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In mathematics, a semigroup with two elements is a semigroup for which the cardinality of the underlying set is two. There are exactly five distinct nonisomorphic semigroups having two elements:

The semigroups LO2 and RO2 are antiisomorphic. O2, and are commutative, and LO2 and RO2 are noncommutative. LO2, RO2 and are bands and also inverse semigroups.

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