A monotonicity formula for the fractional Laplacian and instability results for the Shrira equation

Abstract: By employing a new class of pseudo-differential operators introduced in a previous work, we establish a novel monotonicity formula for the fractional Laplacian $|\nabla_x|<sup>\alpha$ in $\mathbb{R}<sup>n$, with $n \geq 2$ and $\alpha \in [1,2)$, This framework enables us to localize our analysis to specific regions of Euclidean space where monotonicity properties of the $L<sup>2$-mass of solutions to dispersive equations with fractional dispersion are preserved. As an application, we focus on the Shrira equation, proving conditional instability results for its traveling wave solutions in the critical regime. We deduce key virial-type estimates that govern the long-time behavior of the $L<sup>2$-mass. As a consequence, we establish instability results without requiring pointwise decay estimates employed in previous works. Our approach provides a robust and flexible method for monotonicity techniques to higher dimensions, shedding light on the delicate interplay between nonlocal dispersion and spatial localization in nonlinear dispersive models.