Theory Binomial (Isabelle2005: October 2005)

# Theory Binomial

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theory Binomial
imports GCD
begin

    (*  Title:      HOL/Binomial.thy
        ID:         $Id: Binomial.thy,v 1.2 2005/09/20 12:03:37 wenzelm Exp $
        Author:     Lawrence C Paulson
        Copyright   1997  University of Cambridge
    *)

    header{*Binomial Coefficients*}

    theory Binomial
    imports GCD
    begin

    text{*This development is based on the work of Andy Gordon and
    Florian Kammueller*}

    consts
      binomial :: "nat => nat => nat"      (infixl "choose" 65)

    primrec
      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 [rule_format]: "∀k. n < k --> (n choose k) = 0"
    apply (induct "n", auto)
    apply (erule allE)
    apply (erule mp, arith)
    done

    declare binomial_0 [simp del] binomial_Suc [simp del]

    lemma binomial_n_n [simp]: "(n choose n) = 1"
    apply (induct "n")
    apply (simp_all add: binomial_eq_0)
    done

    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 [rule_format]: "k ≤ n --> 0 < (n choose k)"
    by (rule_tac m = n and n = k in 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: "(0 < n choose k) = (k≤n)"
    by (simp add: linorder_not_less [symmetric] binomial_eq_0_iff [symmetric])

    (*Might be more useful if re-oriented*)
    lemma Suc_times_binomial_eq [rule_format]:
         "∀k. k ≤ n --> Suc n * (n choose k) = (Suc n choose Suc k) * Suc k"
    apply (induct "n")
    apply (simp add: binomial_0, clarify)
    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 ≤ 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 ≤ 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

    subsubsection {* 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 ⊆ A & card B = 0} = 1"
      apply (simp cong add: conj_cong add: finite_subset [THEN card_0_eq])
      apply (simp cong add: rev_conj_cong)
      done

    lemma choose_deconstruct: "finite M ==> x ∉ 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 ∉ xa", auto)
      apply (erule rev_mp, subst card_Diff_singleton)
      apply (auto intro: finite_subset)
      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 ∉ 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)
       txt {* finiteness of the two sets *}
       apply (rule_tac [2] B = "Pow (A)" in finite_subset)
       apply (rule_tac B = "Pow (insert x A)" in finite_subset)
       apply fast+
      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 = (∑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} ∪ {n+1} ∪ {1..n}"
        by (auto simp add:atLeastAtMost_def atLeast_def atMost_def)
      have decomp2: "{0..n} = {0} ∪ {1..n}"
        by (auto simp add:atLeastAtMost_def atLeast_def atMost_def)
      have "(a+b::nat)^(n+1) = (a+b) * (∑k=0..n. (n choose k) * a^k * b^(n-k))"
        using Suc by simp
      also have "… =  a*(∑k=0..n. (n choose k) * a^k * b^(n-k)) +
                       b*(∑k=0..n. (n choose k) * a^k * b^(n-k))"
        by(rule nat_distrib)
      also have "… = (∑k=0..n. (n choose k) * a^(k+1) * b^(n-k)) +
                      (∑k=0..n. (n choose k) * a^k * b^(n-k+1))"
        by(simp add: setsum_mult mult_ac)
      also have "… = (∑k=0..n. (n choose k) * a^k * b^(n+1-k)) +
                      (∑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 "… = a^(n+1) + b^(n+1) +
                      (∑k=1..n. (n choose (k - 1)) * a^k * b^(n+1-k)) +
                      (∑k=1..n. (n choose k) * a^k * b^(n+1-k))"
        by(simp add: decomp2)
      also have
        "… = a^(n+1) + b^(n+1) + (∑k=1..n. (n+1 choose k) * a^k * b^(n+1-k))"
        by(simp add: nat_distrib setsum_addf binomial.simps)
      also have "… = (∑k=0..n+1. (n+1 choose k) * a^k * b^(n+1-k))"
        using decomp by simp
      finally show ?case by simp
    qed

    end

lemma binomial_n_0:

      n choose 0 = 1

lemma binomial_0_Suc:

      0 choose Suc k = 0

lemma binomial_Suc_Suc:

      Suc n choose Suc k = n choose k + (n choose Suc k)

lemma binomial_eq_0:

      n < k ==> n choose k = 0

lemma binomial_n_n:

      n choose n = 1

lemma binomial_Suc_n:

      Suc n choose n = Suc n

lemma binomial_1:

      n choose Suc 0 = n

lemma zero_less_binomial:

      k ≤ n ==> 0 < n choose k

lemma binomial_eq_0_iff:

      (n choose k = 0) = (n < k)

lemma zero_less_binomial_iff:

      (0 < n choose k) = (k ≤ n)

lemma Suc_times_binomial_eq:

      k ≤ n ==> Suc n * (n choose k) = (Suc n choose Suc k) * Suc k

lemma binomial_Suc_Suc_eq_times:

      k ≤ n ==> Suc n choose Suc k = Suc n * (n choose k) div Suc k

lemma times_binomial_minus1_eq:

      [| k ≤ n; 0 < k |] ==> (n choose k) * k = n * (n - 1 choose (k - 1))

#### Theorems about @{text "choose"}

lemma card_s_0_eq_empty:

      finite A ==> card {B. B ⊆ A ∧ card B = 0} = 1

lemma choose_deconstruct:

      [| finite M; x ∉ M |]
      ==> {s. s ⊆ insert x M ∧ card s = Suc k} =
          {s. s ⊆ M ∧ card s = Suc k} ∪ {s. ∃t⊆M. card t = k ∧ s = insert x t}

lemma constr_bij:

      [| finite A; x ∉ A |]
      ==> card {B. ∃C⊆A. card C = k ∧ B = insert x C} = card {B. B ⊆ A ∧ card B = k}

lemma n_sub_lemma:

      finite A ==> card {B. B ⊆ A ∧ card B = k} = card A choose k

theorem n_subsets:

      finite A ==> card {B. B ⊆ A ∧ card B = k} = card A choose k

theorem binomial:

      (a + b) ^ n = (∑k = 0..n. (n choose k) * a ^ k * b ^ (n - k))