[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