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In topological groups
Definition: For any neighborhood $U$ of the identity $0,$ there exist finitely many $x_1, \ldots, x_n \in X$ such that $S \subseteq \bigcup_{j=1}^n \left(x_j + U\right) := \left(x_1 + U\right) + \cdots + \left(x_n + U\right).$  |
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