src/HOL/Wellfounded_Relations.thy
author wenzelm
Mon Nov 28 10:58:40 2005 +0100 (2005-11-28)
changeset 18277 6c2b039b4847
parent 15352 cba05842bd7a
child 19404 9bf2cdc9e8e8
permissions -rw-r--r--
added proof of measure_induct_rule;
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(*  ID:   $Id$
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    Author:     Konrad Slind
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    Copyright   1995 TU Munich
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*)
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header {*Well-founded Relations*}
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theory Wellfounded_Relations
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imports Finite_Set
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begin
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text{*Derived WF relations such as inverse image, lexicographic product and
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measure. The simple relational product, in which @{term "(x',y')"} precedes
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@{term "(x,y)"} if @{term "x'<x"} and @{term "y'<y"}, is a subset of the
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lexicographic product, and therefore does not need to be defined separately.*}
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constdefs
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 less_than :: "(nat*nat)set"
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    "less_than == trancl pred_nat"
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 measure   :: "('a => nat) => ('a * 'a)set"
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    "measure == inv_image less_than"
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 lex_prod  :: "[('a*'a)set, ('b*'b)set] => (('a*'b)*('a*'b))set"
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               (infixr "<*lex*>" 80)
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    "ra <*lex*> rb == {((a,b),(a',b')). (a,a') : ra | a=a' & (b,b') : rb}"
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 finite_psubset  :: "('a set * 'a set) set"
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   --{* finite proper subset*}
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    "finite_psubset == {(A,B). A < B & finite B}"
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 same_fst :: "('a => bool) => ('a => ('b * 'b)set) => (('a*'b)*('a*'b))set"
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    "same_fst P R == {((x',y'),(x,y)) . x'=x & P x & (y',y) : R x}"
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   --{*For @{text rec_def} declarations where the first n parameters
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       stay unchanged in the recursive call. 
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       See @{text "Library/While_Combinator.thy"} for an application.*}
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subsection{*Measure Functions make Wellfounded Relations*}
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subsubsection{*`Less than' on the natural numbers*}
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lemma wf_less_than [iff]: "wf less_than"
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by (simp add: less_than_def wf_pred_nat [THEN wf_trancl])
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lemma trans_less_than [iff]: "trans less_than"
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by (simp add: less_than_def trans_trancl)
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lemma less_than_iff [iff]: "((x,y): less_than) = (x<y)"
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by (simp add: less_than_def less_def)
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lemma full_nat_induct:
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  assumes ih: "(!!n. (ALL m. Suc m <= n --> P m) ==> P n)"
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  shows "P n"
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apply (rule wf_less_than [THEN wf_induct])
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apply (rule ih, auto)
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done
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subsubsection{*The Inverse Image into a Wellfounded Relation is Wellfounded.*}
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lemma wf_inv_image [simp,intro!]: "wf(r) ==> wf(inv_image r (f::'a=>'b))"
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apply (simp (no_asm_use) add: inv_image_def wf_eq_minimal)
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apply clarify
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apply (subgoal_tac "EX (w::'b) . w : {w. EX (x::'a) . x: Q & (f x = w) }")
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prefer 2 apply (blast del: allE)
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apply (erule allE)
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apply (erule (1) notE impE)
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apply blast
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done
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subsubsection{*Finally, All Measures are Wellfounded.*}
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lemma wf_measure [iff]: "wf (measure f)"
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apply (unfold measure_def)
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apply (rule wf_less_than [THEN wf_inv_image])
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done
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lemma measure_induct_rule [case_names less]:
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  fixes f :: "'a \<Rightarrow> nat"
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  assumes step: "\<And>x. (\<And>y. f y < f x \<Longrightarrow> P y) \<Longrightarrow> P x"
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  shows "P a"
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proof -
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  have "wf (measure f)" ..
