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A combinatorial proof of q-log-concavity of q-Eulerian numbers

  • Xinmiao Liu,
  • Jiangxia Hou,
  • Fengxia Liu

摘要

Carlitz established a q-analog of the Eulerian numbers \(A_{n,k}(q)\) A n , k ( q ) and defined the relationship \(A_{n,k}(q)=q^{\frac{(n-k)(n-k+1)}{2}}A_{n,k}^{*}(q)\) A n , k ( q ) = q ( n - k ) ( n - k + 1 ) 2 A n , k ( q ) . In this paper, by using the combinatorial interpretation of \(A_{n,k}^{*}(q)\) A n , k ( q ) and constructing injective maps, we prove that \(A_{n,k}^{*}(q)\) A n , k ( q ) and \(A_{n,k}(q)\) A n , k ( q ) are q-log-concave, that is, all the coefficients of the polynomials \(( A_{n,k}^{*}(q)) ^{2}- A_{n,k-1}^{*}(q) A_{n,k+1}^{*}(q) \) ( A n , k ( q ) ) 2 - A n , k - 1 ( q ) A n , k + 1 ( q ) and \((A_{n,k}(q)) ^{2}- A_{n,k-1}(q) A_{n,k+1}(q)\) ( A n , k ( q ) ) 2 - A n , k - 1 ( q ) A n , k + 1 ( q ) are nonnegative for \(1< k <n\) 1 < k < n .