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Interior of a set

From Encyclopedia of Mathematics
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2020 Mathematics Subject Classification: Primary: 54A [MSN][ZBL]

of a set $A$ in a topological space $X$

The interior, or (open kernel), of $A$ is the set of all interior points of $A$: the union of all open sets of $X$ which are subsets of $A$; a point $x \in A$ is interior if there is a neighbourhood $N_x$ contained in $A$ and containing $x$. The interior may be denoted $A^\circ$, $\mathrm{Int} A$ or $\langle A \rangle$.

The interior of $A$ is the complement in $A$ of the boundary of $A$. If $A$ and $B$ are mutually complementary sets in a topological space $X$, that is, if $B = X \setminus A$, then the interior of $A$ is the complement of the closure of $B$: $X \setminus [A] = \langle B \rangle$ and $X \setminus \langle B \rangle = [ A ]$.

The terminology "kernel" is seldom used in this context in the English mathematical literature.

How to Cite This Entry:
Interior of a set. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Interior_of_a_set&oldid=36994
This article was adapted from an original article by S.M. Sirota (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article