  Bootstrap percolation models have been extensively studied during the two
past decades. In this article, we study the following "anisotropic" bootstrap
percolation model: the neighborhood of a point (m,n) is the set
\[\{(m+2,n),(m+1,n),(m,n+1),(m-1,n),(m-2,n),(m,n-1)\}.\] At time 0, sites are
occupied with probability p. At each time step, sites that are occupied remain
occupied, while sites that are not occupied become occupied if and only if
three of more sites in their neighborhood are occupied. We prove that it
exhibits a sharp metastability threshold. This is the first mathematical proof
of a sharp threshold for an anisotropic bootstrap percolation model.
