English

Avoiding colored partitions of two elements in the pattern sense

Combinatorics 2012-06-15 v3

Abstract

Enumeration of pattern-avoiding objects is an active area of study with connections to such disparate regions of mathematics as Schubert varieties and stack-sortable sequences. Recent research in this area has brought attention to colored permutations and colored set partitions. A colored partition of a set SS is a partition of SS with each element receiving a color from the set [k]={1,2,...,k}[k]=\{1,2,...,k\}. Let ΠnCk\Pi_n\wr C_k be the set of partitions of [n][n] with colors from [k][k]. In an earlier work, the authors study pattern avoidance in colored set partitions in the equality sense. Here we study pattern avoidance in colored partitions in the pattern sense. We say that σΠnCk\sigma\in\Pi_n\wr C_k contains πΠmC\pi\in \Pi_m\wr C_\ell in the pattern sense if σ\sigma contains a copy π\pi when the colors are ignored and the colors on this copy of π\pi are order isomorphic to the colors on π\pi. Otherwise we say that σ\sigma avoids π\pi. We focus on patterns from Π2C2\Pi_2\wr C_2 and find that many familiar and some new integer sequences appear. We provide bijective proofs wherever possible, and we provide formulas for computing those sequences that are new.

Keywords

Cite

@article{arxiv.1203.3786,
  title  = {Avoiding colored partitions of two elements in the pattern sense},
  author = {Adam M. Goyt and Lara K. Pudwell},
  journal= {arXiv preprint arXiv:1203.3786},
  year   = {2012}
}

Comments

17 pages; Journal of Integer Sequences 15 (2012) 12.6.2

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