1. M. Bôcher, Higher Algebra, Th. 1, p.170
2. Bôcher, loc. cit., Th. 2, p.171.
3. L. E. Dickson, Modern Algebraic Theories, p. 71.
4. The definitions concerning critical points are as given by M. Morse. See (I) Morse, "Relations between the critical points of a real function of n independent variables," Trans. Amer. Math. Soc., Vol. 27 (1925), pp. 545-596
5. or (II) Morse and Schaack, "The critical point theory under general boundary conditions," Annals of Math., Vol. 55 (1954), pp. 545-571.