Active-sterile neutrino mixing in the ISS(p,q) inverse seesaw models

A class of models with inverse seesaw mechanism is considered for arbitrary numbers p and q of new neutral fermions for each generation of leptons. The models containing a candidate particle for the role of warm dark matter are sorted using the inversion technique of an arbitrary block matrix in terms of Schur complement. Unlike the seesaw type I and II models with keV sterile dark matter neutrinos, in the inverse seesaw models mixing does not depend on the mass of dark matter particle, but depends only on the mass of heavy sterile pseudo-Dirac neutrinos.