Characterization of solutions to dissipative systems with sharp algebraic decay

Abstract

We characterize the set of functions $u\_0\in L^2(R^n)$ such that the solution of the problem $u\_t=\mathcal{L}u$ in $R^n\times (0,\infty )$ starting from $u\_0$ satisfy upper and lower bounds of the form $c(1+t{)}^{-\gamma }\le \Vert u(t)\Vert \_2\le c^{\prime }(1+t{)}^{-\gamma }$.Here $\mathcal{L}$ is in a large class of linear pseudo-differential operator with homogeneous symbol (including the Laplacian, the fractional Laplacian, etc.). Applications to nonlinear PDEs will be discussed: in particular our characterization provides necessary and sufficient conditions on $u\_0$ for a solution of the Navier--Stokes system to satisfy sharp upper-lower decay estimates as above.In doing so, we will revisit and improve the theory of \emph{decay characters} by C. Bjorland, C. Niche, and M.E. Schonbek, by getting advantage of the insight provided by the Littlewood--Paley analysis and the use of Besov spaces.

Post refereeing version. To appear on SIAM J. Math. Anal