# a space $\mathnormal{X}$ is Hausdorff if and only if $\Delta(X)$ is closed

###### Theorem.

A space $X$ is Hausdorff  if and only if

 $\{(x,x)\in X\times X\mid x\in X\}$

is closed in $X\times X$ under the product topology.

###### Proof.

First, some preliminaries: Recall that the diagonal map $\Delta\colon\thinspace X\to X\times X$ is defined as $x\lx@stackrel{{\scriptstyle\Delta}}{{\longmapsto}}(x,x)$. Also recall that in a topology  generated by a basis (like the product topology), a set $Y$ is open if and only if, for every point $y\in Y$, there’s a basis element $B$ with $y\in B\subset Y$. Basis elements for $X\times X$ have the form $U\times V$ where $U,V$ are open sets in $X$.

Now, suppose that $X$ is Hausdorff. We’d like to show its image under $\Delta$ is closed. We can do that by showing that its complement $\Delta(X)^{c}$ is open. $\Delta(X)$ consists of points with equal coordinates, so $\Delta(X)^{c}$ consists of points $(x,y)$ with $x$ and $y$ distinct.

For any $(x,y)\in\Delta(X)^{c}$, the Hausdorff condition gives us disjoint open $U,V\subset X$ with $x\in U,y\in V$. Then $U\times V$ is a basis element containing $(x,y)$. $U$ and $V$ have no points in common, so $U\times V$ contains nothing in the image of the diagonal map: $U\times V$ is contained in $\Delta(X)^{c}$. So $\Delta(X)^{c}$ is open, making $\Delta(X)$ closed.

Now let’s suppose $\Delta(X)$ is closed. Then $\Delta(X)^{c}$ is open. Given any $(x,y)\in\Delta(X)^{c}$, there’s a basis element $U\times V$ with $(x,y)\in U\times V\subset\Delta(X)^{c}$. $U\times V$ lying in $\Delta(X)^{c}$ implies that $U$ and $V$ are disjoint.

If we have $x\neq y$ in $X$, then $(x,y)$ is in $\Delta(X)^{c}$. The basis element containing $(x,y)$ gives us open, disjoint $U,V$ with $x\in U,y\in V$. $X$ is Hausdorff, just like we wanted. ∎

Title a space $\mathnormal{X}$ is Hausdorff if and only if $\Delta(X)$ is closed ASpacemathnormalXIsHausdorffIfAndOnlyIfDeltaXIsClosed 2013-03-22 14:20:47 2013-03-22 14:20:47 mathcam (2727) mathcam (2727) 9 mathcam (2727) Proof msc 54D10 DiagonalEmbedding T2Space ProductTopology SeparatedScheme