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Semigroup
From Groupprops
Definition
QUICK PHRASES: set with associative binary operation, group without identity element and inverses
Symbol-free definition
A semigroup is a set equipped with an associative binary operation.
A semigroup need not have an identity element.
The definition of semigroup does not require it to be nonempty, so the empty set is a semigroup. However, some variants of the definition require a semigroup to be a nonempty set with an associative binary operation.
