Lower bounds for the L^p norms of some Fourier multipliers

Abstract: Quite often we wonder about the sharpness of estimates for certain singular integral operators. In theory, their sharpness can be confirmed by constructing extremizers or approximate extremizers, but, in practice, such extremizers might not be obvious, or they might be impossibly complicated to work with. In this talk we will discuss a reasonably general way of proving lower bounds for the exact $L^p$ norms of unimodular homogeneous Fourier multipliers. We will then apply it to solve three open problems: one by Iwaniec and Martin (from 1996) on the powers of the complex Riesz transform, one by Maz'ya (traced back to the 1970s) on multipliers with smooth phases, and one by Dragičević, Petermichl, and Volberg (from 2006) on the two-dimensional Riesz group. This is joint work with Aleksandar Bulj, Andrea Carbonaro, and Oliver Dragičević.

mathematical physicsanalysis of PDEsclassical analysis and ODEscombinatoricscomplex variablesfunctional analysisinformation theorymetric geometryoptimization and controlprobability

Audience: researchers in the topic

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