In this paper we study several problems concerning the number of homomor-phisms of trees. We begin with an algorithm for the number of homomorphisms from a tree to any graph. By using this algorithm and some transformations on trees, we study various extremal problems about the number of homomorphisms of trees. These applications include a far reaching… (More)
We prove a conjecture of Gessel, which asserts that the joint distribution of descents and inverse descents on permutations has a fascinating refined γ-positivity.
The (q, r)-Eulerian polynomials are the (maj−exc, fix, exc) enumerative polynomials of permutations. Using Shareshian and Wachs' exponential generating function of these Eulerian polynomials, Chung and Graham proved two symmetrical q-Eulerian identities and asked for bijective proofs. We provide such proofs using Foata and Han's three-variable statistic… (More)
We investigate the diagonal generating function of the Jacobi-Stirling numbers of the second kind JS(n + k, n; z) by generalizing the analogous results for the Stir-ling and Legendre-Stirling numbers. More precisely, letting JS(n + k, n; z) = p k,0 (n) + p k,1 (n)z + · · · + p k,k (n)z k , we show that (1 − t) 3k−i+1 n≥0 p k,i (n)t n is a polynomial in t… (More)