View source for Green equivalence relations

{{TEX|done}}{{MSC|20M10}}

''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 [[Principal ideal|Principal ideal]]); $x\mathcal Ry$ and $x\mathcal Jy$ have a similar meaning after  "left"  has been replaced by  "right"  and  "two-sided", respectively; $\mathcal D=\mathcal{L}\lor\mathcal R$ (union in the lattice of equivalence relations); $\mathcal H=\mathcal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|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 [[#References|[1]]].

====References====
<table>
<TR><TD valign="top">[1]</TD> <TD valign="top">  J. Green, "On the structure of semigroups"  ''Ann. of Math.'' , '''54'''  (1951)  pp. 163–172 {{DOI|10.2307/1969317}} {{ZBL|0043.25601}}</TD></TR>
<TR><TD valign="top">[2]</TD> <TD valign="top">  E.S. Lyapin, "Semigroups" , Amer. Math. Soc.  (1974)  (Translated from Russian) {{ZBL|0303.20039}}</TD></TR>
<TR><TD valign="top">[3]</TD> <TD valign="top">  A.H. Clifford, G.B. Preston, "Algebraic theory of semi-groups" , '''1–2''' , Amer. Math. Soc.  (1961–1967) {{ZBL|0111.03403}} {{ZBL|0178.01203}}</TD></TR>
<TR><TD valign="top">[4]</TD> <TD valign="top"> 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}}</TD></TR>
<TR><TD valign="top">[5]</TD> <TD valign="top">  K.H. Hofmann, P.S. Mostert, "Elements of compact semigroups" , C.E. Merrill  (1966) {{ZBL|0161.01901}}</TD></TR>
</table>