Singular metrics of constant negative $Q$-curvature in Euclidean spaces

We study singular metrics of constant negative $Q$-curvature in the Euclidean space ${\mathbb{R}}^n$ for every $n\ge 1$. Precisely, we consider solutions to the problem $$(-\mathrm{\Delta }{)}^{n\mathrm{/}2}u=-e^{nu}\text{on}{\mathbb{R}}^n\mathrm{\backslash }\{0\},$$ under a finite volume condition $\mathrm{\Lambda }:={\int }_{{\mathbb{R}}^n}{e}^{nu}dx$. We classify all singular solutions of the above equation based on their behavior at infinity and zero. As a consequence of this, when $n=1,2$, we show that there is actually no singular solution. Then adapting a variational technique, we obtain that for any $n\ge 3$ and $\mathrm{\Lambda }>0$, the equation admits solutions with prescribed asymptotic behavior. These solutions correspond to metrics of constant negative $Q$-curvature, which are either smooth or have a singularity at the origin of logarithmic or polynomial type. The present paper complements previous works on the case of positive $Q$-curvature, and also sharpens previous results in the nonsingular negative $Q$-curvature case.