src/HOL/Real/HahnBanach/ZornLemma.thy
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HahnBanach update by Gertrud Bauer;
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(*  Title:      HOL/Real/HahnBanach/ZornLemma.thy
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    ID:         $Id$
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    Author:     Gertrud Bauer, TU Munich
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*)
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header {* Zorn's Lemma *};
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theory ZornLemma = Aux + Zorn:;
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text{* 
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Zorn's Lemmas says: if every linear ordered subset of an ordered set 
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$S$ has an upper bound in $S$, then there exists a maximal element in $S$.
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In our application $S$ is a set of sets, ordered by set inclusion. Since 
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the union of a chain of sets is an upperbound for all elements of the 
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chain, the conditions of Zorn's lemma can be modified:
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If $S$ is non-empty, it suffices to show that for every non-empty 
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chain $c$ in $S$ the union of $c$ also lies in $S$:
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*};
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theorem Zorn's_Lemma: 
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  "a:S ==> (!!c. c: chain S ==> EX x. x:c ==> Union c : S) 
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  ==>  EX y: S. ALL z: S. y <= z --> y = z";
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proof (rule Zorn_Lemma2);
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  assume aS: "a:S";
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  assume r: "!!c. c: chain S ==> EX x. x:c ==> Union c : S";
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  show "ALL c:chain S. EX y:S. ALL z:c. z <= y";
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  proof;
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    fix c; assume "c:chain S"; 
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    show "EX y:S. ALL z:c. z <= y";
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    proof (rule case_split);
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      txt{* If $c$ is an empty chain, then every element
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      in $S$ is an upperbound of $c$. *};
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      assume "c={}"; 
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      with aS; show ?thesis; by fast;
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      txt{* If $c$ is non-empty, then $\cup\; c$ 
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      is an upperbound of $c$, that lies in $S$. *};
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    next;
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      assume c: "c~={}";
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      show ?thesis; 
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      proof; 
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        show "ALL z:c. z <= Union c"; by fast;
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        show "Union c : S"; 
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        proof (rule r);
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          from c; show "EX x. x:c"; by fast;  
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        qed;
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      qed;
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    qed;
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  qed;
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qed;
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end;