# Normally-imbedded subspace

2020 Mathematics Subject Classification: *Primary:* 54-XX [MSN][ZBL]

A subspace $A$ of a space $X$ such that for every neighbourhood $U$ of $A$ in $X$ there is a set $H$ that is the union of a countable family of sets closed in $X$ and with $A \subset H \subset U$. If $A$ is normally imbedded in $X$ and $X$ is normally imbedded in $Y$, then $A$ is normally imbedded in $Y$. A normally-imbedded subspace of a normal space is itself normal in the induced topology, which explains the name. Final compactness of a space is equivalent to its being normally imbedded in some (and hence in any) compactification of this space. Quite generally, a normally-imbedded subspace of a finally-compact space is itself finally compact.

#### References

[1] | Yu.M. Smirnov, "On normally-imbedded sets of normal spaces" Mat. Sb. , 29 (1951) pp. 173–176 (In Russian) Zbl 0043.16502 |

#### Comments

A finally-compact space is the same as a Lindelöf space.

**How to Cite This Entry:**

Normally-imbedded subspace.

*Encyclopedia of Mathematics.*URL: http://encyclopediaofmath.org/index.php?title=Normally-imbedded_subspace&oldid=34638