When Pythagoras meets Navier-Stokes

Abstract

In this article, we develop a new method, based on a time decomposition of a Cauchy problem elaborated in [6], to retrieve the well-known $L^\infty ([0,T],L^2(\mathbb{R}^d,\mathbb{R}^d))$ control of the solution of the incompressible Navier-Stokes equation in $\mathbb{R}^d$. We precisely explain how the Pythagorean theorem in $L^2(\mathbb{R}^d,\mathbb{R}^d)$ allows to get the proper energy estimate; however such an argument does not work anymore in $L^p(\mathbb{R}^d,\mathbb{R}^d)$, $p \neq 2$. We also deduce, by similar arguments, an already known $L^\infty ([0,T],L^1(\mathbb{R}^3,\mathbb{R}^3))$ control of vorticity for $d=3$.