Embeddability of restricted linear spaces

In this note we examine the problem of embedding into finite projective planes finite linear spaces with p points and q lines satisfying (q ? p)² ? p and q ? 2.     

Analysis for one-dimensional time-fractional Tricomi-type equations by LDG methods

In this paper, we consider the local discontinuous Galerkin (LDG) finite element method for one-dimensional linear time-fractional Tricomi-type equation (TFTTE), which is obtained from the standard one-dimensional linear Tricomi-type equation by replacing the first-order time derivative with a fractional derivative (of order α, with 1?<?α?≤?2). The proposed LDG is based on LDG finite element method for space and finite difference method for time. We prove that the method is unconditionally stable, and the numerical solution converges to the exact one with order O(h ^k?+?1?+?τ ²), where h, τ and k are the space step size, time step size, polynomial degree, respectively. The comparison of the LDG results with the exact solutions is made, numerical experiments reveal that the LDG is very effective.     

Finiteness in restricted simplicial decomposition

Simplicial decomposition is an important form of decomposition for large non-linear programming problems with linear constraints. Von Hohenbalken has shown that if the number of retained extreme points is n + 1, where n is the number of variables in the problem, the method will reach an optimal simplex after a finite number of master problems have been solved (i.e., after a finite number of major cycles). However, on many practical problems it is infeasible to allocate computer memory for n + 1 extreme points. In this paper, we present a version of simplicial decomposition where the number of retained extreme points is restricted to r, 1 ? r ? n + 1, and prove that if r is sufficiently large, an optimal simplex will be reached in a finite number of major cycles. This result insures rapid convergence when r is properly chosen and the decomposition is implemented using a second order method to solve the master problem.     

Geometry of curves with exceptional secant planes: linear series along the general curve

We study linear series on a general curve of genus g, whose images are exceptional with regard to their secant planes. Working in the framework of an extension of Brill?CNoether theory to pairs of linear series, we prove that a general curve has no linear series with exceptional secant planes, in a very precise sense, whenever the total number of series is finite. Next, we partially solve the problem of computing the number of linear series with exceptional secant planes in a one-parameter family in terms of tautological classes associated with the family, by evaluating our hypothetical formula along judiciously-chosen test families. As an application, we compute the number of linear series with exceptional secant planes on a general curve equipped with a one-dimensional family of linear series. We pay special attention to the extremal case of d-secant (d ? 2)-planes to (2d ? 1)-dimensional series, which appears in the study of Hilbert schemes of points on surfaces. In that case, our formula may be rewritten in terms of hypergeometric series, which allows us both to prove that it is nonzero and to deduce its asymptotics in d.     

Recognizing Intersection Graphs of Linear Uniform Hypergraphs

A hypergraph is linear if any two distinct hyperedges have at most one common vertex. The existence of a polynomial algorithm is shown for deciding whether a graph of minimum degree δ ≥ 19 is the intersection graph of a linear 3-uniform hypergraph. This result improves a corollary of the finite forbidden subgraph characterization proved for δ ≥ 69 by Naik et al. in [8]. Essentially the same methods yield a polynomial recognition algorithm for the intersection graph of a linear r-uniform hypergraph, r ≥ 3, provided the minimum edge-degree of the graphs is at least 2r ² ? 3r + 1. This improves on the cubic bound that follows from the corresponding finite characterization result in [8].     

A classification of linear spaces based on quadrangles,II

A quadrangle in a linear space can have at most 3 diagonal points. Denoting by d(Q) the number of diagonal points of a quadrangle Q, we say that a linear space L is of type T ? {0, 1, 2, 3}, if T is the set of values taken by d(Q) for all quadrangles Q in L. This determines a classification of linear spaces into 16 possible types. In this paper we show that the only type {1, 3} linear space in which all lines have finite cardinality is the one obtained from the projective plane of order 4 by deleting two lines together with all their points except the point of intersection.     

Coprime subdegrees for primitive permutation groups and completely reducible linear groups

In this paper we answer a question of Gabriel Navarro about orbit sizes of a finite linear group H ? GL(V) acting completely reducibly on a vector space V: if the H-orbits containing the vectors a and b have coprime lengths m and n, we prove that the H-orbit containing a + b has length mn. Such groups H are always reducible if n,m > 1. In fact, if H is an irreducible linear group, we show that, for every pair of non-zero vectors, their orbit lengths have a non-trivial common factor. In the more general context of finite primitive permutation groups G, we show that coprime non-identity subdegrees are possible if and only if G is of O’Nan-Scott type AS, PA or TW. In a forthcoming paper we will show that, for a finite primitive permutation group, a set of pairwise coprime subdegrees has size at most 2. Finally, as an application of our results, we prove that a field has at most 2 finite extensions of pairwise coprime indices with the same normal closure.     

Approximate amenability of certain inverse semigroup algebras

In this article, the approximate amenability of semigroup algebra ?¹(S) is investigated, where (S) is a uniformly locally finite inverse semigroup. Indeed, we show that for a uniformly locally finite inverse semigroup (S), the notions of amenability, approximate amenability and bounded approximate amenability of ?¹(S) are equivalent. We use this to give a direct proof of the approximate amenability of ?¹(S) for a Brandt semigroup (S). Moreover, we characterize the approximate amenability of ?¹(S), where S is a uniformly locally finite band semigroup.