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.
The series is called unconditionally convergent to , if
Unconditional convergence is often defined in an equivalent way: A series is unconditionally convergent if for every sequence , with , the series
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 Dvoretzky–Rogers 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.
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