Quasibound states of an electron in a strong magnetic field

Bound and quasibound energy levels of an electron in a strong magnetic field are investigated for a short-range impurity potential and an isotropic quadratic dispersion law. A formula applicable in all fields is obtained for a Born impurity potential. It is shown that there is at least one bound level at any Landau level $\mathcal E_{M0}$ (with angular momentum projection onto the direction of the magnetic field equal to $-M$, the remaining levels being quasibound for $M\geqslant1$).