Permutations with Restricted Patterns and Dyck Paths

  • Authors:
  • C. Krattenthaler

  • Affiliations:
  • Institut für Mathematik der, Universität Wien, Strudlhofgasse 4, A-1090, Vienna, Austria, http://www.mat.univie.ac.at/People/krattf1kratt@ap.univie.ac.atf1

  • Venue:
  • Advances in Applied Mathematics
  • Year:
  • 2001

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Abstract

We exhibit a bijection between 132-avoiding permutations and Dyck paths. Using this bijection, it is shown that all the recently discovered results on generating functions for 132-avoiding permutations with a given number of occurrences of the pattern 12...k follow directly from old results on the enumeration of Motzkin paths, among which is a continued fraction result due to Flajolet. As a bonus, we use these observations to derive further results and a precise asymptotic estimate for the number of 132-avoiding permutations of {1,2,...,n} with exactly r occurrences of the pattern 12...k. Second, we exhibit a bijection between 123-avoiding permutations and Dyck paths. When combined with a result of Roblet and Viennot, this bijection allows us to express the generating function for 123-avoiding permutations with a given number of occurrences of the pattern (k-1)(k-2)...1k in the form of a continued fraction and to derive further results for these permutations.