On the global existence and blowup of smooth solutions to the   multi-dimensional compressible Euler equations with time-depending damping
  In this paper, we are concerned with the global existence and blowup of smooth solutions to the multi-dimensional compressible Euler equations with time-depending damping \begin{equation*}   \partial_t\rho+\operatorname{div}(\rho u)=0, \quad   \partial_t(\rho u)+\operatorname{div}\left(\rho u\otimes u+p\,I_d\right)=-\alpha(t)\rho u, \quad   \rho(0,x)=\bar \rho+\varepsilon\rho_0(x),\quad u(0,x)=\varepsilon u_0(x), \end{equation*} where $x=(x_1, \cdots, x_d)\in\Bbb R^d$ $(d=2,3)$, the frictional coefficient is $\alpha(t)=\frac{\mu}{(1+t)^\lambda}$ with $\lambda\ge0$ and $\mu>0$, $\bar\rho>0$ is a constant, $\rho_0,u_0 \in C_0^\infty(\Bbb R^d)$, $(\rho_0,u_0)\not\equiv 0$, $\rho(0,x)>0$, and $\varepsilon>0$ is sufficiently small. One can totally divide the range of $\lambda\ge0$ and $\mu>0$ into the following four cases:   Case 1: $0\le\lambda<1$, $\mu>0$ for $d=2,3$;   Case 2: $\lambda=1$, $\mu>3-d$ for $d=2,3$;   Case 3: $\lambda=1$, $\mu\le 3-d$ for $d=2$;   Case 4: $\lambda>1$, $\mu>0$ for $d=2,3$.   \noindent We show that there exists a global $C^{\infty}-$smooth solution $(\rho, u)$ in Case 1, and Case 2 with $\operatorname{curl} u_0\equiv 0$, while in Case 3 and Case 4, in general, the solution $(\rho, u)$ blows up in finite time. Therefore, $\lambda=1$ and $\mu=3-d$ appear to be the critical power and critical value, respectively, for the global existence of small amplitude smooth solution $(\rho, u)$ in $d-$dimensional compressible Euler equations with time-depending damping. 
Fei Hou, Huicheng Yin
math.AP