In this paper we describe $${\mathcal {R}}$$ -unipotent semigroups being regular extensions of a left band by an $$\mathcal {R}$$ semigroup T as certain subsemigroups wreath product T. We obtain Szendrei’s result that each E-unitary is embeddable into semidirect group. Further, specialising the first author’s notion $$\lambda $$ -semidirect locally semigroup, provide answer to open question rai...

Decomposability of an algebraic structure into the union of its sub-structures goes back to G. Scorza's Theorem of 1926 for groups. An analogue of this theorem for rings has been recently studied by A. Lucchini in 2012. On the study of this problem for non-group semigroups, the first attempt is due to Clifford's work of 1961 for the regular semigroups. Since then, N.P. Mukherjee in 1972 studied...

We first show that every γ-contractive commuting multioperator is unitarily equivalent to the restriction of S⊕W to an invariant subspace, where S is a backwards multi-shift and W a γ-isometry. We then describe γ-isometries in terms of (γ, 1)-isometries, and establish that under an additional assumption on T , W above can be chosen to be a commuting multioperator of isometries. Our methods prov...

We study the complexity classes P and NP through a semigroup fP (“polynomial-time functions”), consisting of all polynomially balanced polynomial-time computable partial functions. Then P 6= NP iff fP is a non-regular semigroup. The one-way functions considered here are based on worstcase complexity (they are not cryptographic); they are the non-regular elements of fP. We prove various properti...

It is well known that intra-regular semigroups (ordered semigroups) play an essential role in studying the structure of semigroups (ordered semigroups). In this paper we present a structure theorem referring to the decomposition of ordered semigroups into left strongly simple components, that is, into components, which are both simple and left quasi-regular. We prove, among others, that if an o...

By a Boolean inverse semigroup we mean an inverse semigroup whose semilattice of idempotents is a Boolean algebra. We study representations of a given inverse semigroup S in a Boolean inverse semigroup which are tight in a certain well defined technical sense. These representations are supposed to preserve as much as possible any trace of Booleannes present in the semilattice of idempotents of ...

We give a new description of the fundamental double four-spiral semigroup. The fundamental four-spiral semigroup Sp4 and the fundamental double fourspiral semigroup DSp4 were introduced in [1], [3], and [4]. These semigroups are interesting examples of fundamental regular semigroups, and are indispensable building blocks of bisimple, idempotent-generated regular semigroups. Their basic properti...

Let A generate a C0–semigroup T (·) on a Banach space X such that the resolvent R(iτ, A) exists and is uniformly bounded for τ ∈ R. We show that there exists a closed, possibly unbounded projection P on X commuting with T (t). Moreover, T (t)x decays exponentially as t → ∞ for x in the range of P and T (t)x exists and decays exponentially as t→ −∞ for x in kernel of P . The domain of P depends ...