In mathematics, in the field of topology, a topological space
is said to be collectionwise Hausdorff if given any closed discrete subset of
, there is a pairwise disjoint family of open sets with each point of the discrete subset contained in exactly one of the open sets.[1]
Here a subset
being discrete has the usual meaning of being a discrete space with the subspace topology (i.e., all points of
are isolated in
).[nb 1]
Properties
- Every collectionwise normal space is collectionwise Hausdorff. (This follows from the fact that given a closed discrete subset
of
, every singleton
is closed in
and the family of such singletons is a discrete family in
.)
Remarks
- ^ If
is T1 space,
being closed and discrete is equivalent to the family of singletons
being a discrete family of subsets of
(in the sense that every point of
has a neighborhood that meets at most one set in the family). If
is not T1, the family of singletons being a discrete family is a weaker condition. For example, if
with the indiscrete topology,
is discrete but not closed, even though the corresponding family of singletons is a discrete family in
.
References