Covering morphisms

Let (R, m) be a two-dimensional regular local ring and let A be a finitely generated torsion-free R-module. If A is a complete module, then Dan Katz and Vijay Kodiyalam show A satisfies five conditions. They ask whether these five conditions are equivalent without assuming A to be complete. In a previous paper we determined all implications among these five conditions with one exception. In this paper, with an additional hypothesis on the units of R, we resolve the remaining case.     

Deformations of coalgebra morphisms

An algebraic deformation theory of coalgebra morphisms is constructed.     

Initial morphisms and monomorphisms

Initial cones are shown to be mono-cones for several important types of functors, namely those possessing a reducible generator or having a certain topological (1.7) resp. algebraic property (1.8). These results are useful for the computation of the universal initial completion of faithful, topologically algebraic functors, contain results of Börger-Tholen and Hoffmann for certain setbased functors as special cases and allow a complete characterization of the initial cones in quite a few categories.     

On holomorphic harmonic morphisms

We study holomorphic harmonic morphisms from K?hler manifolds to almost Hermitian manifolds. When the codomain is also K?hler we get restrictions on such maps in the case of constant holomorphic curvature. We also prove a Bochner-type formula for holomorphic harmonic morphisms which, under certain curvature conditions of the domain, gives insight to the structure of the vertical distribution. We thus prove that when the domain is compact and non-negatively curved, the vertical distribution is totally geodesic. Received: 28 May 2001     

On p -harmonic morphisms

In this paper, we study the characterisation of p -harmonic morphisms between Riemannian manifolds, in the spirit of Fuglede-Ishihara. After a result establishing that p -harmonic morphisms are precisely horizontally weakly conformal p -harmonic maps, we compare ( 2 -)harmonic morphisms and p -harmonic morphisms ( p>2 ).     

Pure morphisms in pro-categories

Pure epimorphisms in categories pro-C, which essentially are just inverse limits of split epimorphisms in C, were recently studied by J. Dydak and F.R.R. del Portal in connection with Borsuk’s problem of descending chains of retracts of ANRs. We prove that pure epimorphisms are regular epimorphisms whenever C has weak finite limits, or pullbacks, or copowers. This improves the results of the above paper, and the results of the present authors on pure subobjects in accessible categories. We also turn to pure monomorphisms in pro-C, essentially just inverse limits of split monomorphisms in C, and prove that they are regular monomorphisms whenever C has finite products or pushouts.     

Frobenius semisimplicity for convolution morphisms

Mathematische Zeitschrift - This article concerns properties of mixed $$ell $$ -adic complexes on varieties over finite fields, related to the action of the Frobenius automorphism. We establish a...     

The unambiguity of segmented morphisms

This paper studies the ambiguity of morphisms in free monoids. A morphism σ is said to be ambiguous with respect to a string α if there exists a morphism τ which differs from σ for a symbol occurring in α, but nevertheless satisfies τ(α)=σ(α); if there is no such τ then σ is called unambiguous. Motivated by the recent initial paper on the ambiguity of morphisms, we introduce the definition of a so-called segmented morphism σ_n, which, for any , maps every symbol in an infinite alphabet onto a word that consists of n distinct factors in , where and are different letters. For every n, we consider the set U(σ_n) of those finite strings over an infinite alphabet with respect to which σ_n is unambiguous, and we comprehensively describe its relation to any U(σ_m), m≠n.Thus, our work features the first approach to a characterisation of sets of strings with respect to which certain fixed morphisms are unambiguous, and it leads to fairly counter-intuitive insights into the relations between such sets. Furthermore, it shows that, among the widely used homogeneous morphisms, most segmented morphisms are optimal in terms of being unambiguous for a preferably large set of strings. Finally, our paper yields several major improvements of crucial techniques previously used for research on the ambiguity of morphisms.     

Constant morphisms and constant subcategories

In a category supplied with a factorization system for morphisms and a fixed subcategory of constant objects, we introduce suitable notions ofconstant morphism and of the correspondingright andleft constant subcategories. The nature of constant morphisms we use captures two important features of constant subcategories: left-constant subcategories are right-constant in the dual category and the subcategory of constant objects contains relevant information on these subcategories. Furthermore, we present characterizations of constant subcategories in several contexts. Namely, we extend the characterization of disconnectednesses obtained by Huek and Pumplün, via terminal fans, to our context.The author acknowledges financial support by Centro de Matemática da Universidade de Coimbra.