1. B. Riemann,Abh. der Ges. der Wiss. Gött. 13, 1 (1868) [English translation in W. K. Clifford,Collected Mathematical Papers].
2. Felix Klein, A Comparative Review of Recent Researches in Geometry (Erlanger 1872) [English translation inN.Y. Math. Soc. Bull. 2 (July), 215 (1893)].
3. H. Weyl,Philosophy of Mathematics and Natural Science (Princeton Univ. Press, 1949), Chapter 111, 13;The Classical Groups (Princeton Univ. Press, 1946), Chapter 1, 4.
4. G. Cantor,Contributions to the Founding of the Theory of Transfinite Numbers (Dover, 1955).
5. Yu-Lee Lee,Pac. J. Math. 20, 77 (1967).