1. [1] A. N. Tikhonov, On the dependence of the solutions of differential equations on a small parameter, Mat. Sb.22(64) (1948), 193-204.
2. [2] M. I. Vishik and L. A. Lyusternik, Regular degeneration and boundary layer for linear differential equations with small parameter, Uspekhi Mat. Nauk12:5(77) (1957), 3-122.
3. [3] A. V. Bitsadze and A. A. Samarskii, Some elementary generalizations of linear elliptic boundary value problems, Dokl. Akad. Nauk SSSR185(4) (1969), 739-740.
4. [4] V. A. Il’in and E. I. Moiseev, Fist kind nonlocal boundary value problem for Sturm-Liouville operator in differential and difference treatment, Differential Equations23(7) (1987), 1198-1207.
5. [5] V. A. Il’in and E. I. Moiseev, Second kind nonlocal boundary value problem for Sturm-Liouville operator, Differential Equations23(8) (1987), 1422-1431.