Descent in ∗ -autonomous categories

We extend the result of Joyal and Tierney asserting that a morphism of commutative algebras in the ∗-autonomous category of sup-lattices is an effective descent morphism for modules if and only if it is pure, to an arbitrary ∗-autonomous category V (in which the tensor unit is projective) by showing that any V-functor out of V is precomonadic if and only if it is comonadic.     

THE EXPONENTIABLE MORPHISMS IN KELLEY ARE THE OPEN MAPS

Abstract

We show that in the category Kelley of Hausdorff k-spaces a map is exponentiable if and only if it is open and that any open surjection is an effective descent morphism.     

Finite preorders and topological descent II: étale descent

It is known that every effective (global-) descent morphism of topological spaces is an effective étale-descent morphism. On the other hand, in the predecessor of this paper we gave examples of:

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a descent morphism that is not an effective étale-descent morphism;

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an effective étale-descent morphism that is not a descent morphism.

Both of the examples in fact involved only finite topological spaces, i.e. just finite preorders, and now we characterize the effective étale-descent morphisms of preorders/finite topological spaces completely.     

Descent for Regular Epimorphisms in Barr Exact Goursat Categories

We show that the category of regular epimorphisms in a Barr exact Goursat category is almost Barr exact in the sense that (it is a regular category and) every regular epimorphism in it is an effective descent morphism.     

Descent Theory and Morita Theory for Ultrametric Banach Modules

In this paper we consider ultrametric Banach modules over commutative ultrametric Banach algebras with unit. We study the descent problem along a morphism f: R S of such algebras and show that descent morphisms coincide with weak retracts. We give further conditions for having an effective descent morphism or for having a Morita equivalence between the corresponding categories of ultrametric Banach modules.     

Über Morphismen halbmodularer Verbände

Morphisms and weak morphisms extend the concept of strong maps and maps of combinatorial geometry to the class of finite dimensional semimodular lattices. Each lattice which is the image of a semimodular lattice under a morphism is semimodular. In particular, each finite lattice is semimodular if and only if it is the image of a finite distributive lattice under a morphism. Regular and non-singular weak morphisms may be used to characterize modular and distributive lattices. Each morphism gives rise to a geometric closure operator which in turn determines a quotient of a semimodular lattice. A special quotient, the Higgs lift, is constructed and used to show that each morphism decomposes into elementary morphisms, and that each morphism may be factored into an injection and a contraction.

     

On Descent Theory for Monoid Actions

We give some sufficient conditions for a monoid homomorphism to be an effective descent morphism.     

EP morphisms

The concept of an EP matrix is extended to a morphism of a category C with involution. It is shown that an EP morphism has a group inverse iff it has a Moore-Penrose inverse, and in this case the inverses are identical. On the other hand, if a morphism has a Moore-Penrose inverse that is a group inverse, then C is a full subcategory of a category in which φ is EP. Also, if C is an additive category with involution 1 and with 1-biproduct factorization, then a morphism of φ of C is EP iff there is a 1-biproduct J ⊕ K and an invertible morphism θ : J → J such that φ is congruent to a morphism of the form

$\text{θ 0}\text{0 0}\text{: J⊕K → J⊕K.}$

In particular, a square matrix over a principal-ideal domain with involution is EP iff it is congruent to a matrix of the form dg(θ, 0) with θ invertible.     

A spectral characterization of skeletal maps

We prove that a map between two realcompact spaces is skeletal if and only if it is homeomorphic to the limit map of a skeletal morphism between ω-spectra with surjective limit projections.     

The bicategories of corings

To a B-coring and a (B,A)-bimodule that is finitely generated and projective as a right A-module an A-coring is associated. This new coring is termed a base ring extension of a coring by a module. We study how the properties of a bimodule such as separability and the Frobenius properties are reflected in the induced base ring extension coring. Any bimodule that is finitely generated and projective on one side, together with a map of corings over the same base ring, lead to the notion of a module-morphism, which extends the notion of a morphism of corings (over different base rings). A module-morphism of corings induces functors between the categories of comodules. These functors are termed pull-back and push-out functors, respectively, and thus relate categories of comodules of different corings. We study when the pull-back functor is fully faithful and when it is an equivalence. A generalised descent associated to a morphism of corings is introduced. We define a category of module-morphisms, and show that push-out functors are naturally isomorphic to each other if and only if the corresponding module-morphisms are mutually isomorphic. All these topics are studied within a unifying language of bicategories and the extensive use is made of interpretation of corings as comonads in the bicategory Bim of bimodules and module-morphisms as 1-cells in the associated bicategories of comonads in Bim.     

Finely locally injective finely harmonic morphisms

We consider fine topology in the complex plane C and finely harmonic morphisms. We use oriented Jordan curves in the plane to prove that for a finely locally injective finely harmonic morphism f in a fine domain in C, either f or f is a finely holomorphic function. This partially extends result by Fuglede, who considered a kind of continuity for the fine derivatives of the finely harmonic morphism. As a consequence of this we obtain a both necessary and sufficient condition for a function f to be finely holomorphic or finely antiholomorphic. We do not know if the condition of finely local injectivity (q.e.) is automatically fulfilled by any non-constant finely harmonic morphism.     

