A priori Bound of Solutions to a Class of Elliptic/Parabolic Cross Diffusion Systems of $m$ Equations on Two/Three Dimensional Domains. Existence on thin domains

We establish several bounds for solutions to elliptic/parabolic cross-diffusion systems of $m$ equations ($m\ge 2$) on 2d/3d domains $\text{Og}$. 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 $\text{Og}$ in ${\text{RR}}^3$ 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).