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(2+2)-free posets, ascent sequences and pattern avoiding permutations

<12>Interval orders have received significant attention for 42 years, but their enumeration resisted all attempts, until this paper produced one. The key was an encoding of the posets as a structure we introduced, namely ascent sequences. These new sequences are a fundamental contribution as they most transparently embody the recursive structure of interval orders and several other combinatorial structures. This article has become one of the most influential recent papers in enumerative combinatorics, e.g. Ulfarsson (2010) used our work to characterise Gorenstein varieties. Most cited paper of last five years in JCTA, the top combinatorics journal.