  If a sterile neutrino nu_s with an eV-scale mass and a sizeable mixing to the
electron neutrino exists, as indicated by the reactor and gallium neutrino
anomalies, a strong resonance enhancement of nu_e-nu_s oscillations of
atmospheric neutrinos should occur in the TeV energy range. At these energies
neutrino flavour transitions in the 3+1 scheme depend on just one neutrino mass
squared difference and are fully described within a 3-flavour oscillation
framework. We demonstrate that the flavour transitions of atmospheric nu_e can
actually be very accurately described in a 2-flavour framework, with neutrino
flavour evolution governed by an inhomogeneous Schroedinger-like equation.
Evolution equations of this type have not been previously considered in the
theory of neutrino oscillations.
