Weak Hausdorff space

In mathematics, a weak Hausdorff space or weakly Hausdorff space is a topological space where the image of every continuous map from a compact Hausdorff space into the space is closed. In particular, every Hausdorff space is weak Hausdorff. As a separation property, it is stronger than T₁, which is equivalent to the statement that points are closed. Specifically, every weak Hausdorff space is a T₁ space.

The notion was introduced by M. C. McCord to remedy an inconvenience of working with the category of Hausdorff spaces. It is often used in tandem with compactly generated spaces in algebraic topology. For that, see the category of compactly generated weak Hausdorff spaces.

k-Hausdorff spaces

A k-Hausdorff space is a topological space which satisfies any of the following equivalent conditions:

• Each compact subspace is Hausdorff.
• The diagonal $\{(x,x):x\in X\}$ is k-closed in $X\times X$.
• Each compact subspace is closed and strongly locally compact.

In these characterizations:

• A subset $A\subset X$ is k-closed, if $A\cap C$ is closed in $C$ for each compact $C\subset X$.
• A space is strongly locally compact, if for each $x\in X$ and a (not necessarily open) neighborhood $U\subset X$ of $x$, there exists a compact neighborhood $V\subset X$ of $x$ such that $V\subset U$.

Properties

• A k-Hausdorff space is weak Hausdorff. For if $X$ is k-Hausdorff and $f:C\to X$ is a continuous map from a compact space $C$, then $f(C)$ is compact, hence Hausdorff, hence closed.
• A Hausdorff space is k-Hausdorff. For a space is Hausdorff if and only if the diagonal $\{(x,x):x\in X\}$ is closed in $X\times X$, and each closed subset is k-closed.
• A k-Hausdorff space is KC. A space is KC, if each compact subspace is closed.
• A space is Hausdorff-compactly generated weak Hausdorff if and only if it is Hausdorff-compactly generated k-Hausdorff.
• To show that the coherent topology induced by compact Hausdorff subspaces preserves the compact Hausdorff subspaces and their subspace topology requires that the space is k-Hausdorff; weak Hausdorff is not enough. Hence k-Hausdorff can be seen as the more fundamental definition.

Δ-Hausdorff spaces

A Δ-Hausdorff space is a topological space where the image of every path is closed; i.e. if $f:[0,1]\to X$ is continuous, then $f([0,1])$ is closed. Every weak Hausdorff space is Δ-Hausdorff, and every Δ-Hausdorff space is T1. A space is Δ-generated, if its topology is the finest such that each map $f:\Delta ^{n}\to X$ from a topological n-simplex $\Delta ^{n}$ to $X$ is continuous. Δ-Hausdorff spaces are to Δ-generated spaces as weak Hausdorff spaces are to compactly generated spaces.

See also

• Particular point topology