod_reindex_cong[where f="op - n"])
using h kn
apply (simp_all add: inj_on_def image_iff Bex_def expand_set_eq)
apply clarsimp
apply (presburger)
apply presburger
by (simp add: expand_fun_eq ring_simps of_nat_add[symmetric] del: of_nat_add)
have th0: "finite {1..n - Suc h}" "finite {n - h .. n}"
"{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
from eq[symmetric]
have ?thesis using kn
apply (simp add: binomial_fact[OF kn, where ?'a = 'a]
gbinomial_pochhammer field_simps pochhammer_Suc_setprod)
apply (simp add: pochhammer_Suc_setprod fact_altdef_nat h of_nat_setprod setprod_timesf[symmetric] eq' del: One_nat_def power_Suc)
unfolding setprod_Un_disjoint[OF th0, unfolded eq3, of "of_nat:: nat \<Rightarrow> 'a"] eq[unfolded h]
unfolding mult_assoc[symmetric]
unfolding setprod_timesf[symmetric]
apply simp
apply (rule strong_setprod_reindex_cong[where f= "op - n"])
apply (auto simp add: inj_on_def image_iff Bex_def)
apply presburger
apply (subgoal_tac "(of_nat (n - x) :: 'a) = of_nat n - of_nat x")
apply simp
by (rule of_nat_diff, simp)
}
moreover
have "k > n \<or> k = 0 \<or> (k \<le> n \<and> k \<noteq> 0)" by arith
ultimately show ?thesis by blast
qed
lemma gbinomial_1[simp]: "a gchoose 1 = a"
by (simp add: gbinomial_def)
lemma gbinomial_Suc0[simp]: "a gchoose (Suc 0) = a"
by (simp add: gbinomial_def)
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")
proof-
have "?r = ((- 1) ^n * pochhammer (- a) n / of_nat (fact n)) * (of_nat n - (- a + of_nat n))"
unfolding gbinomial_pochhammer
pochhammer_Suc fact_Suc of_nat_mult right_diff_distrib power_Suc
by (simp add: field_simps del: of_nat_Suc)
also have "\<dots> = ?l" unfolding gbinomial_pochhammer
by (simp add: ring_simps)
finally show ?thesis ..
qed
lemma gbinomial_mult_1': "(a gchoose n) * a = of_nat n * (a gchoose n) + of_nat (Suc n) * (a gchoose (Suc n))"
by (simp add: mult_commute gbinomial_mult_1)
lemma gbinomial_Suc: "a gchoose (Suc k) = (setprod (\<lambda>i. a - of_nat i) {0 .. k}) / of_nat (fact (Suc k))"
by (simp add: gbinomial_def)
lemma gbinomial_mult_fact:
"(of_nat (fact (Suc k)) :: 'a) * ((a::'a::field_char_0) gchoose (Suc k)) = (setprod (\<lambda>i. a - of_nat i) {0 .. k})"
unfolding gbinomial_Suc
by (simp_all add: field_simps del: fact_Suc)
lemma gbinomial_mult_fact':
"((a::'a::field_char_0) gchoose (Suc k)) * (of_nat (fact (Suc k)) :: 'a) = (setprod (\<lambda>i. a - of_nat i) {0 .. k})"
using gbinomial_mult_fact[of k a]
apply (subst mult_commute) .
lemma gbinomial_Suc_Suc: "((a::'a::field_char_0) + 1) gchoose (Suc k) = a gchoose k + (a gchoose (Suc k))"
proof-
{assume "k = 0" then have ?thesis by simp}
moreover
{fix h assume h: "k = Suc h"
have eq0: "(\<Prod>i\<in>{1..k}. (a + 1) - of_nat i) = (\<Prod>i\<in>{0..h}. a - of_nat i)"
apply (rule strong_setprod_reindex_cong[where f = Suc])
using h by auto
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)"
unfolding h
apply (simp add: ring_simps del: fact_Suc)
unfolding gbinomial_mult_fact'
apply (subst fact_Suc)
unfolding of_nat_mult
apply (subst mult_commute)
unfolding mult_assoc
unfolding gbinomial_mult_fact
by (simp add: ring_simps)
also have "\<dots> = (\<Prod>i\<in>{0..h}. a - of_nat i) * (a + 1)"
unfolding gbinomial_mult_fact' setprod_nat_ivl_Suc
by (simp add: ring_simps h)
also have "\<dots> = (\<Prod>i\<in>{0..k}. (a + 1) - of_nat i)"
using eq0
unfolding h setprod_nat_ivl_1_Suc
by simp
also have "\<dots> = of_nat (fact (Suc k)) * ((a + 1) gchoose (Suc k))"
unfolding gbinomial_mult_fact ..
finally have ?thesis by (simp del: fact_Suc) }
ultimately show ?thesis by (cases k, auto)
qed
lemma binomial_symmetric: assumes kn: "k \<le> n"
shows "n choose k = n choose (n - k)"
proof-
from kn have kn': "n - k \<le> n" by arith
from binomial_fact_lemma[OF kn] binomial_fact_lemma[OF kn']
have "fact k * fact (n - k) * (n choose k) = fact (n - k) * fact (n - (n - k)) * (n choose (n - k))" by simp
then show ?thesis using kn by simp
qed
end