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In mathematics, a band (also called idempotent semigroup) is a semigroup in which every element is idempotent (in other words equal to its own square). Bands were first studied and named by A. H. Clifford (1954); the lattice of varieties of bands was described independently in the early 1970s by Biryukov, Fennemore and Gerhard. Semilattices, left-zero bands, right-zero bands, rectangular bands, normal bands, and regular bands, specific subclasses of bands which lie near the bottom of this lattice, are of particular interest and are briefly described below.
A left zero band is a band satisfying the equation
whence its Cayley table has constant rows.
Symmetrically, a right zero band is one satisfying
so that the Cayley table has constant columns.
A rectangular band is a band S which satisfies
For example, given arbitrary non-empty sets I and J one can define a semigroup operation on I × J by setting
The resulting semigroup is a rectangular band because
In fact, any rectangular band is isomorphic to one of the above form. Left zero and right zero bands are rectangular bands, and in fact every rectangular band is isomorphic to a direct product of a left zero band and a right zero band. All rectangular bands of prime order are zero bands, either left or right. A rectangular band is said to be purely rectangular if it is not a left or right zero band.
A normal band is a band S satisfying
A regular band is a band S satisfying
When partially ordered by inclusion, varieties of bands naturally form a lattice, in which the meet of two varieties is their intersection and the join of two varieties is the smallest variety that contains both of them. The complete structure of this lattice is known; in particular, it is countable, complete, and distributive. The sublattice of the 13 varieties of regular bands is shown in the figure. The varieties of left-zero bands, semilattices, and right-zero bands are the three atoms (non-trivial minimal elements) of this lattice.