# Urysohn's lemma

by ehrenfest
Tags: lemma, urysohn
 P: 1,998 1. The problem statement, all variables and given/known data Urysohn's lemma My book says that the "if" part of Urysohn's lemma is obvious with no explanation. Can someone explain why? 2. Relevant equations 3. The attempt at a solution
 Math Emeritus Sci Advisor Thanks PF Gold P: 38,708 It would have been a good idea to actually state Urysohn's lemma as it is given in your book. Sometimes statements vary from one book to another. In particular you should note that Urysohn's lemma only applies in NORMAL spaces. What is the definition of a "Normal" topological space?
P: 1,998
 Quote by HallsofIvy It would have been a good idea to actually state Urysohn's lemma as it is given in your book. Sometimes statements vary from one book to another. In particular you should note that Urysohn's lemma only applies in NORMAL spaces. What is the definition of a "Normal" topological space?
Sorry. I meant to put a link to Wikipedia, which has the same statement of Urysohn's Lemma as that in my book.

http://en.wikipedia.org/wiki/Urysohns_lemma

It comes down to whether the sets [0,1/2) and (1/2,1] are open. Apparently this is obvious to other people, but it seems counterintuitive to me because I thought open sets were open intervals.

Emeritus
Sci Advisor
PF Gold
P: 16,101

## Urysohn's lemma

 Quote by ehrenfest Sorry. I meant to put a link to Wikipedia, which has the same statement of Urysohn's Lemma as that in my book. http://en.wikipedia.org/wiki/Urysohns_lemma It comes down to whether the sets [0,1/2) and (1/2,1] are open. Apparently this is obvious to other people, but it seems counterintuitive to me because I thought open sets were open intervals.
[0, 1/2) is not an open subset of R
[0, 1/2) is an open subset of [0, 1].
 P: 1,998 I guess that makes sense, since balls around around 0 can have no negative numbers in them, so they are really just half-balls.

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