Mod Galois representations of solvable image

It is proved that, for a number field and a prime number , there exist only finitely many isomorphism classes of continuous semisimple Galois representations of into of fixed dimension and bounded Artin conductor outside which have solvable images. Some auxiliary results are also proved.

     

-actions whose fixed data has a section

Given a collection of real vector bundles over a closed manifold , suppose that, for some is of the form , where is the trivial one-dimensional bundle. In this paper we prove that if is the fixed data of a -action, then the same is true for the Whitney sum obtained from by replacing by . This stability property is well-known for involutions. Together with techniques previously developed, this result is used to describe, up to bordism, all possible -actions fixing the disjoint union of an even projective space and a point.     

Fitting heights and the character degree graph

We say that the degree graph G\Gamma has bounded Fitting height if there is a bound on the Fitting heights of the solvable groups for which G\Gamma is the degree graph. In this paper, we determine which degree graphs have bounded Fitting height.     

A group of paths in

We define a group structure on the set of compact ``minimal' paths in . We classify all finitely generated subgroups of this group : they are free products of free abelian groups and surface groups. Moreover, each such group occurs in . The subgroups of isomorphic to surface groups arise from certain topological -forms on the corresponding surfaces. We construct examples of such -forms for cohomology classes corresponding to certain eigenvectors for the action on cohomology of a pseudo-Anosov diffeomorphism. Using we construct a non-polygonal tiling problem in , that is, a finite set of tiles whose corresponding tilings are not equivalent to those of any set of polygonal tiles. The group has applications to combinatorial tiling problems of the type: given a set of tiles and a region , can be tiled by translated copies of tiles in ?

     

On diameter and inverse degree of a graph

The inverse degree r(G) of a finite graph G=(V,E) is defined as , where is the degree of vertex v. We establish inequalities concerning the sum of the diameter and the inverse degree of a graph which for the most part are tight. We also find upper bounds on the diameter of a graph in terms of its inverse degree for several important classes of graphs. For these classes, our results improve bounds by Erd?s et al. (1988) [5], and by Dankelmann et al. (2008) [4].     

Corrigenda to ``New primitive whose degree is a Mersenne exponent,' and some new primitive pentanomials

We report an error in our previous paper [#!K1!#], where we announced that we listed all the primitive trinomials over of degree 859433, but there is a bug in the sieve. We missed the primitive trinomial and its reciprocal, as pointed out by Richard Brent et al. We also report some new primitive pentanomials.

     

New primitive whose degree is a Mersenne exponent

All primitive trinomials over with degree 859433 (which is the 33rd Mersenne exponent) are presented. They are and its reciprocal. Also two examples of primitive pentanomials over with degree 86243 (which is the 28th Mersenne exponent) are presented. The sieve used is briefly described.

     

Nilpotency degree of cohomology rings in characteristic

Let be an odd prime number. The purpose of this paper is to provide a -group whose mod- cohomology ring has a nilpotent element satisfying .

     

Inequalities for decomposable forms of degree variables

We consider the number of integral solutions to the inequality , where is a decomposable form of degree in variables. We show that the number of such solutions is finite for all only if the discriminant of is not zero. We get estimates for the number of such solutions that display appropriate behavior in terms of the discriminant. These estimates sharpen recent results of the author for the general case of arbitrary degree.

     

Automorphism group and diameter of a graph

Given a connected graph Γ of order n and diameter d, we establish a tight upper bound for the order of the automorphism group of Γ as a function of n and d, and determine the graphs for which the bound is attained. © 2011 Wiley Periodicals, Inc. J Graph Theory.     

On certain character sums over

Let be the finite field with elements and let denote the ring of polynomials in one variable with coefficients in . Let be a monic polynomial irreducible in . We obtain a bound for the least degree of a monic polynomial irreducible in ( odd) which is a quadratic non-residue modulo . We also find a bound for the least degree of a monic polynomial irreducible in which is a primitive root modulo .

     

A non-homogeneous zero-dimensional is a group

We provide an example of a zero-dimensional (separable metric) absolute Borel set which is not homogeneous, but whose square admits the structure of a topological group. We also construct a zero-dimensional absolute Borel set such that is a homogeneous non-group but is a group. This answers questions of Arhangel'skii and Zhou.

     

Sundual characterizations of the translation group of

We characterize the first three sundual spaces of , with respect to the translation group of .

     

The Brauer group of Sweedler's Hopf algebra

We calculate the Brauer group of the four dimensional Hopf algebra introduced by M. E. Sweedler. This Brauer group is defined with respect to a (quasi-) triangular structure on , given by an element . In this paper is a field . The additive group of is embedded in the Brauer group and it fits in the exact and split sequence of groups: where is the well-known Brauer-Wall group of . The techniques involved are close to the Clifford algebra theory for quaternion or generalized quaternion algebras.