Infinite time blow-up for half-harmonic map flow from $\mathbb{R}$ into $\mathbb{S}^1$

Abstract: We study infinite time blow-up phenomenon for the half-harmonic map flow \begin{equation}\label{e:main00} \left{\begin{array}{ll} u_t = -(-\Delta)^{\frac{1}{2}}u + \left(\frac{1}{2\pi}\int_{\mathbb{R}}\frac{|u(x)-u(s)|^2}{|x-s|^2}ds\right)u\quad\text{ in }\mathbb{R}\times (0, \infty), u(\cdot, 0) = u_0\quad\text{ in }\mathbb{R}, \end{array} \right. \end{equation} with a function $u:\mathbb{R}\times [0, \infty)\to \mathbb{S}^1$. Let $q_1,\cdots, q_k$ be distinct points in $\mathbb{R}$, there exist an initial datum $u_0$ and smooth functions $\xi_j(t)\to q_j$, $0<\mu_j(t)\to 0$, as $t\to +\infty$, $j = 1, \cdots, k$, such that the solution $u_q$ of Problem (\ref{e:main00}) has the form \begin{equation*} u_q =\omega_\infty +\sum_{j= 1}^k \left(\omega (\frac{x-\xi_j(t)}{\mu_j(t)} )-\omega_\infty \right)+\theta(x, t), \end{equation*} where $\omega$ is the canonical least energy half-harmonic map, $\omega_\infty=\begin{pmatrix} 1 \end{pmatrix} $, $\theta(x, t)\to 0$ as $t\to +\infty$, uniformly away from the points $q_j$. In addition, the parameter functions $\mu_j(t)$ decay to $0$ exponentially.