1. Courant, R. and Hilbert, D., Methods of Mathematical Physics, Interscience, 1953. (a. Chapter VII; b. Subsection V. 16. 5.)
2. Wasow, W., Asymptotic Expansions for Ordinary Differential Equations, Interscience Publishers, 1965.
3. Messiah, A., Quantum Mechanics, Vol. 2, Wiley, 1966. (See Equations 170(a, b) in Section XX. 25.)
4. Kato, T., Perturbation Theory for Linear Operators, Springer Verlag, 1966. (a. Subsection I.4.1; b. Subsection I.4.4)
5. Miller, W., Jr., Lie Theory and Special Functions, Academic Press, 1968. (See Equation 3.25.)