Proprietes et applications de la notion de contenu

The notion of content is used to solve certain problems. In the first part, we show that the structural morphism of a content algebra (see the paper of D.E. Rush [22] ) is spectrally open, under mild hypothesis. We show also that a flat module is universally content if and only if it is a Mittag-Leffler’s module in the sense of f2ll . In the second part, using content, we exhibit a kind of localization of a commutative ring A, attached to eyery subset X of Spec(A) i.e. a flat morphism A→ X(A). We can thus show that every quasi-compact, stable under generization subset of a spectra is a spectral image under a flat morphism, in a canonical way. We can also give in certain cases an elementary construction of the maximal flat injective epimor-phism of a ring. Suppose that A is a Noetherian ring and consider pro-perties of Noetherian rings such as factoriality, normality and so on.

Let X be the set of prime ideals of A at which A has the property. If X is stable under generization, the flat morphism A→ X(A) verifies hin general the ring X(A) has lornlly the property and a prime ideal P of A has a prime ideal lying over in X(A) if and only if pthe ring has the property at P.     

Models of Curves and Finite Covers

Let K be a discrete valuation field with ring of integers O _K .Letf : X ! Y be a finite morphism of curves over K. In this article, we study some possible relationships between the models over O K of X and of Y. Three such relationships are listed below. Consider a Galois cover f : X ! Y of degree prime to the characteristic of the residue field, with branch locus B. We show that if Y has semi-stable reduction over K,thenX achieves semi-stable reduction over some explicit tame extension of K.B/.WhenK is strictly henselian, we determine the minimal extension L=K with the property that X L has semi-stable reduction. Let f : X ! Y be a finite morphism, with g.Y/ > 2. We show that if X has a stable model X over O _K ,thenY has a stable model Y over O _K , and the morphism f extends to a morphism X ! Y. ! Y. Finally, given any finite morphism f : X ! Y, is it possible to choose suitable regular models X and Y of X and Y over O _K such that f extends to a finite morphism X ! Y ?As wasshown by Abhyankar, the answer is negative in general. We present counterexamples in rather general situ-ations, with f a cyclic cover of any order > 4. On the other hand, we prove, without any hypotheses on the residual characteristic, that this extension problem has a positive solution when f is cyclic of order 2 or 3.     

一个改进的共轭梯度法及其收敛性

A new conjugate gradient method is proposed in this paper. For any （inexact） line search, our scheme satifies the sufficient descent property. The method is proved to be globally convergent if the restricted Wolfe-Powell line search is used. Preliminary numerical result shows that it is efficient.     

A descent principle for the Dirac-dual-Dirac method

Let G be a torsion-free discrete group with a finite-dimensional classifying space BG. We show that G has a dual-Dirac morphism if and only if a certain coarse (co-)assembly map is an isomorphism. Hence the existence of a dual-Dirac morphism for such groups is a metric, that is, coarse, invariant. We get results for groups with torsion as well.     

A concept of variety for regular biordered sets

We define a bivariety of regular biordered sets to be a nonempty class of regular biordered sets which is closed under taking direct products, regular bimorphic images and relatively regular biordered subsets. It is then shown that there is a complete lattice morphism mapping the complete lattice of all e-varieties of regular semigroups onto the complete lattice of all bivarieties of regular biordered sets; as a corollary we prove that there is a complete lattice morphism mapping the complete lattice of all e-varieties of E-solid regular semigroups onto the complete lattice of all varieties of solid binary algebras. Examples of bivarieties include the class of all solid regular biordered sets and the class of all local semilattices. For each setX with at least two elements, we show that a bivariety contains a free object onX if and only if it consists entirely of solid regular biordered sets or it consists entirely of local semilattices. The author gratefully acknowledges the financial support of an Australian Postgraduate Research Award.     

Tropical hyperelliptic curves

We study the locus of tropical hyperelliptic curves inside the moduli space of tropical curves of genus g. We define a harmonic morphism of metric graphs and prove that a metric graph is hyperelliptic if and only if it admits a harmonic morphism of degree 2 to a metric tree. This generalizes the work of Baker and Norine on combinatorial graphs to the metric case. We then prove that the locus of 2-edge-connected genus g tropical hyperelliptic curves is a (2g?1)-dimensional stacky polyhedral fan whose maximal cells are in bijection with trees on g?1 vertices with maximum valence 3. Finally, we show that the Berkovich skeleton of a classical hyperelliptic plane curve satisfying a certain tropical smoothness condition is a standard ladder of genus g.     

The Levi problem for Riemann domains over Stein spaces with isolated singularities

We consider the following question: Let \({p:Y \rightarrow X}\) be an unbranched Riemann domain and assume that X is a Stein space and p is a Stein morphism. Does it follow that Y is Stein ? We show that the answer is affirmative if X has isolated singularities. This generalizes a result of Andreotti and Narasimhan.     

Epimorphisms of free monoids

A matrix characterization is obtained for the epimorphisms in the category of finitely generated free monoids. It follows from our result that it is effectively decidable whether a given morphism is an epimorphism. The corresponding question for monomorphisms has been answered in the algorithm of Sardinas and Patterson.