Central Fourier Analysis for Lorentz Spaces on Compact Lie Groups.
Let be a locally compact group. Let be the left translation in , given by . We characterize (undre a mild set-theoretical hypothesis) the functions such that the map from into is scalarly measurable (i.e. for , is measurable). We show that it is the case when is measurable for each character , and if is compact, if and only if is Riemann-measurable. We show that is Borel measurable if and only if is left uniformly continuous.Some of the measure-theoretic tools used there...
Given a smooth family of vector fields satisfying Chow-Hörmander’s condition of step 2 and a regularity assumption, we prove that the Sobolev spaces of fractional order constructed by the standard functional analysis can actually be “computed” with a simple formula involving the sub-riemannian distance.Our approach relies on a microlocal analysis of translation operators in an anisotropic context. It also involves classical estimates of the heat-kernel associated to the sub-elliptic Laplacian.