The 1/k-Eulerian polynomials of type B

In this paper, we give a type B analogue of the 1/k-Eulerian polynomials. Properties of this kind of polynomials, including combinatorial interpretations, recurrence relations and gamma-positivity are studied. In particular, we show that the 1/k-Eulerian polynomials of type B are gamma-positive when $k>0$. Moreover, we obtain the corresponding results for derangements of type B. We show that a type B 1/k-derangement polynomials $d_n^B(x;k)$ are bi-gamma-positive when $k\ge 1\mathrm{/}2$. In particular, we get a symmetric decomposition of $d_n^B(x;1\mathrm{/}2)$ in terms of the classical derangement polynomials.