Fr,chet differentiability of molecular distribution functions II: the Ursell function

For a grand canonical ensemble of classical point-like particles at equilibrium in continuous space we investigate the functional relationship between a stable and regular pair potential describing the interaction of the particles and the thermodynamical limit of the Ursell or pair correlation function. For certain admissible perturbations of the pair potential and sufficiently small activity we rigorously establish Frechet differentiability of the Ursell function in the norm. Furthermore, concerning the thermodynamical limit of the pair distribution function we explicitly compute its Fr,chet derivative as a sum of a multiplication operator and an integral operator.