When is a topological group Hausdorff (separated)? - MathOverflow

When is a topological group Hausdorff (separated)?

2012-06-13T17:01:46Z

Does someone knows a good reference for the following result?

"A topological group is Hausdorff if and only if the identity is closed."

I have seen proofs in lecture notes of courses on the web, but I would like a reference in a book or an article, in order to refer to it.

Answer by Greg Marks for When is a topological group Hausdorff (separated)?

2012-06-13T17:31:03Z

You can probably find this result in a million places, one of which is N. Bourbaki, General Topology, Part 1, Chapter 3, Section 1.2, p. 223, Proposition 2.

Answer by Igor Rivin for When is a topological group Hausdorff (separated)?

2012-06-13T17:35:23Z

Bourbaki, General Topology, III.2.5, prop 13. This is from an answer to this question.

Answer by AliReza Olfati for When is a topological group Hausdorff (separated)?

2012-06-13T20:02:59Z

You could find a routin proof in the book "Topological Ring" written by Seth Warner. In this book at page 21 in Theorem 3.4 you could see the following Proposition:

Theorem: Let $G$ be a topological group. The following statements are equivalent:

{$0$} is closed.
{$0$} is an intersection of the neighborhoods of zero.
$G$ is Hausdorff.
$G$ is regular.

You could also find the improvement of it in the book "Topology for analysis" Written by Albert Wilansky. In section 12 at page 243 You could see the following theorem:

THEQREM: Every topological group is completely regular. The following conditions on a topological group $G$ are equivalent:

$G$ is a $T_0$ space.
$G$ is a Tychonoff space.-
$\cap${$U:U$ is a is a neighborhood of $e$}={$e$}

The Proof of Complete regularity Has more details. I think The proof is in the level of Urysohn Lemma.

But if you are interested in the general case, I Suggest You look at the section "uniformity" which were discussed in the following books:

Topology for Analysis: Chapter 11
General topology: Stephen Willard: chapter 9