February 18th, 2009, 12:58 PM  #1 
Newbie Joined: Feb 2009 Posts: 1 Thanks: 0  Closed/Open sets
Hey guys. I'm stuck on a problem and I need some direction/help on how to solve it: Prove that a set U c M is open IFF none of its points are limits of its compliment. Thanks! 
February 18th, 2009, 04:58 PM  #2 
Senior Member Joined: May 2008 From: York, UK Posts: 1,300 Thanks: 0  Re: Closed/Open sets
Hint: A set is open iff for every point there exists an such that the ball yx<\varepsilon\}" /> lies entirely in U. 

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