Nonstandard Types of Mixing and Dark Matter in Models with Heavy Neutral Leptons

The extension of the Standard Model by three right-handed heavyleptons in the physical basis of mass states is considered. Thecosmological restrictions and constraints of direct searches foreach of the three heavy neutral leptons are analyzed togetherwithin the framework of a unified parameterization of the mixingcomponents with active neutrinos. The possible contribution of thenonzero mass of the lightest neutrino is taken into account.Within the framework of this mixing model, the applicability ofphenomenological simplifications for mixing components isanalyzed.