1. Stokes, G. G. 1852,Trans. Camb. Philos. Soc.,9, 399; also 1901Mathematical and Physical Papers of Sir George Stokes (Cambridge, England: Cambridge Univ. Press),3, 233.
2. King, L.V. 1913,Phil. Trans. Roy. Soc. London,A212, 375.
3. Wiener, N., Hopf, E. 1931,Berliner Ber. Math. Phys. Klasse, p. 696.
4. Hopf, E. 1934,Mathematical Problems of Radiative Equilibrium (Cambridge: Cambridge Univ. Press).
5. Ambartsumian, V. A. 1943,C.R. (Doklady), Acad. URSS,38, 257.