Diffusion Approximation for Transport Processes with Boundary Conditions Richard EllisWalter Rosenkrantz Indiana University Mathematics Journal 1977 text pdf 10.1512/iumj.1977.26.26087 10.1512/iumj.1977.26.26087 en Indiana Univ. Math. J. 26 (1977) 1075 - 1096 state-of-the-art mathematics http://iumj.org/access/