Explicit structure of the vanishing viscosity limits with initial data consisting of $\delta$-distributions starting from two point sources

In this article, we consider the one-dimensional zero-pressure gas dynamics system $$u_t + \left( {u^2}/{2} \right)_x = 0,\ \rho_t + (\rho u)_x = 0$$ in the upper-half plane with a linear combination of two $\delta$-distributions $$u|_{t=0} = u_a\ \delta_{x=a} + u_b\ \delta_{x=b},\ \rho|_{t=0} = \rho_c\ \delta_{x=c} + \rho_d\ \delta_{x=d}$$ as initial data. Here $a$, $b$, $c$, $d$ are distinct points on the real line ordered as $a < c < b < d$. Our objective is to provide a detailed analysis of the structure of the vanishing viscosity limits of this system utilizing the corresponding modified adhesion model $$u^\epsilon_t + \left({(u^\epsilon)^2}/{2} \right)_x =\frac{\epsilon}{2} u^\epsilon_{xx},\ \rho^\epsilon_t + (\rho^\epsilon u^\epsilon)_x = \frac{\epsilon}{2} \rho^\epsilon_{xx}.$$ For this purpose, we extensively use the various asymptotic properties of the function erfc$: z \longmapsto \int_{z}^{\infty} e^{-s^2}\ ds$ along with suitable Hopf-Cole transformations.