1. Phil. Trans. Roy. Soc. 1835, Pt. I, p. 95. Lagrange had published them many years earlier without recognizing their value.
2. Jackson, 1475, p. 177.
3. For a condensed explanation of why the term is used, see Sommerfeld (1923), p. 547.
4. F.J.Bloore, J. Phys. A. Math., Nucl. Gen. 6 L7 (1973).
5. W.E.Lamb, Jr., Phys. Rev. 85, 259 (1952). See also K.H. Tang, Ann. of Phys. 101, 62 (1976).