On Future and Past Stability in Incompressible Systems C. Putnam Indiana University Mathematics Journal 1957 text pdf 10.1512/iumj.1957.6.56036 10.1512/iumj.1957.6.56036 en Indiana Univ. Math. J. 6 (1957) 669 - 672 state-of-the-art mathematics http://iumj.org/access/