# Unconditional convergence

Unconditional convergence is a topological property (convergence) related to an algebraical object (sum). It is an extension of the notion of convergence for series of countably many elements to series of arbitrarily many. It has been mostly studied in Banach spaces.

## Definition

Let be a topological vector space. Let be an index set and for all .

The series is called unconditionally convergent to , if

• the indexing set is countable and
• for every permutation of the relation holds: ## Alternative definition

Unconditional convergence is often defined in an equivalent way: A series is unconditionally convergent if for every sequence , with , the series converges.

Every absolutely convergent series is unconditionally convergent, but the converse implication does not hold in general: if X is an infinite dimensional Banach space, then by DvoretzkyRogers theorem there always exists an unconditionally convergent series in this space that is not absolutely convergent. However, when X = Rn, then, by the Riemann series theorem, the series is unconditionally convergent if and only if it is absolutely convergent.