Incompressible Systems L. HelmsC. Putnam Indiana University Mathematics Journal 1958 text pdf 10.1512/iumj.1958.7.57051 10.1512/iumj.1958.7.57051 en Indiana Univ. Math. J. 7 (1958) 901 - 903 state-of-the-art mathematics http://iumj.org/access/