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Bootstrap Percolation in High Dimensions

Published online by Cambridge University Press:  12 August 2010

JÓZSEF BALOGH
Affiliation:
Department of Mathematics, University of Illinois, 1409 W. Green Street, Urbana, IL 61801 and Department of Mathematics, University of California, San Diego, La Jolla, CA 92093 (e-mail: jobal@math.uiuc.edu)
BÉLA BOLLOBÁS
Affiliation:
Trinity College, Cambridge CB2 1TQ, England and Department of Mathematical Sciences, The University of Memphis, Memphis, TN 38152, USA (e-mail: B.Bollobas@dpmms.cam.ac.uk)
ROBERT MORRIS
Affiliation:
IMPA, Estrada Dona Castorina 110, Jardim Botânico, Rio de Janeiro, RJ, Brasil (e-mail: rob@impa.br)
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Abstract

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In r-neighbour bootstrap percolation on a graph G, a set of initially infected vertices A ⊂ V(G) is chosen independently at random, with density p, and new vertices are subsequently infected if they have at least r infected neighbours. The set A is said to percolate if eventually all vertices are infected. Our aim is to understand this process on the grid, [n]d, for arbitrary functions n = n(t), d = d(t) and r = r(t), as t → ∞. The main question is to determine the critical probability pc([n]d, r) at which percolation becomes likely, and to give bounds on the size of the critical window. In this paper we study this problem when r = 2, for all functions n and d satisfying d ≫ log n.

The bootstrap process has been extensively studied on [n]d when d is a fixed constant and 2 ⩽ r ⩽ d, and in these cases pc([n]d, r) has recently been determined up to a factor of 1 + o(1) as n → ∞. At the other end of the scale, Balogh and Bollobás determined pc([2]d, 2) up to a constant factor, and Balogh, Bollobás and Morris determined pc([n]d, d) asymptotically if d ≥ (log log n)2+ϵ, and gave much sharper bounds for the hypercube.

Here we prove the following result. Let λ be the smallest positive root of the equation\[\sum_{k=0}^\infty \frac{(-1)^k \lambda^k}{2^{k^2-k} k!} = 0,\]so λ ≈ 1.166. Then\[ \frac{16\lambda}{d^2} \biggl(1 + \frac{\log d}{\sqrt{d}} \biggr)\: 2^{-2\sqrt{d}} \leq p_c([2]^d,2) \leq \frac{16\lambda}{d^2} \biggl(1 + \frac{5(\log d)^2}{\sqrt{d}} \biggr) \: 2^{-2\sqrt{d}}\]if d is sufficiently large, and moreover\[p_c\bigl([n]^d,2 \bigr) = \bigl(4\lambda + o(1) \bigr) \biggl(\frac{n}{n-1} \biggr)^2 \, \frac{1}{d^2} \, 2^{-2\sqrt{d \log_2 n}}\]as d → ∞, for every function n = n(d) with d ≫ log n.