# Green equivalence relations

2020 Mathematics Subject Classification: *Primary:* 20M10 [MSN][ZBL]

*on a semi-group*

Binary relations $\mathcal L$, $\mathcal R$, $\mathcal J$, $\mathcal D$, $\mathcal H$ defined as follows: $x\mathcal Ly$ means that $x$ and $y$ generate identical left principal ideals (cf. Principal ideal); $x\mathcal Ry$ and $x\mathcal Jy$ have a similar meaning after "left" has been replaced by "right" and "two-sided", respectively; $\mathcal D=\mathcal{L}\lor\mathcal R$ (union in the lattice of equivalence relations); $\mathcal H=\mathcal L\cap\mathcal R$. The relations $\mathcal L$ and $\mathcal R$ are commutative in the sense of multiplication of binary relations, so that $\mathcal D$ coincides with their product. The relation $\mathcal L$ is a right congruence, i.e. is stable from the right: $a\mathcal Lb$ implies $ac\mathcal Lbc$ for all $c$; the relation $\mathcal R$ is a left congruence (stable from the left). An $\mathcal L$-class and an $\mathcal R$-class intersect if and only if they are contained in the same $\mathcal D$-class. All $\mathcal H$-classes in the same $\mathcal R$-class are equipotent. If a $\mathcal D$-class $D$ contains a regular element, then all elements in $D$ are regular and $D$ contains with some given element all elements inverse to it; such a $\mathcal D$-class is said to be regular. In a regular $\mathcal D$-class each $\mathcal L$-class and each $\mathcal R$-class contains an idempotent. Let $H$ be an arbitrary $\mathcal H$-class; then either $H$ is a group (which is the case if and only if $H$ is a maximal subgroup of the given semi-group), or else $H\cap H^2=\emptyset$. All group $\mathcal H$-classes of the same $\mathcal D$-class are isomorphic groups. In the general case $\mathcal D\neq\mathcal J$, but if, for example, some power of each element of the semi-group $S$ belongs to a subgroup (in particular, if $S$ is a periodic semi-group), then $\mathcal D=\mathcal J$. The inclusion of principal left ideals defines in a natural manner a partial order relation on the set of $\mathcal L$-classes; similar considerations are valid for $\mathcal R$-classes and $\mathcal J$-classes. These relations were introduced by J. Green [1].

#### References

[1] | J. Green, "On the structure of semigroups" Ann. of Math. , 54 (1951) pp. 163–172 DOI 10.2307/1969317 Zbl 0043.25601 |

[2] | E.S. Lyapin, "Semigroups" , Amer. Math. Soc. (1974) (Translated from Russian) Zbl 0303.20039 |

[3] | A.H. Clifford, G.B. Preston, "Algebraic theory of semi-groups" , 1–2 , Amer. Math. Soc. (1961–1967) Zbl 0111.03403 Zbl 0178.01203 |

[4] | M.A. Arbib (ed.) Algebraic theory of machines, languages and semigroups Academic Press (1968) ISBN 0120590506 Zbl 0181.01501 Buslenko, N.P. (ed.), Moscow (1975) (In Russian; translated from English) Zbl 0358.94001 |

[5] | K.H. Hofmann, P.S. Mostert, "Elements of compact semigroups" , C.E. Merrill (1966) Zbl 0161.01901 |

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Green equivalence relations.

*Encyclopedia of Mathematics.*URL: http://encyclopediaofmath.org/index.php?title=Green_equivalence_relations&oldid=54699