Continuity and compactness of singular integral operators

In the space of square integrable functions we establish effective sufficient continuity and compactness conditions for singular integral operators with Cauchy kernels on a segment of the real axis.     

Crystal graphs for general linear Lie superalgebras and quasi-symmetric functions

We give a new representation theoretic interpretation of the ring of quasi-symmetric functions. This is obtained by showing that the super analogue of Gessel's fundamental quasi-symmetric function can be realized as the character of a connected crystal for the Lie superalgebra associated to its non-standard Borel subalgebra with a maximal number of odd isotropic simple roots. We also present an algebraic characterization of these super quasi-symmetric functions.     

Cyclotomic factors of the descent set polynomial

We introduce the notion of the descent set polynomial as an alternative way of encoding the sizes of descent classes of permutations. Descent set polynomials exhibit interesting factorization patterns. We explore the question of when particular cyclotomic factors divide these polynomials. As an instance we deduce that the proportion of odd entries in the descent set statistics in the symmetric group Sn only depends on the number on 1's in the binary expansion of n. We observe similar properties for the signed descent set statistics.     

Decomposing complete edge-chromatic graphs and hypergraphs. Revisited

A d-graph is a complete graph whose edges are colored by d colors, that is, partitioned into d subsets some of which might be empty. We say that a d-graph is complementary connected (CC) if the complement to each chromatic component of is connected on V. We prove that every such d-graph contains a sub-d-graph Π or , where Π has four vertices and two non-empty chromatic components each of which is a P₄, while is a three-colored triangle. This statement implies that each Π- and -free d-graph is uniquely decomposable in accordance with a tree whose leaves are the vertices of V and the interior vertices of T are labeled by the colors 1,…d. Such a tree is naturally interpreted as a positional game form (with perfect information and without moves of chance) of d players I={1,…,d} and n outcomes V={v₁,…,v_n}. Thus, we get a one-to-one correspondence between these game forms and Π- and -free d-graphs. As a corollary, we obtain a characterization of the normal forms of positional games with perfect information and, in case d=2, several characterizations of the read-once Boolean functions. These results are not new; in fact, they are 30 and, in case d=2, even 40 years old. Yet, some important proofs did not appear in English.Gyárfás and Simonyi recently proved a similar decomposition theorem for the -free d-graphs. They showed that each -free d-graph can be obtained from the d-graphs with only two non-empty chromatic components by successive substitutions. This theorem is based on results by Gallai, Lovász, Cameron and Edmonds. We obtain some new applications of these results.     

Wilson Function Transforms Related to Racah Coefficients

The irreducible -representations of the Lie algebra consist of discrete series representations, principal unitary series and complementary series. We calculate Racah coefficients for tensor product representations that consist of at least two discrete series representations. We use the explicit expressions for the Clebsch–Gordan coefficients as hypergeometric functions to find explicit expressions for the Racah coefficients. The Racah coefficients are Wilson polynomials and Wilson functions. This leads to natural interpretations of the Wilson function transforms. As an application several sum and integral identities are obtained involving Wilson polynomials and Wilson functions. We also compute Racah coefficients for , which turn out to be Askey–Wilson functions and Askey–Wilson polynomials.This research was done during my stay at the Department of Mathematics at Chalmers University of Technology and Göteborg University in Sweden, supported by a NWO-TALENT stipendium of the Netherlands Organization for Scientific Research (NWO).     

A combinatorial problem from sooner waiting time problems with run and frequency quotas

In this paper, we study the problem of finding the number of integer solutions solving

     

On the local distribution of certain arithmetic functions

Let d(n), σ ₁(n), and φ(n) stand for the number of positive divisors of n, the sum of the positive divisors of n, and Euler’s function, respectively. For each ν ∈, Z, we obtain asymptotic formulas for the number of integers n ⩽ x for which e _n = 2 ^v r for some odd integer m as well as for the number of integers n ⩽ x for which e _n = 2 ^v r for some odd rational number r. Our method also applies when φ(n) is replaced by σ ₁(n), thus, improving upon an earlier result of Bateman, Erdős, Pomerance, and Straus, according to which the set of integers n such that is an integer is of density 1/2. Research supported in part by a grant from NSERC. Research supported by the Applied Number Theory Research Group of the Hungarian Academy of Science and by a grant from OTKA. Published in Lietuvos Matematikos Rinkinys, Vol. 46, No. 3, pp. 315–331, July–September, 2006.     

Stability of a Jensen type equation in the space of generalized functions

We reformulate and solve the stability problem of a Jensen type functional equation

     

First-principle proof of the modified collision boundary conditions for the hard-sphere system

A fundamental issue lying at the foundation of classical statistical mechanics is the determination of the collision boundary conditions that characterize the dynamical evolution of multi-particle probability density functions (PDF) and are applicable to systems of hard-spheres undergoing multiple elastic collisions. In this paper it is proved that, when the deterministic N-body PDF is included in the class of admissible solutions of the Liouville equation, the customary form of collision boundary conditions adopted in previous literature becomes physically inconsistent and must actually be replaced by suitably modified collision boundary conditions.