## Abstract

Babson and Steingrímsson introduced generalized permutation patterns that allow the requirement that two adjacent letters in a pattern must be adjacent in the permutation. Subsequently, Claesson presented a complete solution for the number of permutations avoiding any single pattern of type (1,2) or (2,1). For eight of these twelve patterns the answer is given by the Bell numbers. For the remaining four the answer is given by the Catalan numbers. In the present paper we give a complete solution for the number of permutations avoiding a pair of patterns of type (1,2) or (2,1). We also conjecture the number of permutations avoiding the patterns in any set of three or more such patterns.

Original language | English |
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Pages (from-to) | 17-31 |

Number of pages | 15 |

Journal | Ars Combinatoria |

Volume | 77 |

State | Published - Oct 2005 |

Externally published | Yes |

## Keywords

- Pattern avoidance
- Permutation

## ASJC Scopus subject areas

- General Mathematics