The modulus of Whittaker functions

Abstract

The paper discusses some properties of the modulus $|W_{k,m}(z)|$ of the Whittaker function $W_{k,m}(z)$. In particular, completely monotone functions expressed in terms of $|W_{k,m}(z)|$ are found. The results follow from an integral representation for products of Whittaker functions due to Erd\'elyi (1938).