Ralph McKenzie

Varieties with few subalgebras of powers

The Constraint Satisfaction Problem Dichotomy Conjecture of Feder and Vardi (1999) has in the last 10 years been profitably reformulated as a conjecture about the set SP fin (A) of subalgebras of finite Cartesian powers of a finite universal algebra A. One particular strategy, advanced by Dalmau in his doctoral thesis (2000), has confirmed the conjecture for a certain class of finite algebras A which, among other things, have the property that the number of subalgebras of A n is bounded by an exponential polynomial. In this paper we characterize the finite algebras A with this property, which we call having few subpowers, and develop a representation theory for the subpowers of algebras having few subpowers. Our characterization shows that algebras having few subpowers are the finite members of a newly discovered and surprisingly robust Maltsev class defined by the existence of a special term we call an edge term. We also prove some tight connections between the asymptotic behavior of the number of subalgebras of A n and some related functions on the one hand, and some standard algebraic properties of A on the other hand. The theory developed here was applied to the Constraint Satisfaction Problem Dichotomy Conjecture, completing Dalmaus strategy.

An Elementary Construction of Unsolvable Word Problems in Group Theory

Publisher Summary This chapter presents an elementary construction of unsolvable word problems in group theory and introduces a new approach to constructing finitely presented groups with unsolvable word problems. The argument has a concrete motivation that makes it easy to follow. The chapter also discusses arrow notation and presents a proof of the Novikov–Boone theorem that states that there exists a finitely presented group whose word problem is not recursively solvable.

TARSKI’S FINITE BASIS PROBLEM IS UNDECIDABLE

We exhibit a construction which produces for every Turing machine , an algebra (finite and of finite type) such that the Turing machine halts iff the algebra has a finite basis for its equations.

Para primal varieties: A study of finite axiomatizability and definable principal congruences in locally finite varieties

We introduce new sufficient conditions for a finite algebraU to possess a finite basis of identities. The conditions are that the variety generated byU possess essentially only finitely many subdirectly irreducible algebras, and have definable principal congruences. Both conditions are satisfied if this variety is directly representable by a finite set of finite algebras. One task of the paper is to show that virtually no lattice varieties possess definable principal congruences. However, the main purpose of the paper is to apply the new criterion in proving that every para primal variety (congruence permutable variety generated by finitely many para primal algebras) is finitely axiomatizable. The paper also contains a completely new approach to the structure theory of para primal varieties which complements and extends somewhat the recent work of Clark and Krauss.

The Structure of Decidable Locally Finite Varieties

0 Preliminaries.- 0.1 Languages, structures, algebras, and graphs.- 0.2 Decidability and interpretability.- 0.3 Varieties.- 0.4 Abelian and solvable algebras.- 0.5 Special kinds of varieties.- 0.6 Tame congruence theory.- 0.7 Definable relations in subdirect powers.- 1 Preview: The three sub varieties.- I: Structured Varieties.- 2: a property of the center.- 3: Centerless algebras.- 4: The discriminator subvariety.- 5: The Abelian subvariety.- 6: Transfer principles.- Summary of Part I.- II: Structured Abelian Varieties.- 7: Strongly solvable varieties.- 8: More transfer principles.- 9: Consequences of the transfer principles.- 10: Three interpretations.- 11: From strongly Abelian to essentially unary varieties.- 12: The unary case.- III: The Decomposition.- 13: The decomposition theorem.- 14: Conclusion.- Notation.

THE RESIDUAL BOUNDS OF FINITE ALGEBRAS

We exhibit, for every finite cardinal λ≥3 and also for each of λ=ω, ω1, (2ω)+, a fourelement algebra that generates a precisely residually < λ variety. We exhibit an eight-element simple algebra with eight operations that is inherently non-finitely-based and generates a precisely residually countable variety.

THE RESIDUAL BOUND OF A FINITE ALGEBRA IS NOT COMPUTABLE

We exhibit a construction which produces for every Turing machine , an algebra (finite and of finite type) such that the Turing machine halts iff the variety generated by is residually finite.

Finite equational bases for congruence modular varieties

In this paper it is proved that a variety generated by a finite algebraic system with finitely many operations is finitely axiomatizable, provided that the variety is congruence modular and residually small. This result is an extension to congruence modular varieties of a well known theorem for congruence distributive varieties, due to K. A. Baker. Also, under somewhat less restrictive hypotheses, (which are satisfied by finite groups and rings) it is proved that a finite algebraic system belongs to a finitely axiomatizable locally finite variety.

Narrowness implies uniformity

This paper is about varietiesV of universal algebras which satisfy the following numerical condition on the spectrum: there are only finitely many prime integersp such thatp is a divisor of the cardinality of some finite algebra inV. Such varieties are callednarrow.The variety (or equational class) generated by a classK of similar algebras is denoted by V(K)=HSPK. We define Pr (K) as the set of prime integers which divide the cardinality of a (some) finite member ofK. We callK narrow if Pr (K) is finite. The key result proved here states that for any finite setK of finite algebras of the same type, the following are equivalent: (1) SPK is a narrow class. (2) V(K) has uniform congruence relations. (3) SK has uniform congruences and (3) SK has permuting congruences. (4) Pr (V(K))= Pr(SK).A varietyV is calleddirectly representable if there is a finite setK of finite algebras such thatV= V(K) and such that all finite algebras inV belong to PK. An equivalent definition states thatV is finitely generated and, up to isomorphism,V has only finitely many finite directly indecomposable algebras. Directly representable varieties are narrow and hence congruence modular. The machinery of modular commutators is applied in this paper to derive the following results for any directly representable varietyV. Each finite, directly indecomposable algebra inV is either simple or abelian.V satisfies the commutator identity [x,y]=x·y·[1,1] holding for congruencesx andy over any member ofV. The problem of characterizing finite algebras which generate directly representable varieties is reduced to a problem of ring theory on which there exists an extensive literature: to characterize those finite ringsR with identity element for which the variety of all unitary leftR-modules is directly representable. (In the terminology of [7], the condition is thatR has finite representation type.) We show that the directly representable varieties of groups are precisely the finitely generated abelian varieties, and that a finite, subdirectly irreducible, ring generates a directly representable variety iff the ring is a field or a zero ring.

Commutator theory for relatively modular quasivarieties

We develop a commutator theory for relatively modular quasivarieties that extends the theory for modular varieties. We characterize relatively modular quasivarieties, prove that they have an almost-equational axiomatization and we investigate the lattice of subquasivarieties. We derive the result that every finitely generated, relatively modular quasivariety of semigroups is finitely based