Existence theorems for regular spatially periodic solutions to ersatz Navier-Stokes equations

TL;DR

This paper establishes existence and uniqueness of regular spatially periodic solutions for a Navier-Stokes type system without pressure on a torus, using functional analysis and topological methods.

Contribution

It introduces a new surjectivity criterion for nonlinear mappings related to Navier-Stokes equations, bypassing the need for a priori estimates of derivatives.

Findings

Proves the problem induces an open injective continuous mapping.

Establishes surjectivity for pressure-free Navier-Stokes type equations.

Provides a regular solution existence and uniqueness theorem.

Abstract

The initial problem for the Navier-Stokes type equations over ${\mathbb R}^n \times [0,T]$, $n\geq 2$, with a positive time $T$ in the spatially periodic setting is considered. First, we prove that the problem induces an open injective continuous mapping on scales of specially constructed function spaces of Bo\-chner-Sobolev type over the $n\,$-dimensional torus ${\mathbb T}^n$. Next, rejecting the idea of proving a universal a priori estimate for high-order derivatives, we obtain a surjectivity criterion for the non-linear mapping under the considerations in terms of boundedness for its inverse images of precompact sets. Finally, we prove that the mapping is surjective if we consider the versions of the Navier-Stokes type equations containing no `pressure'{}. This gives a uniqueness and existence theorem for regular solutions to this particular ersatz of the Navier-Stokes type equations. The used techniques consist in proving the closedness of the image by estimating all possible divergent sequences in the preimage and matching the asymptotics. The following facts are essential: i) the torus is a compact closed manifold, ii) the corresponding system is `local'.