Two-Parameter Singular Perturbation Problems for Second-Order Equations R. O'Malley, Jr. Indiana University Mathematics Journal 1967 text pdf 10.1512/iumj.1967.16.16075 10.1512/iumj.1967.16.16075 en Indiana Univ. Math. J. 16 (1967) 1143 - 1164 state-of-the-art mathematics http://iumj.org/access/