On a Theorem by Landesman and Lazer Peter Hess Indiana University Mathematics Journal 1974 text pdf 10.1512/iumj.1974.23.23068 10.1512/iumj.1974.23.23068 en Indiana Univ. Math. J. 23 (1974) 827 - 829 state-of-the-art mathematics http://iumj.org/access/