Non-uniqueness of smooth solutions to the Navier-Stokes equations on torus $\TTT^2$

TL;DR

This paper proves the non-uniqueness of smooth solutions to the two-dimensional Navier-Stokes equations on the torus, extending recent three-dimensional results and introducing new methods to handle two-dimensional complexities.

Contribution

It establishes the existence of multiple smooth solutions in 2D Navier-Stokes, addressing a longstanding open problem and developing novel Fourier mode flow techniques.

Findings

Demonstrates non-uniqueness of smooth solutions in 2D Navier-Stokes

Develops a heat-dominated Fourier mode flow method

Overcomes geometric intersection challenges in 2D flows

Abstract

The local well-posedness theory for the incompressible Navier-Stokes equations in $\BMO^{-1}$ has attracted considerable attention over the past two decades. In a recent breakthrough, Coiculescu and Palasek (Invent. Math., 2025) settled the three-dimensional case by demonstrating the existence of two distinct global solutions, both smooth for $t>0$, evolving from a common initial datum in ${\rm BMO}^{-1}(\mathbb{T}^3)$. However, the two-dimensional case remains open. In this paper, we solve the two-dimensional problem. Unlike its three-dimensional counterpart, the two-dimensional setting presents additional difficulties stemming from the geometric intersections of two-dimensional Mikado flows. To overcome these difficulties, we develop a heat-dominated Fourier mode flow built upon steady two-dimensional Euler flows, and present the proof using a new iterative scheme.