21 Dispersions of the x-th kind; x = 1, 2, 3, 4
(English).
In:
Borůvka, Otakar; Arscott, F. M. (translator): Linear Differential Transformations of the Second Order.
(English).
London:
The English Universities Press, Ltd.,
1971.
0-340-12408-3,
pp. [183]-193