On Singular Perturbations of Nonlinear Operator Equations Melvyn BergerL. Fraenkel Indiana University Mathematics Journal 1971 text pdf 10.1512/iumj.1971.20.20050 10.1512/iumj.1971.20.20050 en Indiana Univ. Math. J. 20 (1971) 623 - 631 state-of-the-art mathematics http://iumj.org/access/