Home > First-countable space

Most "nice" spaces in mathematics are first-countable. In particular, every metric space is first-countable. To see this, note that the set of open balls centered at x with radius 1/n for integers n > 0 form a countable local base at x. An example of a space which is not first-countable is the cofinite topology on an uncountable set (such as the real line).

1 Properties

One of the most important properties of first-countable spaces is that given a subset A, a point x lies in the closure of A if and only if there exists a sequence {x_n} in A which converges to x.

In first-countable spaces, sequential compactness and countable compactness are equivalent properties. However, there exist examples of sequentially compact, first-countable spaces which aren't compact (these are necessarily non-metric spaces). One such space is the order topology on . Every first-countable space is compactly generatedIn topology, a compactly generated space is a topological space X satisfying the following condition: a subspace A is closed in X if and only if A ∩ K is closed in K for all compact subspaces K ⊆ X''. Equivalently, one can replace closed with ope.

Every subspace of a first-countable space is first-countable. Any countable product of a first-countable space is first-countable, although uncountable products need not be.

2 Related topics

• Second-countable spaceIn topology, a second-countable space is a topological space satisfying the "second axiom of countability". Specifically, a space is said to be second countable if its topology has a countable base. Like other countability axioms, the property of being se