Elements of Group Theory
K. N. Srinivasa Rao · Texts and readings in physical sciences · 2006
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.