1.
Simmonds, J.G., Mann, J.E.: A first look at perturbation theory. Dover Publications, Mineola, N.Y. (1998).
2.
Bleistein, N., Handelsman, R.A.: Asymptotic expansions of integrals. Dover Publications, New York (1986).
3.
de Bruijn, M.G.: Asymptotic methods in analysis. Dover, New York (1981).
4.
Tricomi, F.G.: Integral Equations. Dover Publications (2012).