1. A. Messiah, Quantum Mechanics, Vol. II (North Holland, Amsterdam, 1965) Chapt. XIX.IV: Variational Calculations of Transition Amplitudes
2. M.L. Goldberger, K.M. Watson, Collision Theory (John Wiley, New York, 1967) Chapt. 6.7: Approximation Methods, p. 313: Variational Principles
3. J.R. Taylor, Scattering Theory (John Wiley, New York, 1972) Chapt. 14-d: Variational Methods
4. R.G. Newton: Scattering Theory of Waves and Particles, 2nd ed. (Springer, New York, Heidelberg, Berlin, 1985) Chapt. 11.3: Variational Approaches
5. E. Gerjuoy, A.R.P. Rau, L. Spruch, Rev. Mod. Phys. 55, 725 (1983)