1. O. Heaviside:Phil. Trans. Roy. Soc.,183 A, 423 (1893).
2. J. Larmor:Collected Papers (London, 1928).
3. G. Y. Rainich:Trans. Amer. Math. Soc.,27, 106 (1925).
4. H. Bateman:Proc. Math. Soc.,8, 77 (1909);E. Cunningham:Proc. Math. Soc.,8, 77 (1909).
5. L. V. Ovsjannikov:The Group Analysis of Differential Equations (in Russian) (Moscow, 1978).