A note on degenerate Whittaker models for general linear groups

Given a Speh representation π of GL n ( F ) for a non-archimedean local field F, we obtain necessary and sufficient conditions on standard parabolic subgroups for the Jacquet module of π with respect to the parabolic subgroup to be generic. Using this we describe the precise set of characters Θ of the maximal unipotent radical U n of GL n ( F ) such that Hom U n ( π , Θ ) is non-zero. We then describe the behavior of this set under the base change map (with respect to a finite cyclic extension of F of prime degree).