In algebra, a presentation of a monoid (or a presentation of a semigroup) is a description of a monoid (or a semigroup) in terms of a set Σ of generators and a set of relations on the free monoid Σ∗ (or the free semigroup Σ+ ) generated by Σ . The monoid is then presented as the quotient of the free monoid (or the free semigroup) by these relations. This is an analogue of a group presentation in group theory. As a mathematical structure, a monoid presentation is identical to a string rewriting system (also known as a semi-Thue system). Every monoid may be presented by a semi-Thue system (possibly over an infinite alphabet). A presentation should not be confused with a representation.

Construction

The relations are given as a (finite) binary relation R on Σ∗ . To form the quotient monoid, these relations are extended to monoid congruences as follows: First, one takes the symmetric closure R ∪ R−1 of R. This is then extended to a symmetric relation E ⊂ Σ∗ × Σ∗ by defining x ~E y if and only if x = sut and y = svt for some strings u, v, s, t ∈ Σ∗ with (u,v) ∈ R ∪ R−1 . Finally, one takes the reflexive and transitive closure of E, which then is a monoid congruence. In the typical situation, the relation R is simply given as a set of equations, so that. Thus, for example, is the equational presentation for the bicyclic monoid, and is the plactic monoid of degree 2 (it has infinite order). Elements of this plactic monoid may be written as for integers i, j, k, as the relations show that ba commutes with both a and b.

Inverse monoids and semigroups

Presentations of inverse monoids and semigroups can be defined in a similar way using a pair where is the free monoid with involution on X, and is a binary relation between words. We denote by (respectively ) the equivalence relation (respectively, the congruence) generated by T. We use this pair of objects to define an inverse monoid Let \rho_X be the Wagner congruence on X, we define the inverse monoid presented by (X;T) as In the previous discussion, if we replace everywhere with we obtain a presentation (for an inverse semigroup) (X;T) and an inverse semigroup presented by (X;T). A trivial but important example is the free inverse monoid (or free inverse semigroup) on X, that is usually denoted by (respectively ) and is defined by or