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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)

- Zhicong Lin
- 2016

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.

Article is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use. The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. Abstract. Let hom(G, H) denote the number of homomorphisms from a graph G to… (More)

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)

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