Stieltjes, Poisson and other integral representations for functions of   Lambert $W$
  We show that many functions containing $W$ are Stieltjes functions. Explicit Stieltjes integrals are given for functions $1/W(z)$, $W(z)/z$, and others. We also prove a generalization of a conjecture of Jackson, Procacci & Sokal. Integral representations of $W$ and related functions are also given which are associated with the properties of their being Pick or Bernstein functions. Representations based on Poisson and Burniston--Siewert integrals are given as well. 
German A. Kalugin, David J. Jeffrey and Robert M. Corless
math.CV