Some stability problem for the Navier–Stokes equations in the periodic case

The Navier-Stokes motions in a box with periodic boundary conditions are considered. First the existence of global regular two-dimensional solutions is proved. The solutions are such that continuous with respect to time norms are controlled by the same constant for all $t\in\mathbb{R}_+$. Assuming that the initial velocity and the external force are sufficiently close to the initial velocity and the external force of the two-dimensional solutions we prove existence of global three-dimensional regular solutions which remain close to the two-dimensional solutions for all time. In this way we mean stability of two-dimensional solutions.