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{\bf Sophie Huczynska and Vincent Vatter}
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{\bf Grid Classes and the Fibonacci Dichotomy for Restricted Permutations}
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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.
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