1. O. Heaviside, Electrician 17, 212 (1886)
2. reprinted as Electrical Papers, Vol. II (Macmillan & Co., London--New York, 1892), p. 64
3. A. E. Kennelly, Trans. Am. Inst. Electr. Eng. 10, 172 (1893);
4. A. Šabanović, K. Ohnishi, Motion Control Systems (John Wiley and Sons (Asia) Pte Ltd, 2011);
5. L. E. Kinsler, A. R. Frey, A. B. Coppens, J. V. Sanders, Fundamentals of Acoustics, 4th ed. (John Wiley and Sons Ltd, 2000)