Classification of spherical metrics on tori with four singularities, I: half periods

Classifying the spherical metrics on a torus $E_{\tau }$ with $4\pi$ conic angle at each half period point, ${\omega }_k\mathrm{/}2,k=0,1,2,3$, is equivalent to classify solutions of the following curvature equation \begin{align}\label{eq0731093154} Δu+e^u=4π\sum_{k=0}^3δ_{\frac{ω_k}{2}}$\text{ on }$E_τ\end{align} where $\tau \in \mathbb{H}:=\{z\in \mathbb{C}\mid \mathrm{I}\mathrm{m}z>0\}$ and ${\delta }_p$ is the Dirac measure at $p\in E_{\tau }$. By constructing a multiple Green function $$G_2(z_1,z_2;\tau ):=G(z_1-z_2;\tau )-\frac{1}{2}\sum _{j=0}^3(G(z_1-\frac{{\omega }_j}{2};\tau )+G(z_2-\frac{{\omega }_j}{2};\tau )),$$ in terms of the Green function $G(z;\tau )$ on $E_{\tau }$, we classify the solutions of (\ref{eq0731093154}) into two types: \emph{special} and \emph{non-special}. Furthermore, we obtain the following conclusion about the solutions of (\ref{eq0731093154}): \begin{enumerate} \item any special solution is an even function and the set of special solutions is isomorphic to $SL(2,\mathbb{C})\mathrm{/}SU(2)$ for all $\tau \in \mathbb{H}$. \item a non-special solution exists if and only if $\tau \in \mathcal{E}$. Moreover, if $\tau \in \mathcal{E}$, then there are six one-parameter families of nonspecial solutions. \end{enumerate} where $$\mathcal{E}:=\{\tau \in \mathbb{H}\mid G(z;\tau )\text{has exactly 5 critical points.}\}\mathrm{.}$$ The set $\mathcal{E}$ is completely determined in \cite{CLW2018, Lin}, which is a union of countable many open triangular domains. As a byproduct, we completely determine and classify the critical points of $G_2$ and then obtain the degeneracy criterion of critical points for $G_2$, which may be of independent interest.