ON WEAKLY AMPLE SEMIGROUPS

Let$\def \xmlpi #1{}\def \mathsfbi #1{\boldsymbol {\mathsf {#1}}}\let \le =\leqslant \let \leq =\leqslant \let \ge =\geqslant \let \geq =\geqslant \def \Pr {\mathit {Pr}}\def \Fr {\mathit {Fr}}\def \Rey {\mathit {Re}}S$be a semigroup. Elements$a,b$of$S$are$\widetilde{\mathscr{R}}$-related if they have the same idempotent left identities. Then$S$is weakly left ample if (1) idempotents of$S$commute, (2) $\widetilde{\mathscr{R}}$is a left congruence, (3) for any$a \in S$,$a$is$\widetilde{\mathscr{R}}$-related to a (unique) idempotent, say$a^+$, and (4) for any element$a$and idempotent$e$of$S$,$ae=(ae)^+a$. Elements$a,b$of$S$are$\mathscr{R}^*$-related if, for any$x,y \in S^1$,$xa=ya$if and only if$xb=yb$. Then$S$is left ample if it satisfies (1), (3) and (4) relative to$\mathscr{R}^*$instead of$\widetilde{\mathscr{R}}$. Further,$S$is (weakly) ample if it is both (weakly) left and right ample. We establish several characterizations of these classes of semigroups. For weakly left ample ones we provide a construction of all such semigroups with zero all of whose nonzero idempotents are primitive. Among characterizations of weakly ample semigroups figure (strong) semilattices of unipotent monoids, and among those for ample semigroups, (strong) semilattices of cancellative monoids. This describes the structure of these two classes of semigroups in an optimal way, while, for the ‘one-sided’ case, the problem of structure remains open.

On right self-injective regular semigroups, II

It is shown that a semigroup is right self-injective and a band of groups if and only if it is isomorphic to the spined product of a self-injective semilattice of groups and a right self-injective band. A necessary and sufficient condition for a band to be right self-injective is given. It is shown that a left [right] self-injective semigroup has the [anti-] representation extension property and the right [left] congruence extension property.

On semigroups whose nontrlvial left congruence classes are left ideals

An equivalence relation δ on a semigroup S is called a left congruence of S if (u, v) Ε λ implies that (su, sv) Ε λ for every s in S. With every set ℒ of pairwise disjoint left ideals (i. e. subsets L of S such that SL⊂EL), one can associate the left congruence {(u, v)|u = v or there exists an L in ℒ such that u Ε L and v Ε L}. Thus every nonempty left ideal is a left congruence class (i. e. an equivalence class of some left congruence). A left congruence has the form just described if and only if all its nontrivial classes (i. e. its classes containing at least two elements) are left ideals. Such a left congruence is called a Rees left congruence if there is at most one nontrivial class. The identity relation on S is a Rees left congruence since the empty set is a left ideal by definition.

On Semigroups of Transformations Acting Transitively on a Set

We call a semigroup S transitive if S is isomorphic to a semigroup T of transformations of some set M into itself, where T acts on M transitively, that is in such a manner that for all x, y ∊ M we have Xπ = y for some transformation π∊T. In [4] the author showed that S is transitive if and only if there exists a right congruence σ (i.e., an equivalence relation for which a σ b always implies ac σ bc for all c ∊ S) on S, satisfying:(1)There exists a left identity modulo σ, that is an element e such that ea σ a for all a ∊ S .(2)Each σ-class meets each right ideal, or, equivalently, for all a, b ∊ S we have ac σ b for some c ∊ S .(3)The relation σ contains ( i. e. , is less fine than) no left congruence except the identity relation (in which each class consists of a single element).