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  then show ?thesis
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  proof induct
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    case (less x)
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    show ?case
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    proof (rule step)
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      fix y
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      assume "f y < f x"
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      then have "(y, x) \<in> measure f"
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        by (simp add: measure_def inv_image_def)
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      then show "P y" by (rule less)
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    qed
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  qed
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qed
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lemma measure_induct:
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  fixes f :: "'a \<Rightarrow> nat"
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  shows "(\<And>x. \<forall>y. f y < f x \<longrightarrow> P y \<Longrightarrow> P x) \<Longrightarrow> P a"
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  by (rule measure_induct_rule [of f P a]) iprover
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subsection{*Other Ways of Constructing Wellfounded Relations*}
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text{*Wellfoundedness of lexicographic combinations*}
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lemma wf_lex_prod [intro!]: "[| wf(ra); wf(rb) |] ==> wf(ra <*lex*> rb)"
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apply (unfold wf_def lex_prod_def) 
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apply (rule allI, rule impI)
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apply (simp (no_asm_use) only: split_paired_All)
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apply (drule spec, erule mp) 
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apply (rule allI, rule impI)
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apply (drule spec, erule mp, blast) 
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done
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text{*Transitivity of WF combinators.*}
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lemma trans_lex_prod [intro!]: 
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    "[| trans R1; trans R2 |] ==> trans (R1 <*lex*> R2)"
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by (unfold trans_def lex_prod_def, blast) 
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subsubsection{*Wellfoundedness of proper subset on finite sets.*}
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lemma wf_finite_psubset: "wf(finite_psubset)"
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apply (unfold finite_psubset_def)
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apply (rule wf_measure [THEN wf_subset])
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apply (simp add: measure_def inv_image_def less_than_def less_def [symmetric])
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apply (fast elim!: psubset_card_mono)
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done
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lemma trans_finite_psubset: "trans finite_psubset"
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by (simp add: finite_psubset_def psubset_def trans_def, blast)
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subsubsection{*Wellfoundedness of finite acyclic relations*}
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text{*This proof belongs in this theory because it needs Finite.*}
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lemma finite_acyclic_wf [rule_format]: "finite r ==> acyclic r --> wf r"
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apply (erule finite_induct, blast)
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apply (simp (no_asm_simp) only: split_tupled_all)
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apply simp
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done
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lemma finite_acyclic_wf_converse: "[|finite r; acyclic r|] ==> wf (r^-1)"
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apply (erule finite_converse [THEN iffD2, THEN finite_acyclic_wf])
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apply (erule acyclic_converse [THEN iffD2])
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done
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lemma wf_iff_acyclic_if_finite: "finite r ==> wf r = acyclic r"
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by (blast intro: finite_acyclic_wf wf_acyclic)
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subsubsection{*Wellfoundedness of @{term same_fst}*}
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lemma same_fstI [intro!]:
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     "[| P x; (y',y) : R x |] ==> ((x,y'),(x,y)) : same_fst P R"
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by (simp add: same_fst_def)
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lemma wf_same_fst:
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  assumes prem: "(!!x. P x ==> wf(R x))"
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  shows "wf(same_fst P R)"
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apply (simp cong del: imp_cong add: wf_def same_fst_def)
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apply (intro strip)
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apply (rename_tac a b)
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apply (case_tac "wf (R a)")
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 apply (erule_tac a = b in wf_induct, blast)
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apply (blast intro: prem)
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done
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subsection{*Weakly decreasing sequences (w.r.t. some well-founded order) 
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   stabilize.*}
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text{*This material does not appear to be used any longer.*}
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lemma lemma1: "[| ALL i. (f (Suc i), f i) : r^* |] ==> (f (i+k), f i) : r^*"
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apply (induct_tac "k", simp_all)
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apply (blast intro: rtrancl_trans)
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done
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lemma lemma2: "[| ALL i. (f (Suc i), f i) : r^*; wf (r^+) |]  
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      ==> ALL m. f m = x --> (EX i. ALL k. f (m+i+k) = f (m+i))"
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apply (erule wf_induct, clarify)
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apply (case_tac "EX j. (f (m+j), f m) : r^+")
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 apply clarify
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 apply (subgoal_tac "EX i. ALL k. f ((m+j) +i+k) = f ( (m+j) +i) ")
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  apply clarify
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  apply (rule_tac x = "j+i" in exI)
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  apply (simp add: add_ac, blast)
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apply (rule_tac x = 0 in exI, clarsimp)
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apply (drule_tac i = m and k = k in lemma1)
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apply (blast elim: rtranclE dest: rtrancl_into_trancl1)
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done
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lemma wf_weak_decr_stable: "[| ALL i. (f (Suc i), f i) : r^*; wf (r^+) |]  
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      ==> EX i. ALL k. f (i+k) = f i"
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apply (drule_tac x = 0 in lemma2 [THEN spec], auto)
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done
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(* special case of the theorem above: <= *)
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lemma weak_decr_stable:
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     "ALL i. f (Suc i) <= ((f i)::nat) ==> EX i. ALL k. f (i+k) = f i"
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apply (rule_tac r = pred_nat in wf_weak_decr_stable)
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apply (simp add: pred_nat_trancl_eq_le)
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apply (intro wf_trancl wf_pred_nat)
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done
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ML
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{*
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val less_than_def = thm "less_than_def";
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val measure_def = thm "measure_def";
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val lex_prod_def = thm "lex_prod_def";
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val finite_psubset_def = thm "finite_psubset_def";
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val wf_less_than = thm "wf_less_than";
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val trans_less_than = thm "trans_less_than";
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val less_than_iff = thm "less_than_iff";
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val full_nat_induct = thm "full_nat_induct";
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val wf_inv_image = thm "wf_inv_image";
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val wf_measure = thm "wf_measure";
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val measure_induct = thm "measure_induct";
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val wf_lex_prod = thm "wf_lex_prod";
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val trans_lex_prod = thm "trans_lex_prod";
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val wf_finite_psubset = thm "wf_finite_psubset";
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val trans_finite_psubset = thm "trans_finite_psubset";
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val finite_acyclic_wf = thm "finite_acyclic_wf";
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val finite_acyclic_wf_converse = thm "finite_acyclic_wf_converse";
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val wf_iff_acyclic_if_finite = thm "wf_iff_acyclic_if_finite";
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val wf_weak_decr_stable = thm "wf_weak_decr_stable";
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val weak_decr_stable = thm "weak_decr_stable";
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val same_fstI = thm "same_fstI";
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val wf_same_fst = thm "wf_same_fst";
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*}
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end