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Published online by Cambridge University Press: 26 June 2025
We prove that a solution to the 3D Navier–Stokes or magneto-hydrodynamics equations does not blow up at t = T provided $\displaystyle \limsup_{q \to \infty} \int_{\mathcal{T}_q}^T \|\Delta_q(\nabla \times u)\|_\infty \, dt$ is small enough, where u is the velocity,
$\Delta_q$ is the Littlewood–Paley projection and
$\mathcal T_q$ is a certain sequence such that
$\mathcal T_q \to T$ as
$q \to \infty$. This improves many existing regularity criteria.