Second-order equations for fermions on Schwarzschild, Reissner–Nordström, Kerr, and Kerr–Newman space–times

We obtain relativistic self-adjoint second-order equations for fermions in Schwarzschild, Reissner–Nordström, Kerr, and Kerr–Newman gravitational and electromagnetic fields. Second-order equations with effective potentials and spinor wave functions extend opportunities for obtaining regular solutions of quantum mechanics equations for spin-$1/2$ particles.