We develop refined Strichartz estimates at
regularity for a class of time-dependent Schrödinger operators. Such refinements
quantify near-optimizers of the Strichartz estimate and play a pivotal part in the global
theory of mass-critical NLS. On one hand, the harmonic analysis is quite subtle in the
-critical
setting due to an enormous group of symmetries, while on the other hand,
the space-time Fourier analysis employed by the existing approaches to the
constant-coefficient equation are not adapted to nontranslation-invariant
situations, especially with potentials as large as those considered in this
article.
Using phase-space techniques, we reduce to proving certain analogues of (adjoint)
bilinear Fourier restriction estimates. Then we extend Tao’s bilinear restriction
estimate for paraboloids to more general Schrödinger operators. As a particular
application, the resulting inverse Strichartz theorem and profile decompositions
constitute a key harmonic analysis input for studying large-data solutions to the
-critical
NLS with a harmonic oscillator potential in dimensions
. This
article builds on recent work of Killip, Visan, and the author in one space
dimension.