1. The German mathematician Karl Weierstrass (1815–1897) showed in 1858 that the motion of a dynamical system could always be expressed in terms of normal coordinates
2. Misner, W., Thorne, K.S., Wheeler, J.: Gravitation, pp. 1022–1034. W. H. Freeman, San Fransisco (1973)
3. Byers, N.: Israel Mathematical Conference Proceedings, vol. 12 (1999)
4. Goldstein H., Poole C., Safko J. (2002) Classical Mechanics, p. 250. Addison-Wesley, Reading
5. Marion, J.B., Thornton, S.T.: Classical Dynamics of Particles and Systems, 3rd edn, p. 458. HBJ (1988)