Drexel dragonThe Math ForumDonate to the Math Forum

Ask Dr. Math - Questions and Answers from our Archives
_____________________________________________
Associated Topics || Dr. Math Home || Search Dr. Math
_____________________________________________

Closed Sets


Date: 02/27/99 at 07:37:29
From: Michael
Subject: Closed Sets Problem

We showed in class that a set is closed if and only if it contains all 
of its limit points. I want to know how to use this property to show 
that a union of finite closed sets and the intersection of any number 
of closed sets are closed.


Date: 02/27/99 at 07:50:43
From: Doctor Allan
Subject: Re: Closed sets problem

Assume the union of closed sets C_1, C_2,..., C_n is not closed. This 
means that there is a limit point, x,  outside the union. All limit 
points of the union must belong to one of the C_i's (why?), so x must 
belong to one of the C_i's. Does it belong to C_1? No, because if it 
did, it would belong to the union as well. In addition, it does not 
belong to any of the other C_i's, yielding a contradiction. Therefore 
the union must be closed.

Similar reasoning can be used for your question on intersection.

Sincerely,

- Doctor Allan, The Math Forum
  http://mathforum.org/dr.math/   
    
Associated Topics:
College Logic
High School Logic
High School Sets

Search the Dr. Math Library:


Find items containing (put spaces between keywords):
 
Click only once for faster results:

[ Choose "whole words" when searching for a word like age.]

all keywords, in any order at least one, that exact phrase
parts of words whole words

Submit your own question to Dr. Math

[Privacy Policy] [Terms of Use]

_____________________________________
Math Forum Home || Math Library || Quick Reference || Math Forum Search
_____________________________________

Ask Dr. MathTM
© 1994-2013 The Math Forum
http://mathforum.org/dr.math/