1. R. Fueter; Comm. Math. Helv., 7, 307 (1934-35), 8, 371 (1935-37)
2. 1. 'The relation of Maxwell's equations to the theory of functions of a biquaternion variable': K. Imaeda: Prog. Theor. Phys., 5, 133 (1950). The Memoir of the Faculty of Liberal Arts and Education, Yamanashi Univ., 2, 111(1951). The latter is included in 'Quaternionic Formulation of Classical Electrodynamics and Theory of Functions of a Biquaternions Variable
3. 2. Report-FPL-83, Feb. 1983. Other references are: D. Hestenes: Space-Time Algebra, Gordon and Breach, New York (1966), (And earlier references on the quaternionic form of Maxwell's equations). R. Hermann: Spinors, Clifford and Cayley Algebras, Math Sci Press, Brooklyn, Ma (1974). D. Hestenes and G. Sobczyk: Clifford Algebra to Geometric Calculus, D. Reidel Pub. Co., Dordrecht (1984).
4. The e.(i = 1, 2, 3) satisfy the same relations as Pauli matrices. As to the geometric interpretation: see D. Hestenes Space-Time Algebra loc. cit.
5. D. Riabouchinsky: Compt. Rend., 19, 1139 (1924). M. Goto: Journal Inst. Electrical Engineers of Japan (in Japanese) 742 (1929) and 275 (1929).