A drawing of a graph G in the plane has the vertices represented by distinct points and the edges represented by polygonal lines joining their endpoints such that:
The crossing number cr(G) of G is the minimum number of crossings in all drawings of G in the plane.
The d-dimensional (hyper)cube Qd is the graph whose vertices are all binary sequences of length d, and two of the sequences are adjacent in Qd if they differ in precisely one coordinate.
Conjecture (Erdős and Guy ): lim cr(Qd) / 4d = 5/32 (as n tends to infinity).
It is known that cr(Qd) = 0 for d = 1,2,3 and that cr(Q4) = 8. No other exact values are known. Madej  proved that cr(Qd) is at most 4d/6 + o(4d/6). Faria and de Figueiredo  improved the upper bound to (165/1024) 4d. Sykora and Vrto  proved that 4d/20 + o(4d/20) is a lower bound on cr(Qd).
 P. Erdős and R.K. Guy, Crossing number problems, Amer. Math. Monthly 80 (1973) 52-58.
 L. Faria, C.M.H. de Figueiredo, On Eggleton and Guy's conjectured upper bound for the crossing number of the n-cube, Math. Slovaca 50 (2000) 271-287.
 T. Madej, Bounds for the crossing number of the n-cube, J. Graph Theory 15 (1991) 81-97.
 O. Sykora and I. Vrto, On crossing numbers of hypercubes and cube connected cycles, BIT 33 (1993) 232-237.
Send comments to Bojan.Mohar@uni-lj.si