Asymptotic Stability of Stationary Solutions of a Free Boundary Problem   Modeling the Growth of Tumors with Fluid Tissues
  This paper aims at proving asymptotic stability of the radial stationary solution of a free boundary problem modeling the growth of nonnecrotic tumors with fluid-like tissues. In a previous paper we considered the case where the nutrient concentration $\sigma$ satisfies the stationary diffusion equation $\Delta\sigma=f(\sigma)$, and proved that there exists a threshold value $\gamma_*>0$ for the surface tension coefficient $\gamma$, such that the radial stationary solution is asymptotically stable in case $\gamma>\gamma_*$, while unstable in case $\gamma<\gamma_*$. In this paper we extend this result to the case where $\sigma$ satisfies the non-stationary diffusion equation $\epsln\partial_t\sigma=\Delta\sigma-f(\sigma)$. We prove that for the same threshold value $\gamma_*$ as above, for every $\gamma>\gamma_*$ there is a corresponding constant $\epsln_0(\gamma)>0$ such that for any $0<\epsln<\epsln_0(\gamma)$ the radial stationary solution is asymptotically stable with respect to small enough non-radial perturbations, while for $0<\gamma<\gamma_*$ and $\epsln$ sufficiently small it is unstable under non-radial perturbations. 
Junde Wu and Shangbin Cui