author  paulson 
Tue, 09 Jul 2002 23:05:26 +0200  
changeset 13328  703de709a64b 
parent 13269  3ba9be497c33 
child 14171  0cab06e3bbd0 
permissions  rwrr 
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(* Title: ZF/AC.thy 
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ID: $Id$ 
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Author: Lawrence C Paulson, Cambridge University Computer Laboratory 
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Copyright 1994 University of Cambridge 
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*) 
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header{*The Axiom of Choice*} 
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theory AC = Main: 
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text{*This definition comes from Halmos (1960), page 59.*} 
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axioms AC: "[ a: A; !!x. x:A ==> (EX y. y:B(x)) ] ==> EX z. z : Pi(A,B)" 
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(*The same as AC, but no premise a \<in> A*) 

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lemma AC_Pi: "[ !!x. x \<in> A ==> (\<exists>y. y \<in> B(x)) ] ==> \<exists>z. z \<in> Pi(A,B)" 

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apply (case_tac "A=0") 

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apply (simp add: Pi_empty1) 
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(*The nontrivial case*) 
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apply (blast intro: AC) 

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done 

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(*Using dtac, this has the advantage of DELETING the universal quantifier*) 

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lemma AC_ball_Pi: "\<forall>x \<in> A. \<exists>y. y \<in> B(x) ==> \<exists>y. y \<in> Pi(A,B)" 

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apply (rule AC_Pi) 

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apply (erule bspec, assumption) 
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done 
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lemma AC_Pi_Pow: "\<exists>f. f \<in> (\<Pi>X \<in> Pow(C){0}. X)" 

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apply (rule_tac B1 = "%x. x" in AC_Pi [THEN exE]) 

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apply (erule_tac [2] exI, blast) 
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done 
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lemma AC_func: 
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"[ !!x. x \<in> A ==> (\<exists>y. y \<in> x) ] ==> \<exists>f \<in> A>Union(A). \<forall>x \<in> A. f`x \<in> x" 

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apply (rule_tac B1 = "%x. x" in AC_Pi [THEN exE]) 

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prefer 2 apply (blast dest: apply_type intro: Pi_type, blast) 
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done 
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lemma non_empty_family: "[ 0 \<notin> A; x \<in> A ] ==> \<exists>y. y \<in> x" 

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by (subgoal_tac "x \<noteq> 0", blast+) 
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lemma AC_func0: "0 \<notin> A ==> \<exists>f \<in> A>Union(A). \<forall>x \<in> A. f`x \<in> x" 
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apply (rule AC_func) 

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apply (simp_all add: non_empty_family) 

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done 

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lemma AC_func_Pow: "\<exists>f \<in> (Pow(C){0}) > C. \<forall>x \<in> Pow(C){0}. f`x \<in> x" 

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apply (rule AC_func0 [THEN bexE]) 

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apply (rule_tac [2] bexI) 

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prefer 2 apply assumption 
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apply (erule_tac [2] fun_weaken_type, blast+) 

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done 
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lemma AC_Pi0: "0 \<notin> A ==> \<exists>f. f \<in> (\<Pi>x \<in> A. x)" 

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apply (rule AC_Pi) 

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apply (simp_all add: non_empty_family) 

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done 

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end 