[1] CENKL B.:
On the G-structure of higher order. Čas. pěst. mat. 90 (1963), 26-32.
MR 0192435
[2] CLARK R. S.:
Special geometric object fields on a differentiable manifold. Tensor 13 (1965), 60-70.
MR 0155263
[3] GUILLEMIN, SINGER: Diff. equations and G-atructures. Proc. of US Japan Seminar in Dif. Geom. Kyoto (1965), 34-36.