English

Connections between certain numbers related to derangements and $r$-permutations

Combinatorics 2025-11-18 v2 Number Theory

Abstract

For non-negative integer parameters r,u,m,nr,u,m,n define \begin{align*} \cal{D}(r,u,m,n) := \big\{\ \sigma\in \cal{S}_{r+n}\ \big|\ \sigma(x)=y \textrm{ for exactly } u \textrm{ pairs } (x,y) \textrm{ such that } 1\leq x,y\leq r \textrm{ and } \sigma(t)=t \textrm{ for exactly } m \textrm{ elements } r+1\leq t\leq r+n\ \big\} \end{align*} and \begin{align*} \cal{D}_{r,u,m}(n) := \big\{\ \sigma\in \cal{S}_{r+n}\ \big|\ \forall_{1\leq x<y\leq r} \ x \textrm{ and } y \textrm{ are in disjoint cycles of } \sigma \textrm{ and } \sigma(z)=z \textrm{ for exactly } u \textrm{ elements } 1\leq z\leq r, \textrm{ and } \sigma(t)=t \textrm{ for exactly } m \textrm{ elements } r+1\leq t\leq r+n\ \big\}, \end{align*} where Sn\mathcal{S}_{n} denotes the set of all the permutations of {1,,n}\{1,\ldots ,n\}. In this paper we study connections between the sets D(r,u,m,n)\mathcal{D}(r,u,m,n), Dr,u,m(n)\mathcal{D}_{r,u,m}(n), and the sets of (some classes of) rr-derangements. We rely mostly on counting arguments.

Keywords

Cite

@article{arxiv.2411.19294,
  title  = {Connections between certain numbers related to derangements and $r$-permutations},
  author = {Piotr Miska and Błażej Żmija},
  journal= {arXiv preprint arXiv:2411.19294},
  year   = {2025}
}

Comments

Published in Australasian Journal of Combinatorics