Convolution operators preserving the set of totally positive sequences

Abstract

A real sequence $(a_k)_{k=0}^\infty$ is called {\it totally positive} if all minors of the infinite Toeplitz matrix $ \left\| a_{j-i} \right\|_{i, j =0}^\infty$ are nonnegative (here $a_k=0$ for $k<0$). In this paper, which continues our earlier work \cite{kv}, we investigate the set of real sequences $(b_k)_{k=0}^\infty$ with the property that for every totally positive sequence $(a_k)_{k=0}^\infty,$ the sequense of termwise products $(a_k b_k)_{k=0}^\infty$ is also totally positive. In particular, we show that for every totally positive sequence $(a_k)_{k=0}^\infty$ the sequence $\left(a_k a^{-k (k-1)}\right)_{k=0}^\infty$ is totally positive whenever $a^2\geq 3{.}503.$ We also propose several open problems concerning convolution operators that preserve total positivity.