Two Hydrodynamic Comparison Theorems James Serrin Indiana University Mathematics Journal 1952 text pdf 10.1512/iumj.1952.1.51017 10.1512/iumj.1952.1.51017 en Indiana Univ. Math. J. 1 (1952) 563 - 572 state-of-the-art mathematics http://iumj.org/access/