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de Bruijn, M. G. 1981. Asymptotic Methods in Analysis. 3rd ed. New York: Dover.
Simmonds, James G., and James E. Mann. 1998. A First Look at Perturbation Theory. 2nd ed. Mineola, N.Y.: Dover Publications.
Tricomi, F. G. 2012. Integral Equations. Dover Publications.