Convex Subset of Topological Vector Space containing Zero Vector in Interior is Absorbing Set
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Theorem
Let $\GF \in \set {\R, \C}$.
Let $\struct {X, \tau}$ be a topological vector space over $\GF$.
Let $C \subseteq X$ be a convex set such that ${\mathbf 0}_X \in C^\circ$, where $C^\circ$ denotes the interior of $C$.
Then $C$ is an absorbing set.
Proof
Let $x \in X$.
Let $V$ be an open neighborhood of ${\mathbf 0}_X$ contained in $C$.
Then from Multiple of Vector in Topological Vector Space Converges, we have:
- $\ds \frac x n \to {\mathbf 0}_X$
So by the definition of a convergent sequence, for some $N \in \N$ we have:
- $\ds \frac x N \in V$
Then:
- $\ds x \in N V \subseteq N C$
Since $x \in X$ was arbitrary, we can apply Characterization of Convex Absorbing Set in Vector Space and obtain that:
- $C$ is absorbing set.
$\blacksquare$