[1] H. SACHS:
On spatial analogue of Kuratowski's Theorem on planar graphs - an open problem. in: Graph Theory, Lagów 1981 (ed. M. Horowiecki et al.), Lecture Botes In Mathematics, Vol. 1018, Springer-Verlag 1983, pp. 230-241.
MR 0730653
[2] P. SEYMOUR N. ROBERTSON: (to appear).
[3] W. MADER:
Existenz gewisser Konfigurationen in n-gesättigten Graphen und in Graphen genügend grosser Kantendichte. Math. Ann. 194 (1971), 295-312.
MR 0289344
[4] I. JACOBSEN:
On certain homomorphism properties I, II. Math. Scand. 32 (1972), 379-404 and 52 (1983), 229-261.
MR 0702956