Circuits Through Cocircuits In A Graph With Extensions To Matroids

We show that for any k-connected graph having cocircumference c*, there is a cycle which intersects every cocycle of size c*-k + 2 or greater. We use this to show that in a 2-connected graph, there is a family of at most c* cycles for which each edge of the graph belongs to at least two cycles in the family. This settles a question raised by Oxley. A certain result known for cycles and cocycles in graphs is extended to matroids. It is shown that for a k-connected regular matroid having circumference c ≥ 2k if C₁ and C₂ are disjoint circuits satisfying r(C₁)+r(C₂)=r(C₁∪C₂), then |C₁|+|C₂|≤2(c-k + 1).     

A simple proof of an approximation theorem of H. Minkowski

It is proved that every convex bodyC inR ⁿ can be approximated by a sequenceC _k of convex bodies, whose boundary is the intersection of a level set of a homogeneous polynomial of degree 2k and a hyperplane. The Minkowski functional ofC _k is given explictly. Some further nice properties of the approximantsC _k are proved.Supported in part by BSF and Erwin Schrödinger Auslandsstipendium J0630.     

A New Approach to Optimization Under Monotonic Constraint

A new efficient branch and bound method is proposed for solving convex programs with an additional monotonic nonconvex constraint. Computational experiments demonstrated that this method is quite practical for solving rank k reverse convex programs with much higher values of k than previously considered in the literature and can be applied to a wider class of nonconvex problems.     

Arrangements of oriented hyperplanes

An arrangement ofn oriented hyperplanes or half-spaces dividesE ^d into a certain number of convex cells. We study the numberc _k of cells which are covered by exactlyk half-spaces and derive an upper bound onc _k for givenn andd.     

On monomialk-Buchsbaum curves inP r

This paper is deveted to the study of projective monomialk-Buchsbaum curves C. First, using the theory of affine semigroup rings, we give a criterion forC to bek-Buchsbaum. Then we give some lower and upper bounds for the numberk _c such thatC is strictlyk _c-Buchsbaum. For some classes of monomial curves we can computek _C explicity.     

On two finite covering problems of Bambah,Rogers, Woods and Zassenhaus

Bambah, Rogers, Woods, and Zassenhaus considered the general problem of covering planar convex bodiesC byk translates of a centrally-symmetric convex bodyK ofE ² with the ramification that these translates cover the convex hullC _k of their centres. They proved interesting inequalities for the volume ofC andC _k . In the present paper some analogous results in euclideand-spaceE ^d are given. It turns out that on one hand extremal configurations ford5 are of quite different type than in the planar case. On the other hand inequalities similar to the planar ones seem to exist in general. Inequalities in both directions for the volume and other quermass-integrals are given.     

Global C2‐Estimates for Convex Solutions of Curvature Equations

We establish a C² a priori estimate for convex hypersurfaces whose principal curvatures κ=(κ₁,…, κ_n) satisfy σ_k(κ(X))=f(X,ν(X)), the Weingarten curvature equation. We also obtain such an estimate for admissible 2‐convex hypersurfaces in the case k=2. Our estimates resolve a longstanding problem in geometric fully nonlinear elliptic equations.© 2015 Wiley Periodicals, Inc.     

Expectation of random polytopes

A random polytopeP _n in a convex bodyC is the convex hull ofn identically and independently distributed points inC. Its expectation is a convex body in the interior ofC. We study the deviation of the expectation ofP _n fromC asn→∞: while forC of classC ^k+1,k≥1, precise asymptotic expansions for the deviation exist, the behaviour of the deviation is extremely irregular for most convex bodiesC of classC ¹. Dedicated to my teacher and friend Professor Edmund Hlawka on the occasion of his 80^th birthday     

Differential calculus over general base fields and rings

We present an axiomatic approach to finite- and infinite-dimensional differential calculus over arbitrary infinite fields (and, more generally, suitable rings). The corresponding basic theory of manifolds and Lie groups is developed. Special attention is paid to the case of mappings between topological vector spaces over non-discrete topological fields, in particular ultrametric fields or the fields of real and complex numbers. In the latter case, a theory of differentiable mappings between general, not necessarily locally convex spaces is obtained, which in the locally convex case is equivalent to Keller's C^k_c-theory.     

A homotopy continuation method for solving normal equations

In this paper, we present a continuation method for solving normal equations generated byC ² functions and polyhedral convex sets. We embed the normal map into a homotopyH, and study the existence and characteristics of curves inH ¹(0) starting at a specificd point. We prove the convergence of such curves to a solution of the normal equation under some conditions on the polyhedral convex setC and the functionf. We prove that the curve will have finite are length if the normal map, associated with the derivative df(·) and the critical coneK, is coherently oriented at each zero of the normal mapf _c inside a certain ball of ⁿ . © 1998 The Mathematical Programming Society, Inc. Published by Elsevier Science B.V.This research was performed at the Department of Industrial Engineering, University of Wisconsin-Madison, Madison, WI, USA.     

Towards a Mathematical Theory of Super‐resolution

This paper develops a mathematical theory of super‐resolution. Broadly speaking, super‐resolution is the problem of recovering the fine details of an object—the high end of its spectrum—from coarse scale information only—from samples at the low end of the spectrum. Suppose we have many point sources at unknown locations in [0,1] and with unknown complex‐valued amplitudes. We only observe Fourier samples of this object up to a frequency cutoff f_c. We show that one can super‐resolve these point sources with infinite precision—i.e., recover the exact locations and amplitudes—by solving a simple convex optimization problem, which can essentially be reformulated as a semidefinite program. This holds provided that the distance between sources is at least 2/f_c. This result extends to higher dimensions and other models. In one dimension, for instance, it is possible to recover a piecewise smooth function by resolving the discontinuity points with infinite precision as well. We also show that the theory and methods are robust to noise. In particular, in the discrete setting we develop some theoretical results explaining how the accuracy of the super‐resolved signal is expected to degrade when both the noise level and the super‐resolution factor vary. © 2014 Wiley Periodicals, Inc.     

