On a Problem in Hilbert Space with Applications J. BlumD. Hanson Indiana University Mathematics Journal 1962 text pdf 10.1512/iumj.1962.11.11031 10.1512/iumj.1962.11.11031 en Indiana Univ. Math. J. 11 (1962) 497 - 501 state-of-the-art mathematics http://iumj.org/access/