# A formula for the Möbius function of the permutation poset based on a topological decomposition

Smith, Jason P. (2017) A formula for the Möbius function of the permutation poset based on a topological decomposition. Advances in Applied Mathematics, 91. pp. 98-114. ISSN 0196-8858 (https://doi.org/10.1016/j.aam.2017.06.002)  Preview Text. Filename: Smith_AAM_2017_formula_for_the_mobius_function_of_the_permutation_poset_based_on_a_topological_decomposition.pdf Accepted Author Manuscript License: Download (194kB)| Preview

## Abstract

We present a two term formula for the Möbius function of intervals in the poset of all permutations, ordered by pattern containment. The first term in this formula is the number of so called normal occurrences of one permutation in another. Our definition of normal occurrences is similar to those that have appeared in several variations in the literature on the Möbius function of this and other posets, but simpler than most of them. The second term in the formula is complicated, but we conjecture that it equals zero for a significant proportion of intervals. We present some cases where the second term vanishes and others where it is nonzero. Computing the Möbius function recursively from its definition has exponential complexity, whereas the computation of the first term in our formula is polynomial and the exponential part is isolated to the second term, which seems to often vanish. We also present a result on the Möbius function of posets connected by a poset fibration.

#### ORCID iDs

;
• Item type: Article 61343 DateEvent31 October 2017Published3 July 2017Published Online20 June 2017Accepted Möbius function, posets, permutations, pattern containment, combinatorics, Mathematics, Discrete Mathematics and Combinatorics Science > Mathematics Faculty of Science > Computer and Information Sciences Pure Administrator 25 Jul 2017 09:26 01 Oct 2022 03:22 https://strathprints.strath.ac.uk/id/eprint/61343