A digit reversal property for Stern polynomials

Abstract

We consider the following polynomial generalization of Stern's diatomic series: let $s_1(x,y)=1$, and for $n\ge 1$ set $s_{2n}(x,y)=s_n(x,y)$ and $s_{2n+1}(x,y)=x{s}_n(x,y)+y{s}_{n+1}(x,y)$. The coefficient $[x^i{y}^j]s_n(x,y)$ is the number of hyperbinary expansions of $n-1$ with exactly $i$ occurrences of the digit $2$ and $j$ occurrences of $0$. We prove that the polynomials $s_n$ are invariant under \emph{digit reversal}, that is, $s_n=s_{n^R}$, where $n^R$ is obtained from $n$ by reversing the binary expansion of $n$.

arXiv comment5 pages, accepted for publication in INTEGERS