On the Existence of Subsonic Flows of a Compressible Fluid Max Shiffman Indiana University Mathematics Journal 1952 text pdf 10.1512/iumj.1952.1.51020 10.1512/iumj.1952.1.51020 en Indiana Univ. Math. J. 1 (1952) 605 - 652 state-of-the-art mathematics http://iumj.org/access/