We study a new geometric bootstrap percolation model, line percolation, on the d-dimensional integer grid [n] d . In line percolation with infection parameter r, infection spreads from a subset A ⊂ [n] d of initially infected lattice points as follows: if there exists an axis-parallel line L with r or more infected lattice points on it, then every lattice point of [n] d on L gets infected, and we repeat this until the infection can no longer spread. The elements of the set A are usually chosen independently, with some density p, and the main question is to determine p c (n, r, d), the density at which percolation (infection of the entire grid) becomes likely. In this paper, we determine p c (n, r, 2) up to a multiplicative factor of 1 + o(1) and p c (n, r, 3) up to a multiplicative constant as n → ∞ for every fixed r ∈ N. We also determine the size of the minimal percolating sets in all dimensions and for all values of the infection parameter.
K E Y W O R D Sbootstrap percolation, critical probability, polynomial method
INTRODUCTIONBootstrap percolation models and arguments have been used to study a range of phenomena in various areas, ranging from crack formation and the dynamics of glasses to neural nets and economics; see [4,12,20] for a small sample of such applications. In this paper, we introduce and study a new geometric bootstrap percolation model defined on the d-dimensional integer grid [n] d which we call line percolation; here, we write [n] for the set {1, 2, . . . , n}. For v ∈ [n] d , let L(v) denote the set of d Random Struct Alg. 2018;52:597-616.wileyonlinelibrary.com/journal/rsa