Urysohn extension theorem
Let be a topological space, and and the rings of continuous functions and bounded
continuous functions
respectively.
Urysohn Extension Theorem. A subset is -embedded (http://planetmath.org/CEmbedding) if and only if any two completely separated sets in are completely separated in as well.
Remarks.
- •
Suppose that is a metric space and is closed in . If are completely separated sets in , then they are contained in disjoint zero sets
and in . Since and are closed in , and is closed in , and are closed in . Since is a metric space, and are zero sets in . Since and and are disjoint, and are completely separated in as well. By Urysohn Extension Theorem, any bounded continuous function defined on can be extended to a continuous function on , which is the statement of the metric space version of the Tietze extension theorem.
- •
However, the above argument
does not generalize to normal spaces
, so can not be used to prove the generalized version of the Tietze extension theorem. Urysohn’s lemma is required to prove this more general result.