Fractional derivatives are commonly used to model a variety of phenomena, but… what does it mean to have a logarithmic derivative? And what would it be used for?

In this talk we focus on the logarithmic Laplacian, a pseudodifferential operator that appears as a first order expansion of the fractional Laplacian of order 2s as s goes to zero. This operator can also be represented as an integrodifferential operator with a zero order kernel.

We will discuss how this operator can be used to study the behavior of linear and nonlinear fractional problems in the small order limit. This analysis will also reveal a deep and interesting mathematical structure behind the set of solutions of Dirichlet logarithmic problems.

We present new existence results for nontrivial solutions of some biharmonic Nonlinear Schrödinger equation in $\mathbb{R}^N$ that are based on a constrained minimization approach. Here the main difficulty comes from the fact that spherical rearrangements need not decrease the energy so that more sophisticated arguments are needed to overcome the lack of compactness. A new and intrisically motivated tool is given by a new class of Gagliardo-Nirenberg inequalities where, essentially, the Laplacian in the classical Gagliardo-Nirenberg inequality is replaced by the Helmholtz operator. Having explained the relevance of such inequalities for our analysis, we comment on their proofs and related questions from Harmonic Analysis. Finally, we shall mention a symmetry-breaking phenomenon related to our results that was recently observed by Lenzmann and Weth. Accordingly, the talk covers topics from the Calculus of Variations as well as Harmonic Analysis or, more specifically, Fourier Restriction Theory.

In this talk we introduce models of short wave-long wave interactions in the relativistic setting. In this context the nonlinear Schrödinger equation is no longer adequate for describing short waves and is replaced by a nonlinear Dirac equation. Two specific examples are considered: the case where the long waves are governed by a scalar conservation law; and the case where the long waves are governed by the augmented Born-Infeld equations in electromagnetism. This is a joint work with JOÃO PAULO DIAS.