A categorical genealogy for the congruence distributive property.
A characterization of arithmetical varieties by two-element subsets
A characterization of distributive lattices by tolerance lattices
A characterization of quasivarieties of partial algebras by means of limits and products.
A characterization of tolerance-distributive tree semilattices
A characterization of varieties of inverse semigroups.
A common generalization of permutability and 0-permutability
A concept of variety for regular biordered sets.
A duality between infinitary varieties and algebraic theories
A duality between -ary varieties and -ary algebraic theories is proved as a direct generalization of the finitary case studied by the first author, F.W. Lawvere and J. Rosick’y. We also prove that for every uncountable cardinal , whenever -small products commute with -colimits in , then must be a -filtered category. We nevertheless introduce the concept of -sifted colimits so that morphisms between -ary varieties (defined to be -ary, regular right adjoints) are precisely the functors...
A duality for isotropic median algebras
We establish categorical dualities between varieties of isotropic median algebras and suitable categories of operational and relational topological structures. We follow a general duality theory of B.A. Davey and H. Werner. The duality results are used to describe free isotropic median algebras. If the number of free generators is less than five, the description is detailed.
A finite axiomatization of nondeterministic regular expressions
A Finite Axiomatization of Nondeterministic Regular Expressions
An alternative (tree-based) semantics for a class of regular expressions is proposed that assigns a central rôle to the + operator and thus to nondeterminism and nondeterministic choice. For the new semantics a consistent and complete axiomatization is obtained from the original axiomatization of regular expressions by Salomaa and by Kozen by dropping the idempotence law for + and the distribution law of • over +.
A Galois theory with stable units for simplicial sets.
A general existence theorem on partial algebras and its special cases
A generalization of Płonka sums
A generalization of the direct product of universal algebras
A multiplication of -varieties of orthodox semigroups
We define semantically a partial multiplication on the lattice of all e–varieties of regular semigroups. In the case that the first factor is an e–variety of orthodox semigroups we describe our multiplication syntactically in terms of biinvariant congruences.
A multiplication of e-varieties of regular -solid semigroups by inverse semigroup varieties
A multiplication of e-varieties of regular -solid semigroups by inverse semigroup varieties is described both semantically and syntactically. The associativity of the multiplication is also proved.
A new example of a limit variety.
A new proof of Reiterman's theorem