The Banach limit is a mathematical concept that is particularly useful in functional analysis and the study of sequences and series. It is a continuous linear functional that extends the notion of limits to bounded sequences. Specifically, the Banach limit can be defined on the space of bounded sequences, denoted as \(\ell^\infty\). ### Key Properties: 1. **Limit for Bounded Sequences:** The Banach limit exists for any bounded sequence \((a_n)\).
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A Banach limit is a positive linear functional on the real l-infinity sequence space that extends the ordinary limit on the convergent sequence space, has operator norm one, and is invariant under the left shift on bounded sequences. The Hahn-Banach theorem constructs one by separating the constant sequence from the subspace , whose distance from is one. The functional is generally not unique.