We explain how the moments of the (weight function of the) Askey Wilson polynomials are related to the enumeration of the staircase tableaux introduced by the first and fourth authors [11, 12]. This gives us a direct combinatorial formula for these moments, which is related to, but more elegant than the formula given in [11]. Then we use techniques developed by Ismail and the third author to gi...

Tree structures are used extensively in domains such as computational biology, pattern recognition, XML databases, computer networks, and so on. One important problem in mining databases of trees is to find frequently occurring subtrees. However, because of the combinatorial explosion, the number of frequent subtrees usually grows exponentially with the size of the subtrees. In this paper, we p...

A wide variety of new results in combinatorial enumeration have been obtained recently. Many of these results were prompted by new problems in computer science, while others answered old questions in combinatorics and other fields. The purpose of this note is to survey a subset of these new results, namely those dealing with tree enumeration. A selection of tree enumeration results is presented...

In this paper we outline a Matrix Ansatz approach to some problems of combinatorial enumeration. The idea is that many interesting quantities can be expressed in terms of products of matrices, where the matrices obey certain relations. We illustrate this approach with applications to moments of orthogonal polynomials, permutations, signed permutations, and tableaux. To Dennis Stanton with admir...

This paper presents a new systematic approach for the uniform random generation of combinatorial objects. The method is based on the notion of object grammars which give recursive descriptions of objects and generalize contextfree grammars. The application of particular valuations to these grammars leads to enumeration and random generation of objects according to non algebraic parameters.

An asymptotic estimate is given for the coe cients of products of large powers of generating functions. This theorem and another local limit theorem which is useful for conditioning are applied to various combinatorial enumeration problems that involve multivariate Lagrange inversion. 1991 AMS Class. No. Primary: 41A63 Secondary: 05A16, 05C05, 41A60 the electronic journal of combinatorics 6 (19...

In this paper we introduce polynomials associated with uniform oriented matroids whose coefficients enumerate cells in the corresponding arrangements. These polynomials are quite useful in the study of many enumeration problems of combinatorial geometry, such as counting faces of polytopes, counting Radon partitions, counting k-sets, and so forth. We also describe some striking equations relati...

We describe a framework for systematic enumeration of families combinatorial structures which possess a certain regularity. More precisely, we describe how to obtain the differential equations satisfied by their generating series. These differential equations are then used to determine the initial terms in the counting sequence and for asymptotic analysis. The key tool is the scalar product for...

We consider the problem of counting transitive factorizations of permutations; that is, we study tuples (σr , . . . , σ1) of permutations on {1, . . . , n} such that (1) the product σr · · · σ1 is equal to a given target permutation π , and (2) the group generated by the factors σi acts transitively on {1, . . . , n}. This problem is widely known as the Hurwitz Enumeration Problem, since an enc...

Welschinger invariants of the real projective plane can be computed via the enumeration of enriched graphs, called marked floor diagrams. By a purely combinatorial study of these objects, we establish a Caporaso-Harris type formula which allows one to compute Welschinger invariants for configurations of points with any number of complex conjugated points.