Biordered set

A biordered set ("boset") is a mathematical object that occurs in the description of the structure of the set of idempotents in a semigroup. The concept and the terminology were developed by K S S Nambooripad in the early 1970s.
The defining properties of a biordered set are expressed in terms of two quasiorders defined on the set and hence the name biordered set. Patrick Jordan, while a master's student at University of Sydney, introduced in 2002 the term boset as an abbreviation of biordered set.
According to Mohan S. Putcha, "The axioms defining a biordered set are quite complicated. However, considering the general nature of semigroups, it is rather surprising that such a finite axiomatization is even possible." Since the publication of the original definition of the biordered set by Nambooripad, several variations in the definition have been proposed. David Easdown simplified the definition and formulated the axioms in a special arrow notation invented by him.
The set of idempotents in a semigroup is a biordered set and every biordered set is the set of idempotents of some semigroup.
A regular biordered set is a biordered set with an additional property. The set of idempotents in a regular semigroup is a regular biordered set, and every regular biordered set is the set of idempotents of some regular semigroup. 
Definition.
The formal definition of a biordered set given by Nambooripad requires some preliminaries. 
Preliminaries.
If "X" and "Y" be sets and ρ⊆ "X" × "Y", let ρ ( "y" ) = { "x" ∈ "X" : "x" ρ "y" }. 
Let "E" be a set in which a partial binary operation, indicated by juxtaposition, is defined. If "D""E" is the domain of the partial binary operation on "E" then "D""E" is a relation on "E" and ("e","f") is in "D""E" if and only if the product "ef" exists in "E". The following relations can be defined in "E":
If "T" is any statement about "E" involving the partial binary operation and the above relations in "E", one can define the left-right dual of "T" denoted by "T"*. If "D""E" is symmetric then "T"* is meaningful whenever "T" is. 
Formal definition.
The set "E" is called a biordered set if the following axioms and their duals hold for arbitrary elements "e", "f", "g", etc. in "E".
In "M" ( "e", "f" ) = ω"l" ( "e" ) ∩ ω"r" ( "f" ) (the "M"-set of "e" and "f" in that order), define a relation formula_6 by 
Then the set
is called the sandwich set of "e" and "f" in that order. 
"M"-biordered sets and regular biordered sets.
We say that a biordered set "E" is an "M"-biordered set if "M" ( "e", "f" ) ≠ ∅ for all "e" and "f" in "E". 
Also, "E" is called a regular biordered set if "S" ( "e", "f" ) ≠ ∅ for all "e" and "f" in "E".
In 2012 Roman S. Gigoń gave a simple proof that "M"-biordered sets arise from "E"-inversive semigroups.
Subobjects and morphisms.
Biordered subsets.
A subset "F" of a biordered set "E" is a biordered subset (subboset) of "E" if "F" is a biordered set under the partial binary operation inherited from "E". 
For any "e" in "E" the sets ω"r" ( "e" ), ω"l" ( "e" ) and ω ( "e" ) are biordered subsets of "E".
Bimorphisms.
A mapping φ : "E" → "F" between two biordered sets "E" and "F" is a biordered set homomorphism (also called a bimorphism) if for all ( "e", "f" ) in "D""E" we have ( "e"φ ) ( "f"φ ) = ( "ef" )φ.
Illustrative examples.
Vector space example.
Let "V" be a vector space and 
where "V" = "A" ⊕ "B" means that "A" and "B" are subspaces of "V" and "V" is the internal direct sum of "A" and "B". 
The partial binary operation ⋆ on E defined by 
makes "E" a biordered set. The quasiorders in "E" are characterised as follows: 
Biordered set of a semigroup.
The set "E" of idempotents in a semigroup "S" becomes a biordered set if a partial binary operation is defined in "E" as follows: "ef" is defined in "E" if and only if "ef" = "e" or "ef"= "f" or "fe" = "e" or "fe" = "f" holds in "S". If "S" is a regular semigroup then "E" is a regular biordered set.
As a concrete example, let "S" be the semigroup of all mappings of "X" = { 1, 2, 3 } into itself. Let the symbol ("abc") denote the map for which 1 → "a", 2 → "b", and 3 → "c". The set "E" of idempotents in "S" contains the following elements:
The following table (taking composition of mappings in the diagram order) describes the partial binary operation in "E". An X in a cell indicates that the corresponding multiplication is not defined. 