Certainly, one of the most fundamental early results in universal algebra is Birkhoff’s 1944 decomposition theorem which states that every (universal) algebra d may be represented as a subdirect product of subdirectly irreducible algebras of the same type. It is clear that for d finite such decompositions may be obtained effectively, at least in principle. At first sight, therefore, one might b...

We describe the structure of a free loop of rank n in the variety of automorphic Moufang loops as a subdirect product of a free group and a free commutative Moufang loop, both of rank n. In particular, the variety of automorphic Moufang loops is the join of the variety of groups and the variety of commutative Moufang loops.

We characterize the distributive lattices of Jacobson rings and prove that if a semiring is a distributive lattice of Jacobson rings, then, up to isomorphism, it is equal to the subdirect product of a distributive lattice and a Jacobson ring. Also, we give a general method to construct distributive lattices of Jacobson rings.

Subdirect representations are investigated in varieties which are defined by operations of not necessarily finite arity. It is shown that, in this context, Birkhoff’s Subdirect Representation Theorem does not hold. However, a class of unranked varieties is identified which admit subdirect representations by subdirectly irreducibles and then even are residually small.

We establish a general criterion for the finite presentability of subdirect products of groups and use this to characterize finitely presented residually free groups. We prove that, for all n ∈ N, a residually free group is of type FPn if and only if it is of type Fn. New families of subdirect products of free groups are constructed, including the first examples of finitely presented subgroups ...

Let $H \leq S_n$ be an intransitive group with orbits $\Omega_1, \Omega_2, \ldots ,\Omega_k$. Then certainly $H$ is a subdirect product of the direct its projections on each orbit, $H|_{\Omega_1} \times H|_{\Omega_2} H|_{\Omega_k}$. Here we provide polynomial time algorithm for computing finest partition $P$ $H$-orbits such that = \prod_{c \in P} H|_c$ and demonstrate usefulness in some applica...

Conditions are given which guarantee that the k-subdirect sum of S-strictly diagonally dominant matrices (S-SDD) is also S-SDD. The same situation is analyzed for SDD matrices. The converse is also studied: given an SDD matrix C with the structure of a k-subdirect sum and positive diagonal entries, it is shown that there are two SDD matrices whose subdirect sum is C. AMS subject classifications...

The paper is in two parts. In Part I we describe a construction of a certain kind of subdirect product of a family of rings. We endow the index set of the family with the partial order structure of an SFP domain, as introduced by Plotkin, and provide a commuting system of homomorphisms between those rings whose indices are related in the ordering. We then take the subdirect product consisting o...

In [5], a representation of bounded lattices within so-called standard topological contexts has been developed. Based on the theory of formal concept analysis [14] it includes Stone’s representation of Boolean algebras by totally disconnected compact spaces [10], Priestley’s representation of bounded distributive lattices by totally order disconnected compact spaces [7] as well as Urquhardt’s r...