# regular spaces

• Mar 10th 2009, 10:22 PM
Andreamet
regular spaces
Prove that the product of two regular spaces is regular.
• Mar 11th 2009, 01:19 AM
aliceinwonderland
Quote:

Originally Posted by Andreamet
Prove that the product of two regular spaces is regular.

Lemma 1. A $T_1$-space is regular iff for each a in X and each open set U containing a ,there is an open set W containing a whose closure is contained in U.

Let $X_1, X_2$ be regular spaces.
We need to show $X = X_1 \times X_2$ is regular.

Pick an arbitrary point a in X and an open set U containing a. By lemma 1, it is sufficient to show that there is an open set V in X containing a whose closure is contained in U. Let $p_{i}$ be a canonical projection mapping $p_i:X \rightarrow X_i, i=1,2$;let $\bigcap _{i=1,2} p_{i}^{-1}(U_{i})$ be a basic open set in X which contains a and is contained in U.

Using a regularity and choose a V as $V = \bigcap _{i=1,2} p_{i}^{-1}(V_{i})$, where $p_{i}(a) \in V_{i}, \bar{V_{i}} \subset U_{i}.$

Since a V contains a and $\bigcap _{i=1,2} p_{i}^{-1}(\overline{V_{i}}) \subset \bigcap _{i=1,2} p_{i}^{-1}(U_{i})$, we conclude that $\bar{V}$ is contained in U.

Thus, X is regular.

A similar proof can be found here as well.