Spin Structures on Lefschetz Fibrations

Necessary and sufficient conditions for Lefschetz fibrationsover D² and S² to carry spin structures are presented. The conditionsare given in terms of vanishing cycles and homology sections.     

球面导数，高阶导数与正现族

本文应用陈怀惠和顾永兴关于Ｚａｌｃｍａｎ不正规性条件的改进结果，推广和加强了Ｌａｐｐａｎ的一个正规定则．Ｌａｐｐａｎ证明：若亚纯函数族中的所有函数的球面导数的幂方（大于２）在紧集上的积分一致有界，则该族是正则的．本文证明，把积分限制在函数值的模小于给定常数的子集上，结论仍然成立．同时，用高阶导数的积分替代球面导数的积分，得到十分一般的结果．另外对幂方为２的情形也进行了讨论．     

A Note on Fibrations

In this paper we generalize a theorem of McAuley-Tulley [2] to show that small homotopies of any topological space can be lifted to small homotopies for any Hurewicz fibration between arbitrary topological spaces. Received: July 7, 2005. Revised: July 13, 2006.     

Fibrations of bigroupoids

Using Brown's construction (J. Algebra 15 (1970) 103) of an exact 6-term sequence for a fibration of groupoids we show how an exact 9-term sequence can be associated to a fibration of bigroupoids. Applications to topology and algebra are given.     

Hyperbolic Fibrations and q-Clans

We show there is a bijection between regular hyperbolic fibrations with constant back half and normalized q-clans. Thus there is also a bijection with flocks of a quadratic cone, once a conic of the flock has been specified. This yields a plethora of two-dimensional translation planes of even and odd order which arise from spreads admitting a regular elliptic cover.     

Fibrations and nullifications

The paper considers some aspects of the preservation of a fibrationF→E→B by a localization functorL, meaning thatLF→LE→LB is also a fibration. We obtain necessary and sufficient conditions for this, in caseL is equivalent to a nullification functorP _W . Applications include the fact that if such a fibration is preserved by a nullification, then so too are all of its pullbacks. Another application is a homological criterion for a fibration to be preserved. No similar results are known for general localization functors.     

Fibrations of topological stacks

In this note we define fibrations of topological stacks and establish their main properties. When restricted to topological spaces, our notion of fibration coincides with the classical one. We prove various standard results about fibrations (long exact sequence for homotopy groups, Leray–Serre and Eilenberg–Moore spectral sequences, etc.). We prove various criteria for a morphism of topological stacks to be a fibration, and use these to produce examples of fibrations. We prove that every morphism of topological stacks factors through a fibration and construct the homotopy fiber of a morphism of topological stacks. As an immediate consequence of the machinery we develop, we also prove van Kampen?s theorem for fundamental groups of topological stacks.     

Fibrations of Simplicial Sets

There are infinitely many variants of the notion of Kan fibration that, together with suitable choices of cofibrations and the usual notion of weak equivalence of simplicial sets, satisfy Quillen’s axioms for a homotopy model category. The combinatorics underlying these fibrations is purely finitary and seems interesting both for its own sake and for its interaction with homotopy types. To show that these notions of fibration are indeed distinct, one needs to understand how iterates of Kan’s Ex functor act on graphs and on nerves of small categories.     

Fibrations of Genus Two on Complex Surfaces

This work is devoted to the study of fibrations of genus 2 by using as its main tool the theory of singular holomorphic foliations. In particular we obtain a sharp differentiable version of Matsumoto–Montesinos theory. In the case of isotrivial fibrations, these methods are powerful enough to provide a detailed global picture of the both the ambient surface and of the structure of the fibrations itself.     

On Some Families of Smooth Affine Spherical Varieties of Full Rank

Let G be a complex connected reductive group. Losev has shown that a smooth affine spherical G-variety X is uniquely determined by its weight monoid, which is the set of irreducible representations of G that occur in the coordinate ring of X. In this paper we use a combinatorial characterization of the weight monoids of smooth affine spherical varieties to classify:(a) all such varieties for G = SL(2) × C~×and(b) all such varieties for G simple which have a G-saturated weight monoid of full rank. We also use the characterization and Knop's classification theorem for multiplicity free Hamiltonian manifolds to give a new proof of Woodward's result that every reflective Delzant polytope is the moment polytope of such a manifold.     

On Lifting Fibrations of Genus One

We investigate the problem of lifting fibrations of genus one on algebraic surfaces of Kodaira dimension zero. We prove that fibrations on the following surfaces lift: Enriques surfaces, K3 surfaces covering Enriques surfaces, certain hyperelliptic, and quasi-hyperelliptic surfaces.     

On Milnor Fibrations of Arrangements

We use covering space theory and homology with local coefficientsto study the Milnor fiber of a homogeneous polynomial. Thesetechniques are applied in the context of hyperplane arrangements,yielding an explicit algorithm for computing the Betti numbersof the Milnor fiber of an arbitrary real central arrangementin C³, as well as the dimensions of the eigenspaces of the algebraicmonodromy. We also obtain combinatorial formulas for these invariantsof the Milnor fiber of a generic arrangement of arbitrary dimensionusing these methods.     

Fibrations with Hopfian properties

This study explores the homotopy-theoretic meeting-point of topics in differential topology, combinatorial group theory and algebraicK-theory. The first two are due to H. Hopf and date from around 1930. The third arose in the author’s characterisation of plus-constructive fibrations. LetF ( _→ ^ί )E →B be a fibration such thati induces an isomorphism of homology with trivial integer coefficients; what is the effect ofi on fundamental groups? In particular, when one passes to hypoabelianisations by factoring out perfect radicals, doesi induce an epimorphism? Numerous conditions are determined which force an affirmative answer. On the other hand, negative examples of a non-finitary nature are also provided. This leaves the question open in the finitely generated case, where it forms a homological version of the dual to Hopf’s original, famous question in group theory.     

Fibrations as triple algebras

The standard conversion of a map to a fibration is shown to be a triple. The algebras of this triple are fibrations with restrictions. Up to a category of fractions, however, the algebras are equivalent to the category of fibrations.     

Variational Sequences in Mechanics on Grassmann Fibrations

Extension of the variational sequence theory in mechanics to the first order Grassmann fibrations of 1-dimensional submanifolds is presented. The correspondence with the variational theory of parameter-invariant problems on manifolds is discussed in terms of the theory of jets (slit tangent bundles) and contact elements. In particular, the Helmholtz expressions for parameter-invariant variational problems, measuring local variationality of differential forms and differential equations, are given in the canonical and adapted coordinates. The methods can easily be extended to higher order variational problems.     

一类分离Hash函数族的结构

Hash函数族是从有限集到有限集的函数的集合,任何一种编码都可以表示为一个Hash函数族,完全Hash函数族在密码学中有很重要的应用.利用矩阵和图论的方法研究了一类分离Hash函数族的结构,得到了几个分离Hash函数族的结构定理.     

Fibrations of Campana general type on surfaces

We construct complex surfaces with genus two fibrations over \mathbb P¹{\mathbb P^1} having special fibres such that the minimum of the multiplicities of the components is ≥ 2 whereas the g.c.d is 1. We can then produce new examples of fibred surfaces without multiple fibres which are of “general type” according to the definition of Campana. We prove that these surfaces are of general type and simply connected; and we compute in some cases their invariants. Moreover, we extend the construction obtaining general type fibrations of any even genus on simply connected surfaces. All our examples are defined over number fields.