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unary operation (monadic operation) defined on a set S. A function from the domain S into S itself. The identity function is unary. Other examples are the operations of negation in arithmetic or logic and of taking complements in set theory or in Boolean algebra. Although basically functions, unary operations are frequently represented using a special notation, e.g. ¬A or A′. When the set S is finite, a truth table can be used to define the meaning of the operation.