Jane G. Pitkethly

Binary homomorphisms and natural dualities.

Abstract We investigate ways in which certain binary homomorphisms of a finite algebra can guarantee its dualisability. Of particular interest are those binary homomorphisms which are lattice, flat-semilattice or group operations. We prove that a finite algebra which has a pair of lattice operations amongst its binary homomorphisms is dualisable. As an application of this result, we find that every finite unary algebra can be embedded into a dualisable algebra. We develop some general tools which we use to prove the dualisability of a large number of unary algebras. For example, we show that the endomorphisms of a finite cyclic group are the operations of a dualisable unary algebra.

THE COMPLEXITY OF DUALISABILITY: THREE-ELEMENT UNARY ALGEBRAS

We solve the dualisability problem in the class of three-element unary algebras. Our aim in tackling this class is to demonstrate the difficulty of the general dualisability problem. We also want to investigate the extent to which the dualisability of a finite algebra is a finiteness condition on the quasi-variety it generates.

THE LATTICE OF ALTER EGOS

We introduce a new Galois connection for partial operations on a finite set, which induces a natural quasi-order on the collection of all partial algebras on this set. The quasi-order is compatible with the basic concepts of natural duality theory, and we use it to turn the set of all alter egos of a given finite algebra into a doubly algebraic lattice. The Galois connection provides a framework for us to develop further the theory of natural dualities for partial algebras. The development unifies several fundamental concepts from duality theory and reveals a new understanding of full dualities, particularly at the finite level.

On the Betti Numbers of Nilpotent Lie Algebras of Small Dimension

The work of Golod and Safarevic on class field towers motivated the conjecture that b2 > b2 1/4 for nilpotent Lie algebras of dimension at least 3, where b i denotes the i th Betti number. Using a new lower bound for b 2 and a characterization of Lie algebras of the form g/Z(g), we prove this conjecture for 2-step algebras. We also give the Betti numbers of nilpotent Lie algebras of dimension at most 7 and use them to establish the conjecture for all nilpotent Lie algebras whose centres have codimension ≤ 7.

UNCOUNTABLY MANY DUALISABLE ALGEBRAS

Fix a finite set M with at least three elements. We find uncountably many different clones on M, each of which is the clone of term functions of a strongly dualisable algebra. This provides a solution to the Finite Type Problem of natural duality theory: there are finite algebras that are dualisable but not via a structure of finite type.

COUNTING THE RELATIONS COMPATIBLE WITH AN ALGEBRA

We investigate when a finite algebra admits only a finite number of compatible relations (modulo a natural equivalence). This finiteness condition is closely related to others in the literature, and arises naturally in duality theory. We find necessary conditions for a finite algebra to admit only finitely many compatible relations, as well as a family of examples of such algebras.

How finite is a three-element unary algebra?

We show that, within the class of three-element unary algebras, there is a tight connection between a finitely based quasi-equational theory, finite rank, enough algebraic operations (from natural duality theory) and a special injectivity condition.

The Homomorphism Lattice Induced by a Finite Algebra

Each finite algebra A induces a lattice LA via the quasi-order → on the finite members of the variety generated by A, where B →C if there exists a homomorphism from B to C. In this paper, we introduce the question: ‘Which lattices arise as the homomorphism lattice LA induced by a finite algebra A?’ Our main result is that each finite distributive lattice arises as LQ, for some quasi-primal algebra Q. We also obtain representations of some other classes of lattices as homomorphism lattices, including all finite partition lattices, all finite subspace lattices and all lattices of the form L ⊕1, where L is an interval in the subgroup lattice of a finite group.

New-from-old full dualities via axiomatisation

We clarify what it means for two full dualities based on the same algebra to be different. Our main theorem gives conditions on two different alter egos of a finite algebra under which, if one yields a full duality, then the other does too. We use this theorem to obtain a better understanding of several important examples from the theory of natural dualities. Throughout the paper, a fundamental role is played by the universal Horn theory of the dual classes.