  While free and weakly interacting nonrelativistic particles are described by
a Gross-Pitaevskii equation, which is a nonlinear self-interacting
Schr\"odinger equation, the phenomena in the strong- coupling limit are
governed by an effective action that is extremized by a double-fractional
generalization of this equation. Its particle orbits perform L\`evy walks
rather than Gaussian random walks.
