  In this paper, we develop approximative two-flavor neutrino oscillation
formulas including subleading nonstandard interaction effects. Especially, the
limit when the small mass-squared difference approaches zero is investigated.
The approximate formulas are also tested against numerical simulations in order
to determine their accuracy and they will probably be most useful in the GeV
energy region, which is the energy region where most upcoming neutrino
oscillation experiments will be operating. Naturally, it is important to have
analytical formulas in order to interpret the physics behind the degeneracies
between standard and nonstandard parameters.
