# Hausdorffness is product-closed

This article gives the statement, and possibly proof, of a topological space property (i.e., Hausdorff space) satisfying a topological space metaproperty (i.e., product-closed property of topological spaces)
View all topological space metaproperty satisfactions | View all topological space metaproperty dissatisfactions
Get more facts about Hausdorff space |Get facts that use property satisfaction of Hausdorff space | Get facts that use property satisfaction of Hausdorff space|Get more facts about product-closed property of topological spaces

## Statement

### Verbal statement

An arbitrary (finite or infinite) product of Hausdorff spaces, when endowed with the product topology, is also a Hausdorff space.

## Definitions used

### Hausdorff space

Further information: Hausdorff space

A topological space $X$ is Hausdorff if given distinct points $a,b \in X$, there exist disjoint open sets $U, V$ containing $a$ and $b$.

### Product topology

Further information: Product topology

Suppose $I$ is an indexing set, and $X_i$ a family of topological spaces, $i \in I$. Then if $X$ is the Cartesian product of the $X_i$s, the product topology on $X$ is a topology with subbasis given by all the open cylinders: all sets of the form $\prod_i A_i$ such that for all but one $i$, $A_i = X_i$, and for the one exceptional $i$, $A_i$ is an open subset of $X_i$.

## Proof

### Proof outline

The proof has the following three steps:

• Write down both points in the product space as tuples
• Find a coordinate where they differ, and separate the projections on that coordinate, by disjoint open sets in that coordinate (this is where we use that each space is Hausdorff)
• Use the open cylinders corresponding to these disjoint open subsets, to separate the original two points

## References

### Textbook references

• Topology (2nd edition) by James R. Munkres, More info, Page 100, Theorem 17.11, Page 101, Exercise 11, and Page 196, Theorem 31.2 (a)