# Blow-up for the 3-dimensional axially symmetric harmonic map flow into S2

## TL;DR

This paper constructs the first known finite-time blow-up solutions for the 3D harmonic map flow into S^2 with singularities along a curve, demonstrating a space-codimension 2 blow-up set as predicted by theory.

## Contribution

It provides the first explicit examples of blow-up solutions with a space-codimension 2 singular set in 3D harmonic map flow into S^2.

## Key findings

- Blow-up occurs exactly at a prescribed curve in the domain.
- The blow-up set has codimension 2, matching theoretical predictions.
- Constructed solutions exhibit a delta measure concentration on the curve.

## Abstract

We construct finite time blow-up solutions to the 3-dimensional harmonic map flow into the sphere $S^2$, \begin{align*} u_t & = \Delta u + |\nabla u|^2 u \quad \text{in } \Omega\times(0,T) \\ u &= u_b \quad \text{on } \partial \Omega\times(0,T) \\ u(\cdot,0) &= u_0 \quad \text{in } \Omega , \end{align*} with $u(x,t): \bar \Omega\times [0,T) \to S^2$. Here $\Omega$ is a bounded, smooth axially symmetric domain in $\mathbb{R}^3$. We prove that for any circle $\Gamma \subset \Omega$ with the same axial symmetry, and any sufficiently small $T>0$ there exist initial and boundary conditions such that $u(x,t)$ blows-up exactly at time $T$ and precisely on the curve $\Gamma$, in fact $$ |\nabla u(\cdot ,t)|^2 \rightharpoonup |\nabla u_*|^2 + 8\pi \delta_\Gamma \text{ as } t\to T . $$ for a regular function $u_*(x)$, where $\delta_\Gamma$ denotes the Dirac measure supported on the curve. This the first example of a blow-up solution with a space-codimension 2 singular set, the maximal dimension predicted in the partial regularity theory by Chen-Struwe and Cheng.