# Finite semigroups¶

class sage.categories.finite_semigroups.FiniteSemigroups(base_category)

The category of finite (multiplicative) semigroups.

A finite semigroup is a finite set endowed with an associative binary operation $$*$$.

Warning

Finite semigroups in Sage used to be automatically endowed with an enumerated set structure; the default enumeration is then obtained by iteratively multiplying the semigroup generators. This forced any finite semigroup to either implement an enumeration, or provide semigroup generators; this was often inconvenient.

Instead, finite semigroups that provide a distinguished finite set of generators with semigroup_generators() should now explicitly declare themselves in the category of finitely generated semigroups:

sage: Semigroups().FinitelyGenerated()
Category of finitely generated semigroups


This is a backward incompatible change.

EXAMPLES:

sage: C = FiniteSemigroups(); C
Category of finite semigroups
sage: C.super_categories()
[Category of semigroups, Category of finite sets]
sage: sorted(C.axioms())
['Associative', 'Finite']
sage: C.example()
An example of a finite semigroup: the left regular band generated by ('a', 'b', 'c', 'd')

class ParentMethods
idempotents()

Returns the idempotents of the semigroup

EXAMPLES:

sage: S = FiniteSemigroups().example(alphabet=('x','y'))
sage: sorted(S.idempotents())
['x', 'xy', 'y', 'yx']

j_classes()

Returns the $$J$$-classes of the semigroup.

Two elements $$u$$ and $$v$$ of a monoid are in the same $$J$$-class if $$u$$ divides $$v$$ and $$v$$ divides $$u$$.

OUTPUT:

All the $J$-classes of self, as a list of lists.

EXAMPLES:

sage: S = FiniteSemigroups().example(alphabet=('a','b', 'c'))
sage: sorted(map(sorted, S.j_classes()))
[['a'], ['ab', 'ba'], ['abc', 'acb', 'bac', 'bca', 'cab', 'cba'], ['ac', 'ca'], ['b'], ['bc', 'cb'], ['c']]

j_classes_of_idempotents()

Returns all the idempotents of self, grouped by J-class.

OUTPUT:

a list of lists.

EXAMPLES:

sage: S = FiniteSemigroups().example(alphabet=('a','b', 'c'))
sage: sorted(map(sorted, S.j_classes_of_idempotents()))
[['a'], ['ab', 'ba'], ['abc', 'acb', 'bac', 'bca', 'cab', 'cba'], ['ac', 'ca'], ['b'], ['bc', 'cb'], ['c']]

j_transversal_of_idempotents()

Returns a list of one idempotent per regular J-class

EXAMPLES:

sage: S = FiniteSemigroups().example(alphabet=('a','b', 'c'))
sage: sorted(S.j_transversal_of_idempotents())
['a', 'acb', 'b', 'ba', 'bc', 'c', 'ca']