We establish several bounds for solutions to elliptic/parabolic cross-diffusion systems of $m$ equations ($m\ge2$) on 2d/3d domains $\Og$. We settle the existence and global existence problems in these cases and also provide new counter-examples for the case of general dimensions. Most importantly, we prove that when $m=N=3$, the thinness of $\Og$ in $\RR^3$ is sufficient and necessary. When $m$ is arbitrary and $N=3$, we establish global existence results for nonlinear cross-diffusion systems (the case of the scalar semilinear equation was considered in the literature but the classical methods do not seem to be applicable here).
Categories: math.AP math-ph math.MP