Xerxes Posted October 21, 2005 Share Posted October 21, 2005 Say I have a set X and a topology T on X = {X {} A} i.e A is an open subset of T. Then the complement of A is X - A = Ac, which is closed. Now the interior of A, int(A) is the largest open set (or the union of all open sets) contained in A which is A, and the closure of A, cl(A) is the smallest closed set in {X {} Ac} containing A which is X. So if bd(A) = cl(A) - int(A), we have that bd(A) = X - A = Ac. Similarly, the closure of Ac is the smallest closed set containing Ac, which is Ac = X - A. So, by an alternative definition of the boundary of A, cl(A) intersect cl(Ac) = X intersect (X - A) which is X - A = Ac. I've tried it out on a number of arbitrary topologies of my own devising, and the answer is always the same, except where sets in T are both open and closed or neither. Surely it's not right in general, though? Hmm. Does that make sense? (just back from the pub) Link to comment Share on other sites More sharing options...
Dave Posted October 21, 2005 Share Posted October 21, 2005 Say I have a set X and a topology T on X = {X {} A} i.e A is an open subset of T. Then the complement of A is X - A = Ac, which is closed. I'm assuming that this means [imath]\mathcal{T} = \left\{ X, \phi, A \right\}[/imath] for some [imath]A \subset X[/imath]? Or am I being silly? Link to comment Share on other sites More sharing options...
Xerxes Posted October 21, 2005 Author Share Posted October 21, 2005 I'm assuming that this means [imath]\mathcal{T} = \left\{ X, \phi, A \right\}[/imath] for some [imath]A \subset X[/imath']? Or am I being silly? Yes that's exactly what I meant! Was there any ambiguity in not bothering with LaTeX? As to substance......? Link to comment Share on other sites More sharing options...
Dave Posted October 21, 2005 Share Posted October 21, 2005 I can't see an error in your reasoning. I'll have a look at it tomorrow when I'm not so tired Link to comment Share on other sites More sharing options...
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