Remarks on the fermi contact interaction

It is shown by rigorous mathematical methods that the so-called Fermi contact interaction does not define any selfadjoint Hamiltonian, and the only possible form of contact interaction localized strictly at r = 0 is found. However, the Fermi contact interaction retains sense when used as a bilinear form in the perturbation theory for Dirac electrons. Some results obtained earlier by other authors are confirmed and interpreted by our analysis.