Quantitative bounds for bounded solutions to the Navier-Stokes equations in endpoint critical Besov spaces

Abstract

Building on Tao's quantitative regularity theory and triple-logarithmic blow-up estimate in $L^3$ in \cite{Tao_20}, we consider classical solutions $(u,P)$ of the three-dimensional incompressible Navier--Stokes equations on $[0,T]\times {\mathbb{R}}^3$. For $3<p<\infty$, under simultaneous uniform control of the two scaling-critical quantities $\Vert u{\Vert }_{L_T^{\infty }({\dot{B}}_{p,\infty }^{-1+\frac{3}{p}})}$ and $\Vert |D{|}^{-1+\frac{3}{p}}u{\Vert }_{L_T^{\infty }(L^p)}$, we obtain explicit quantitative estimates for all spatial derivatives of $u$. As a consequence, we derive a mixed blow-up criterion coupling a double exponential of the critical Besov norm with the $L^p$ norm of $|D{|}^{-1+\frac{3}{p}}u$, which forces quantified growth of at least one of these two critical quantities near any finite blow-up time. The low regularity and lack of dyadic summability in the endpoint Besov space are handled through a finite iterative decomposition that successively improves spatial integrability and produces an energy-class remainder, together with refined nonlinear energy estimates. The nonlocal signed quantity $|D{|}^{-1+\frac{3}{p}}u$ is treated by localized mean-zero vector tests and almost orthogonality across geometrically separated concentration scales.