Grid Classes and the Fibonacci Dichotomy for Restricted Permutations

Abstract

We introduce and characterise grid classes, which are natural generalisations of other well-studied permutation classes. This characterisation allows us to give a new, short proof of the Fibonacci dichotomy: the number of permutations of length $n$ in a permutation class is either at least as large as the $n$th Fibonacci number or is eventually polynomial.