An update on the Lp-Lq norms of spectral multipliers on unimodular Lie groups

This note gives a wide-ranging update on the multiplier theorems by Akylzhanov and the second author [J. Funct. Anal., 278 (2020), 108324]. The proofs of the latter crucially rely on Lp-Lq norm estimates for spectral projectors of left-invariant weighted subcoercive operators on unimodular Lie groups, such as Laplacians, sub-Laplacians, and Rockland operators. By relating spectral projectors to heat kernels, explicit estimates of the Lp-Lq norms can be immediately exploited for a much wider range of (connected unimodular) Lie groups and operators than previously known. The comparison with previously established bounds by the authors show that the heat kernel estimates are sharp. As an application, it is shown that several consequences of the multiplier theorems, such as time asymptotics for the Lp-Lq norms of the heat kernels and Sobolev-type embeddings, are then automatic for the considered operators