Boundary Value Problems for Linear Systems of Ordinary Differential Equations Involving Many Small Parameters R. O'Malley, Jr. Indiana University Mathematics Journal 1969 text pdf 10.1512/iumj.1969.18.18061 10.1512/iumj.1969.18.18061 en Indiana Univ. Math. J. 18 (1969) 835 - 855 state-of-the-art mathematics http://iumj.org/access/