1. Bender, C.M., Orszag, S.A.: Advanced Mathematical Methods for Scientists and Engineers. Mc Graw-Hill, New York (1978)
2. Kevorkian, J., Cole, J.D.: Perturbation Methods in Applied Mathematics. Springer, New York (1981)
3. Nayfeh, A.H.: Perturbation Methods. Wiley, New York (1973)
4. O’ Malley R.E.: Introduction to Singular Perturbations. New York, Academic Press (1974)
5. Doolan, E.P., Miller, J.J.H., Schilders, W.H.A.: Uniform Numerical Methods for Problems with Initial and Boundary Layers. Boole Press, Dublin (1980)