A Free Boundary Problem: The Flow of Two Immiscible Fluids in a One-Dimensional Porous Medium, I L. Evans Indiana University Mathematics Journal 1977 text pdf 10.1512/iumj.1977.26.26074 10.1512/iumj.1977.26.26074 en Indiana Univ. Math. J. 26 (1977) 915 - 932 state-of-the-art mathematics http://iumj.org/access/