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The nonlocal Liouville-type equation in $\mathbb{R}$ and conformal immersions of the disk with boundary singularities

In this paper we perform a blow-up and quantization analysis of the fractional Liouville equation in dimension $1$. More precisely, given a sequence $u_k :\mathbb{R} \to \mathbb{R}$ of solutions to \begin{equation} (-Δ)^\frac{1}{2} u_k =K_ke^{u_k}\quad \text{in }\mathbb{R}, \end{equation} with $K_k$ bounded in $L^\infty$ and $e^{u_k}$ bounded in $L^1$ uniformly with respect to $k$, we show that up to extracting a subsequence $u_k$ can blow-up at (at most) finitely many points $B=\{a_1,\dots, a_N\}$ and either (i) $u_k\to u_\infty$ in $W^{1,p}_{loc}(\mathbb{R}\setminus B)$ and $K_ke^{u_k} \stackrel{*}{\rightharpoondown} K_\infty e^{u_\infty}+ \sum_{j=1}^N πδ_{a_j}$, or (ii) $u_k\to-\infty$ uniformly locally in $\mathbb{R}\setminus B$ and $K_k e^{u_k}\stackrel{*}{\rightharpoondown} \sum_{j=1}^N α_j δ_{a_j}$ with $α_j\ge π$ for every $j$. This result, resting on the geometric interpretation and analysis provided in a recent collaboration of the authors with T. Rivière and on a classical work of Blank about immersions of the disk into the plane, is a fractional counterpart of the celebrated works of Brézis-Merle and Li-Shafrir on the $2$-dimensional Liouville equation, but providing sharp quantization estimates ($α_j=π$ and $α_j\ge π$) which are not known in dimension $2$ under the weak assumption that $(K_k)$ be bounded in $L^\infty$ and is allowed to change sign.