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Register a new userExistence of cube terms in finite algebras
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Added by
Alexandr Kazda
Publication date
2019
fields
Informatics Engineering
and research's language is
English
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We study the problem of whether a given finite algebra with finitely many basic operations contains a cube term; we give both structural and algorithmic results. We show that if such an algebra has a cube term then it has a cube term of dimension at most $N$, where the number $N$ depends on the arities of basic operations of the algebra and the size of the basic set. For finite idempotent algebras we give a tight bound on $N$ that, in the special case of algebras with more than $binom{|A|}2$ basic operations, improves an earlier result of K. Kearnes and A. Szendrei. On the algorithmic side, we show that deciding the existence of cube terms is in P for idempotent algebras and in EXPTIME in general. Since an algebra contains a $k$-ary near unanimity operation if and only if it contains a $k$-dimensional cube term and generates a congruence distributive variety, our algorithm also lets us decide whether a given finite algebra has a near unanimity operation.
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We show that for a fixed positive integer k one can efficiently decide if a finite algebra A admits a k-ary weak near unanimity operation by looking at the local behavior of the terms of A. We also observe that the problem of deciding if a given finite algebra has a quasi Taylor operation is solvable in polynomial time by looking, essentially, for local quasi Siggers operations.
In this paper we investigate the computational complexity of deciding if a given finite algebraic structure satisfies a fixed (strong) Maltsev condition $Sigma$. Our goal in this paper is to show that $Sigma$-testing can be accomplished in polynomial time when the algebras tested are idempotent and the Maltsev condition $Sigma$ can be described using paths. Examples of such path conditions are having a Maltsev term, having a majority operation, and having a chain of Jonsson (or Gumm) terms of fixed length.
The subalgebra membership problem is the problem of deciding if a given element belongs to an algebra given by a set of generators. This is one of the best established computational problems in algebra. We consider a variant of this problem, which is motivated by recent progress in the Constraint Satisfaction Problem, and is often referred to as the Subpower Membership Problem (SMP). In the SMP we are given a set of tuples in a direct product of algebras from a fixed finite set $mathcal{K}$ of finite algebras, and are asked whether or not a given tuple belongs to the subalgebra of the direct product generated by a given set. Our main result is that the subpower membership problem SMP($mathcal{K}$) is in P if $mathcal{K}$ is a finite set of finite algebras with a cube term, provided $mathcal{K}$ is contained in a residually small variety. We also prove that for any finite set of finite algebras $mathcal{K}$ in a variety with a cube term, each one of the problems SMP($mathcal{K}$), SMP($mathbb{HS} mathcal{K}$), and finding compact representations for subpowers in $mathcal{K}$, is polynomial time reducible to any of the others, and the first two lie in NP.
This paper investigates the computational complexity of deciding if a given finite idempotent algebra has a ternary term operation $m$ that satisfies the minority equations $m(y,x,x) approx m(x,y,x) approx m(x,x,y) approx y$. We show that a common polynomial-time approach to testing for this type of condition will not work in this case and that this decision problem lies in the class NP.
For some Maltsev conditions $Sigma$ it is enough to check if a finite algebra $mathbf A$ satisfies $Sigma$ locally on subsets of bounded size, in order to decide, whether $mathbf A$ satisfies $Sigma$ (globally). This local-global property is the main known source of tractability results for deciding Maltsev conditions. In this paper we investigate the local-global property for the existence of a $G$-term, i.e. an $n$-ary term that is invariant under permuting its variables according to a permutation group $G leq$ Sym($n$). Our results imply in particular that all cyclic loop conditions (in the sense of Bodirsky, Starke, and Vucaj) have the local-global property (and thus can be decided in polynomial time), while symmetric terms of arity $n>2$ fail to have it.
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