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internal point (Definition)
Definition 1   Let $X$ be a vector space and $S \subset X$ . Then $x \in S$ is called an internal point of $S$ if and only if the intersection of each line in $X$ through $x$ and $S$ contains a small interval around $x$ .

That is $x$ is an internal point of $S$ if whenever $y \in X$ there exists an $\epsilon > 0$ such that $x + ty \in S$ for all $t < \epsilon$ .

If $X$ is a topological vector space and $x$ is in the interior of $S$ , then it is an internal point, but the converse is not true in general. However if $S \subset {\mathbb{R}}^n$ is a convex set then all internal points are interior points and vice versa.

Bibliography

1
H. L. Royden. Real Analysis. Prentice-Hall, Englewood Cliffs, New Jersey, 1988




"internal point" is owned by jirka.
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Cross-references: interior points, convex set, converse, interior, topological vector space, interval, contains, line, intersection, vector space

This is version 2 of internal point, born on 2004-06-15, modified 2005-03-07.
Object id is 5923, canonical name is InternalPoint.
Accessed 2109 times total.

Classification:
AMS MSC52A99 (Convex and discrete geometry :: General convexity :: Miscellaneous)

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