# The Crossing Number of the Hypercube

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 $Q_d$ is the graph whose vertices are all binary sequences of length $d$, and two of the sequences are adjacent in $Q_d$ if they differ in precisely one coordinate.

\begin{conjecture} $\displaystyle \lim \frac{cr(Q_d)}{4^d} = \frac{5}{32}$ \end{conjecture}

It is known that $cr(Q_d) = 0$ for $d = 1,2,3$ and that $cr(Q_4) = 8$. No other exact values are known. Madej [M] proved that $cr(Q_d) \le 4^d/6 + o(4^d/6)$. Faria and de Figueiredo [FF] improved the upper bound to $(165/1024) 4^d$. Sykora and Vrto [SV] proved that $4^d/20 + o(4^d/20)$ is a lower bound on $cr(Q_d)$.

## Bibliography

*[EG] P. Erdős and R.K. Guy, Crossing number problems, Amer. Math. Monthly 80 (1973) 52-58.

[FF] 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.

[M] T. Madej, Bounds for the crossing number of the $n$-cube, J. Graph Theory 15 (1991) 81-97.

[SV] O. Sykora and I. Vrto, On crossing numbers of hypercubes and cube connected cycles, BIT 33 (1993) 232-237.

* indicates original appearance(s) of problem.

## Improved upper bound

I came accross a paper \doi[Faria, Herrera de Figueiredo, Sykora, Vrto: An improved upper bound on the crossing number of the hypercube]{10.1002/jgt.20330} that proves half of this, getting the correct upper bound.