Sachs (1983). Sachs, Horst (1983), "On a spatial analogue of Kuratowski's Theorem on planar graphs – an open problem", in Horowiecki, M.; Kennedy, J. W.; Sysło, M. M. (eds.), Graph Theory: Proceedings of a Conference held in Łagów, Poland, February 10–13, 1981, Lecture Notes in Mathematics, vol. 1018, Springer-Verlag, pp. 230–241, doi:10.1007/BFb0071633, ISBN978-3-540-12687-4.