# Balanced set

In linear algebra and related areas of mathematics a balanced set, circled set or disk in a vector space (over a field K with an absolute value function ${\displaystyle |\cdot |}$) is a set S such that for all scalars ${\displaystyle \alpha }$ with ${\displaystyle |\alpha |\leqslant 1}$

${\displaystyle \alpha S\subseteq S}$

where

${\displaystyle \alpha S:=\{\alpha x\mid x\in S\}.}$

The balanced hull or balanced envelope for a set S is the smallest balanced set containing S. It can be constructed as the intersection of all balanced sets containing S.

## Examples

• The open and closed balls centered at 0 in a normed vector space are balanced sets.
• Any subspace of a real or complex vector space is a balanced set.
• The cartesian product of a family of balanced sets is balanced in the product space of the corresponding vector spaces (over the same field K).
• Consider ${\displaystyle \mathbb {C} ,}$ the field of complex numbers, as a 1-dimensional vector space. The balanced sets are ${\displaystyle \mathbb {C} }$ itself, the empty set and the open and closed discs centered at zero. Contrariwise, in the two dimensional Euclidean space there are many more balanced sets: any line segment with midpoint at the origin will do. As a result, ${\displaystyle \mathbb {C} }$ and ${\displaystyle \mathbb {R} ^{2}}$ are entirely different as far as their vector space structure is concerned.
• If ${\displaystyle p}$ is a semi-norm on a linear space ${\displaystyle X,}$ then for any constant ${\displaystyle c>0,}$ the set
${\displaystyle \{x\in X\mid p(x)\leqslant c\}}$
is balanced.

## Properties

• The union and intersection of balanced sets is a balanced set.
• The closure of a balanced set is balanced.
• The union of ${\displaystyle \{0\}}$ and the interior of a balanced set is balanced.
• A set is absolutely convex if and only if it is convex and balanced