In mathematics, in semigroup theory, a Rees factor semigroup (also called Rees quotient semigroup or just Rees factor), named after David Rees, is a certain semigroup constructed using a semigroup and an ideal of the semigroup.
Contents
Let S be a semigroup and I be an ideal of S. Using S and I one can construct a new semigroup by collapsing I into a single element while the elements of S outside of I retain their identity. The new semigroup obtained in this way is called the Rees factor semigroup of S modulo I and is denoted by S/I.
The concept of Rees factor semigroup was introduced by David Rees in 1940.
Formal definition
A subset
is an equivalence relation in
The congruence
Example
Consider the semigroup S = { a, b, c, d, e } with the binary operation defined by the following Cayley table:
Let I = { a, d } which is a subset of S. Since
SI = { aa, ba, ca, da, ea, ad, bd, cd, dd, ed } = { a, d } ⊆ IIS = { aa, da, ab, db, ac, dc, ad, dd, ae, de } = { a, d } ⊆ Ithe set I is an ideal of S. The Rees factor semigroup of S modulo I is the set S/I = { b, c, e, I } with the binary operation defined by the following Cayley table:
Ideal extension
A semigroup S is called an ideal extension of a semigroup A by a semigroup B if A is an ideal of S and the Rees factor semigroup S/A is isomorphic to B.
Some of the cases that have been studied extensively include: ideal extensions of completely simple semigroups, of a group by a completely 0-simple semigroup, of a commutative semigroup with cancellation by a group with added zero. In general, the problem of describing all ideal extensions of a semigroup is still open.