2-adic Valuations of Coefficients of the Fifth and Ninth Powers of the Thue--Morse Generating Function

Abstract

Let $T(x)={\prod }_{k=0}^{\infty }(1-x^{2^k})$ be the generating function of the Thue--Morse sequence, and write $T(x{)}^m={\sum }_{n\ge 0}{t}_m(n)x^n$. We prove exact formulas for the $2$-adic valuations of the coefficients $t_5(n)$ and $t_9(n)$: \[ ν_2\bigl(t_5(4n+j)\bigr) =4\Bigl\lceil\tfrac{ν_2(n+1)}{2}\Bigr\rceil-\bigl(ν_2(n+1)\bmod 2\bigr), \quad j\in\{0,1,2,3\}, \] \[ ν_2\bigl(t_9(8n+j)\bigr) =5\Bigl\lceil\tfrac{ν_2(n+1)}{2}\Bigr\rceil-2\bigl(ν_2(n+1)\bmod 2\bigr), \quad j\in\{0,1,\ldots,7\}. \] These formulas confirm Conjecture~5.2 of Gawron--Miska--Ulas~\cite{ga} for $m=5$ and $m=9$, and imply that $t_5(n)\ne 0$ and $t_9(n)\ne 0$ for every $n\ge 0$. A key structural ingredient is a closed-form formula for the determinant of a family of matrices with binomial-coefficient entries.