Prasad, Phoolan: Nonlinear hyperbolic waves in multi-dimensions, s. 104. CRC Press, 2001. ISBN 9781584880721. Google Book. (englanniksi)
Hecht, Eugene: Schaum’s outline of theory and problems of optics, s. 36. McGraw-Hill Professional, 1975. ISBN 9780070277304. Google Book. (englanniksi)
