1. G. C. Debney, R. P. Kerr, and A. Schild, J. Math. Phys. 10, 1842 (1969).
2. R. Penrose and W. Rindler, Spinors and Space-Time, Vol. II (Cambridge Univ. Press, Cambridge, 1986).
3. V. V. Kassandrov and J. A. Rizkallah, Twistor and “Weak“ Gauge Structures in the Framework of Quaternionic Analysis, gr-qc/0012109.
4. V. V. Kassandrov, in: Space-Time Structure. Algebra and Geometry, Ed. by D. G. Pavlov et al. (Lilia-Print, Moscow, 2007), p. 441, arXiv: 0710.2895.
5. V. V. Kassandrov, Algebraic Structure of Space-Time and Algebrodynamics (People Fried. Univ. Press, 1992), (in Russian).