1. Fokker. Jan. 27 1917. Jan. 27, Verslag d. Amsterdamer Akad. For further bibliography, compare Klein's second Note,Göttinger Nachrichten, July 19, 1918. A recently published work by Kneser (Math.Zschr. 2) deals with the setting up of invariants by a similar method
2. Lie, in “Grundlagen fur die Theorie der Unendlichen kontinuierlichen Transformationsgruppen” (Foundations of the theory of infinite continuous groups of transformations),Ber. d. K. Sachs. Ges. d. Wiss. 1891 (cited asGrundlagen., defines the infinite continuous group as a group of transformations which are given by the most general solutions of a system of partial differential equations, provided these solutions do not depend only on a finite number of parameters. One of the above-mentioned types differing from the finite group will thus be thereby obtained; whereas conversely the limiting case of infinitely many parameters need not necessarily satisfy a system of differential equations