# Beta-dual space

In functional analysis and related areas of mathematics, the beta-dual or β-dual is a certain linear subspace of the algebraic dual of a sequence space.

## Definition

Given a sequence space X the β-dual of X is defined as

${\displaystyle X^{\beta }:=\left\{x\in X\ :\ \sum _{i=1}^{\infty }x_{i}y_{i}<\infty \quad \forall y\in X\right\}.}$

If X is an FK-space then each y in Xβ defines a continuous linear form on X

${\displaystyle f_{y}(x):=\sum _{i=1}^{\infty }x_{i}y_{i}\qquad x\in X.}$

## Examples

• ${\displaystyle c_{0}^{\beta }=\ell ^{1}}$
• ${\displaystyle (\ell ^{1})^{\beta }=\ell ^{\infty }}$
• ${\displaystyle \omega ^{\beta }=\emptyset }$

## Properties

The beta-dual of an FK-space E is a linear subspace of the continuous dual of E. If E is an FK-AK space then the beta dual is linear isomorphic to the continuous dual.