Positively Invariant Regions for Systems of Nonlinear Diffusion Equations K. ChuehC. ConleyJ. Smoller Indiana University Mathematics Journal 1977 text pdf 10.1512/iumj.1977.26.26029 10.1512/iumj.1977.26.26029 en Indiana Univ. Math. J. 26 (1977) 373 - 392 state-of-the-art mathematics http://iumj.org/access/