# Extremal $4$-Neighbour Bootstrap Percolation in the Hypercube

\begin{problem} Determine the smallest percolating set for the $4$-neighbour bootstrap process in the hypercube. \end{problem}

% You may use many features of TeX, such as % arbitrary math (between $...$ and $$...$$) % \begin{theorem}...\end{theorem} environment, also works for question, problem, conjecture, ... % % Our special features: % Links to wikipedia: \Def {mathematics} or \Def[coloring]{Graph_coloring} % General web links: \href [The On-Line Encyclopedia of Integer Sequences]{http://www.research.att.com/~njas/sequences/}

The \emph{$r$-neighbour bootstrap process} starts with an initial set of "infected" vertices in a graph and, at each step, a healthy vertex becomes infected if it has at least $r$ infected neighbours. Say that the initial set of infected vertices \emph{percolates} if every vertex of $G$ is eventually infected. Let $m(G,r)$ denote the smallest percolating set in $G$ under the $r$-neighbour process.

Let $Q_d$ denote the hypercube of dimension $d$. Balogh and Bollobás [BB] proved the following.

\begin{theorem}[Balogh and Bollobás] $m(Q_d,2) = \left\lceil \frac{d}{2}\right\rceil +1$ for all $d\geq 2$. \end{theorem}

They also conjectured that $m(Q_d,r) = \frac{1+o(1)}{r}\binom{d}{r-1}$ for fixed $r$ and $d\to\infty$. This conjecture was proved by Morrison and Noel [MN], who also showed the following.

\begin{theorem}[Morrison and Noel] $m(Q_d,3) = \left\lceil \frac{d(d+3)}{6} \right\rceil +1$ for all $d\geq 3$. \end{theorem}

It seems possible that one could obtain a general formula for $m(Q_d,r)$ for all $r$ and $d\geq r$. However, the precise formula for $m(Q_d,r)$ (in terms of $d$) is not known for any fixed $r\geq4$. A solution to this problem may have applications in proving probabilistic results for bootstrap percolation in the hypercube; see [BBM].

## Bibliography

% Example: %*[B] Claude Berge, Farbung von Graphen, deren samtliche bzw. deren ungerade Kreise starr sind, Wiss. Z. Martin-Luther-Univ. Halle-Wittenberg Math.-Natur. Reihe 10 (1961), 114. % %[CRS] Maria Chudnovsky, Neil Robertson, Paul Seymour, Robin Thomas: \arxiv[The strong perfect graph theorem]{math.CO/0212070}, % Ann. of Math. (2) 164 (2006), no. 1, 51--229. \MRhref{MR2233847} % % (Put an empty line between individual entries)

[BB] J. Balogh and B. Bollobás, Bootstrap percolation on the hypercube, Probab. Theory Related Fields 134 (2006), no. 4, 624–648.

[BBM] J. Balogh, B. Bollobás and R. Morris, Bootstrap percolation in high dimensions, Combin. Probab. Comput. 19 (2010), no. 5-6, 643–692.

[MN] N. Morrison and J. A. Noel, Extremal Bounds for Bootstrap Percolation in the Hypercube, preprint, arXiv:1506.04686v1.

* indicates original appearance(s) of problem.