Absolutely convex set

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A set C in a real or complex vector space is said to be absolutely convex or disked if it is convex and balanced (circled), in which case it is called a disk.

Properties

A set $C$  is absolutely convex if and only if for any points $x_{1},\,x_{2}$  in $C$  and any numbers $\lambda _{1},\,\lambda _{2}$  satisfying $|\lambda _{1}|+|\lambda _{2}|\leq 1$  the sum $\lambda _{1}x_{1}+\lambda _{2}x_{2}$  belongs to $C$ .

The intersection of arbitrarily many absolutely convex sets is again absolutely convex; however, unions of absolutely convex sets need not be absolutely convex anymore.

Absolutely convex hull

Since the intersection of any collection of absolutely convex sets is absolutely convex, one can define for any subset A of a vector space its absolutely convex hull as the intersection of all absolutely convex sets containing A, analogous to the well-known construction of the convex hull.

More explicitly, one can define the absolutely convex hull of the set A via

${\mbox{absconv}}A=\left\{\sum _{i=1}^{n}\lambda _{i}x_{i}:n\in \mathbb {N} ,\,x_{i}\in A,\,\sum _{i=1}^{n}|\lambda _{i}|\leq 1\right\},$

where the λi are elements of the underlying field.

The absolutely convex hull of a bounded set in a topological vector space is again bounded.