1. Birkhoff, G. D., & R. E. Langer: The boundary problem and developments associated with a system of ordinary differential equations of first order. Proc. Amer. Acad. Arts Sci. 58 (2), 51–128 (1923).
2. Chapin, G. G., Jr.: One and two point boundary value problems for ordinary linear differential equations containing a parameter. Ph. D. thesis, University of Minnesota, 1959.
3. Coddington, E., & N. Levinson: A boundary value problem for a non-linear differential equation with a small parameter. Proc. Amer. Math. Soc. 3, 73–81 (1952).
4. Gradstein, I. S.: Linear equations with small parameters. Math. Sbornik N. S. 27 [Russian], 69, 47–68 (1950). Also Translation Number 82, Amer. Math. Soc. 1953.
5. Harris, W. A., Jr.: A boundary value problem for a system of ordinary linear differential equations involving powers of a parameter. Technical Report 3, OOR Project Number 1469, Department of Math., Univ. of Minn. 1958, 137 p.