Factorable congruences and strict refinement.
Finite atomistic lattices that can be represented as lattices of quasivarieties
We prove that a finite atomistic lattice can be represented as a lattice of quasivarieties if and only if it is isomorphic to the lattice of all subsemilattices of a finite semilattice. This settles a conjecture that appeared in the context of [11].
Finitely generated almost universal varieties of -lattices
A concrete category is (algebraically) universal if any category of algebras has a full embedding into , and is almost universal if there is a class of -objects such that all non-constant homomorphisms between them form a universal category. The main result of this paper fully characterizes the finitely generated varieties of -lattices which are almost universal.
Finitely generated congruence distributive quasivarieties of algebras
Finitely generated universal varieties of distributive double -algebras
Finite-to-finite universal varieties of distributive double -algebras
Formulas and ultraproducts in categories
Free algebras in varieties
We define varieties of algebras for an arbitrary endofunctor on a cocomplete category using pairs of natural transformations. This approach is proved to be equivalent to one of equational classes defined by equation arrows. Free algebras in the varieties are investigated and their existence is proved under the assumptions of accessibility.
Free Suslin algebras
From hereditary classes to varieties in abstract model theory and partial algebra