Let H∞(D) denote the algebra of bounded analytic functions on the open unit disc in the complex plane. For a function g ∈ L∞(D), the Hankel-type operator Sg is defined by Sg(f) = gf +H∞(D). We give here an overview of the study of the symbol of the Hankel-type operator, with emphasis on those symbols for which the operator is compact, weakly compact, or completely continuous. We conclude with a...

Having as starting point Barr’s description of topological spaces as lax algebras for the ultrafilter monad [2], in this paper we present further topological examples of lax algebras – such as quasi-metric spaces, approach spaces and quasi-uniform spaces – and show that, in a suitable setting, the categories of lax algebras have indeed a topological nature. Furthermore, we generalize to this se...

Khoussainov, B. Recursive “nary algebras and trees, Annals of Pure and Applied Logic 67 (1994) 213-268. A unary algebra is an algebraic system d = (A,f,, ,L). wheref,, . ,fm are unary operations on A and n EW. In the paper we develop the theory ofeffective “nary algebras. We investigate well-known questions of constructive (recursive) model theory with respect to the class of unary algebras. In...

We have two polynomial time results for the uniform word problem for a quasivariety Q: (a) The uniform word problem for Q can be solved in polynomial time iff one can find a certain congruence on finite partial algebras in polynomial time. (b) Let Q* be the relational class determined by Q. If any universal Horn class between the universal closure S(Q*) and the weak embedding closure S(Q*) of Q...

Let A = (A;F ) be an algebra and ConA its congruence lattice. Recall that A is congruence uniform (see e.g. [1], [5]) if for each Θ ∈ ConA and every a, b ∈ A, card[a]Θ = card[b]Θ. Examples of congruence uniform algebras are e.g. groups, rings or Boolean algebras. A variety V is congruence uniform if each A ∈ V has this property. It was proved by W. Taylor [5] that every congruence uniform varie...