Existence results for Toda systems with sign-changing prescribed functions: Part I

Let $(M,g)$ be a compact Riemann surface with area $1$, we shall study the Toda system $$\begin{cases} -\Delta u_1 = 2\rho_1(h_1e^{u_1}-1) - \rho_2(h_2e^{u_2}-1),\\ -\Delta u_2 = 2\rho_2(h_2e^{u_2}-1) - \rho_1(h_1e^{u_1}-1), \end{cases}$$ on $(M,g)$ with $\rho_1=4\pi$, $\rho_2\in(0,4\pi)$, $h_1$ and $h_2$ are two smooth functions on $M$. In Jost-Lin-Wang's celebrated article (Comm. Pure Appl. Math., 59 (2006), no. 4, 526--558), they obtained a sufficient condition for the existence of this Toda system when $h_1$ and $h_2$ are both positive. In this paper, we shall improve this result to the case $h_1$ and $h_2$ can change signs. We shall pursue a variational method and use the standard blowup analysis. Among other things, the main contribution in our proof is to show the blowup can only happen at one point where $h_1$ is positive.