Elements of Group Theory
A set S is a collection of entities called elements of the set. If a is an element belonging to the set S, we write a ∈ S (read a belongs to S or is contained in S). If it does not we write a ∉ S. Equivalently one also writes S ∋ a or S ∌ a for these relations.
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- 6.L. Pontrjagin, Topological groups (ch1), Princeton, Princeton University Press, 1946.Google Scholar