A Variational Theorem for Nonlinear Differential Equations David Westreich Indiana University Mathematics Journal 1976 text pdf 10.1512/iumj.1976.25.25091 10.1512/iumj.1976.25.25091 en Indiana Univ. Math. J. 25 (1976) 1149 - 1157 state-of-the-art mathematics http://iumj.org/access/