The Partitioned Version of the Erdős—Szekeres Theorem

   Abstract. Let k≥ 4 . A finite planar point set X is called a convex k -clustering if it is a disjoint union of k sets X ₁ , . . . ,X _k of equal sizes such that x ₁ x ₂ . . . x _k is a convex k -gon for each choice of x ₁ ∈ X ₁ , . . . ,x _k ∈ X _k . Answering a question of Gil Kalai, we show that for every k≥ 4 there are two constants c=c(k) , c'=c'(k) such that the following holds. If X is a finite set of points in general position in the plane, then it has a subset X' of size at most c' such that X \ X' can be partitioned into at most c convex k -clusterings. The special case k=4 was proved earlier by Pór. Our result strengthens the so-called positive fraction Erdos—Szekeres theorem proved by Barany and Valtr. The proof gives reasonable estimates on c and c' , and it works also in higher dimensions. We also improve the previous constants for the positive fraction Erdos—Szekeres theorem obtained by Pach and Solymosi.     

Mild solutions for abstract fractional differential equations

We propose a unified functional analytic approach to derive a variation of constants formula for a wide class of fractional differential equations using results on (a,?k)-regularized families of bounded and linear operators, which covers as particular cases the theories of C ₀-semigroups and cosine families. Using this approach we study the existence of mild solutions to fractional differential equation with nonlocal conditions. We also investigate the asymptotic behaviour of mild solutions to abstract composite fractional relaxation equations. We include in our analysis the Basset and Bagley–Torvik equations.     

<Emphasis Type="Italic">k</Emphasis>-Idempotency of linear combinations of an idempotent matrix and a tripotent matrix that commute

A complete solution is established to the problem of characterizing all situations in which a linear combination C = c ₁ A+c ₂ B of an idempotent matrix A and a tripotent matrix B is k-idempotent. As a special case of this, a set of necessary and sufficient conditions for a linear combination C = c ₁ A+c ₂ B to be k-idempotent when A and B are idempotent matrices, is also studied in this paper.     

On the power of the zeros of Bessel functions

For ν≥0 let c_νk be the k-th positive zero of the cylinder functionC _v(t)=J _v(t)cosα-Y _v(t)sinα, 0≤α>π whereJ _ν(t) andY _ν(t) denote the Bessel functions of the first and the second kind, respectively. We prove thatC _v,k ^1+H(x) is convex as a function of ν, ifc _νk≥x>0 and ν≥0, whereH(x) is specified in Theorem 1.1.     

An algorithm for selecting a good value for the parameter c in radial basis function interpolation

The accuracy of many schemes for interpolating scattered data with radial basis functions depends on a shape parameter c of the radial basis function. In this paper we study the effect of c on the quality of fit of the multiquadric, inverse multiquadric and Gaussian interpolants. We show, numerically, that the value of the optimal c (the value of c that minimizes the interpolation error) depends on the number and distribution of data points, on the data vector, and on the precision of the computation. We present an algorithm for selecting a good value for c that implicitly takes all the above considerations into account. The algorithm selects c by minimizing a cost function that imitates the error between the radial interpolant and the (unknown) function from which the data vector was sampled. The cost function is defined by taking some norm of the error vector E = (E ₁, ... , E_N)^T where E _k = E_k = f_k - S^k x_k) and S _k is the interpolant to a reduced data set obtained by removing the point x _k and the corresponding data value f _k from the original data set. The cost function can be defined for any radial basis function and any dimension. We present the results of many numerical experiments involving interpolation of two dimensional data sets by the multiquadric, inverse multiquadric and Gaussian interpolants and we show that our algorithm consistently produces good values for the parameter c. This revised version was published online in June 2006 with corrections to the Cover Date.     

A Generalisation of Tverberg’s Theorem

We will prove the following generalisation of Tverberg’s Theorem: given a set S⊂ℝ ^d of (r+1)(k−1)(d+1)+1 points, there is a partition of S in k sets A ₁,A ₂,…,A _k such that for any C⊂S of at most r points, the convex hulls of A ₁\C,A ₂\C,…,A _k \C are intersecting. This was conjectured first by Natalia García-Colín (Ph.D. thesis, University College of London, 2007).     

A note on Solodov and Tseng’s methods for maximal monotone mappings

This paper considers the problem of finding a zero of the sum of a single-valued Lipschitz continuous mapping A and a maximal monotone mapping B in a closed convex set C. We first give some projection-type methods and extend a modified projection method proposed by Solodov and Tseng for the special case of B=N_C to this problem, then we give a refinement of Tseng’s method that replaces P_C by P_Ck. Finally, convergence of these methods is established.     

On the number of cycles of length k in a maximal planar graph

Let G be a maximal planar graph with p vertices, and let C_k(G) denote the number of cycles of length k in G. We first present tight bounds for C₃(G) and C₄(G) in terms of p. We then give bounds for C_k(G) when 5 ≤ k ≤ p, and consider in particular bounds for C_p(G), in terms of p. Some conjectures and unsolved problems are stated.