  We consider a quantum above-barrier reflection of a Bose-Einstein condensate
by a one-dimensional rectangular potential barrier, or by a potential well, for
nonlinear Schroedinger equation (Gross-Pitaevskii equation) with a small
nonlinearity. The most interesting case is realized in resonances when the
reflection coefficient is equal to zero for the linear Schroedinger equation.
Then the reflection is determined only by small nonlinear term in the
Gross-Pitaevskii equation. A simple analytic expression has been obtained for
the reflection coefficient produced only by the nonlinearity. An analytical
condition is found when common action of potential barrier and nonlinearity
produces a zero reflection coefficient. The reflection coefficient is derived
analytically in the vicinity of resonances which are shifted by nonlinearity.
