Bernstein and Stieltjes properties of some functions involving logarithmic functions

Abstract

We show that $\frac1{g(x)}$, with $g(x)=\frac{x-1}{x\ln x}$, is a complete Bernstein function and give its Lévy measure explicitly. For $h(x)=\frac{(x^2-1)\ln(x+1)}{x^2\ln x}$, we prove that $h$ is not a Stieltjes function, so $\frac1h$ is not a complete Bernstein function, but $\frac1h$ is a Bernstein function. The latter rests on the positivity of a function built from the convolution of the exponential integrals $E_1$ and $E_2$. We also give a related completely monotonic function that is not a Stieltjes function.