Nonlinear Problems with Exactly Three Solutions Melvyn Berger Indiana University Mathematics Journal 1979 text pdf 10.1512/iumj.1979.28.28047 10.1512/iumj.1979.28.28047 en Indiana Univ. Math. J. 28 (1979) 689 - 698 state-of-the-art mathematics http://iumj.org/access/