English

Restricted Stirling and Lah number matrices and their inverses

Combinatorics 2018-01-01 v2

Abstract

Given RNR \subseteq \mathbb{N} let {nk}R{n \brace k}_R, [nk]R{n \brack k}_R, and L(n,k)RL(n,k)_R be the number of ways of partitioning the set [n][n] into kk non-empty subsets, cycles and lists, respectively, with each block having cardinality in RR. We refer to these as the RR-restricted Stirling numbers of the second and first kind and the RR-restricted Lah numbers, respectively. Note that the classical Stirling numbers of the second kind and first kind, and Lah numbers are {nk}={nk}N{n \brace k} = {n \brace k}_{\mathbb{N}}, [nk]=[nk]N{n \brack k} = {n \brack k}_{\mathbb{N}} and L(n,k)=L(n,k)NL(n,k) = L(n,k)_{\mathbb{N}}, respectively. The matrices [{nk}]n,k1[{n \brace k}]_{n,k \geq 1}, [[nk]]n,k1[{n \brack k}]_{n,k \geq 1} and [L(n,k)]n,k1[L(n,k)]_{n,k \geq 1} have inverses [(1)nk[nk]]n,k1[(-1)^{n-k}{n \brack k}]_{n,k \geq 1}, [(1)nk{nk}]n,k1[(-1)^{n-k} {n \brace k}]_{n,k \geq 1} and [(1)nkL(n,k)]n,k1[(-1)^{n-k} L(n,k)]_{n,k \geq 1} respectively. The inverse matrices [{nk}R]n,k11[{n \brace k}_R]^{-1}_{n,k \geq 1}, [[nk]R]n,k11[{n \brack k}_R]^{-1}_{n,k \geq 1} and [L(n,k)R]n,k11[L(n,k)_R]^{-1}_{n,k \geq 1} exist if and only if 1R1 \in R. We express each entry of each of these matrices as the difference between the cardinalities of two explicitly defined families of labeled forests. In particular the entries of [{nk}[r]]n,k11[{n \brace k}_{[r]}]^{-1}_{n,k \geq 1} have combinatorial interpretations, affirmatively answering a question of Choi, Long, Ng and Smith from 2006. If 1,2R1,2 \in R and if for all nRn \in R with nn odd and n3n \geq 3, we have n±1Rn \pm 1 \in R, we additionally show that each entry of [{nk}R]n,k11[{n \brace k}_R]^{-1}_{n,k \geq 1}, [[nk]R]n,k11[{n \brack k}_R]^{-1}_{n,k \geq 1} and [L(n,k)R]n,k11[L(n,k)_R]^{-1}_{n,k \geq 1} is up to an explicit sign the cardinality of a single explicitly defined family of labeled forests. Our results also provide combinatorial interpretations of the kkth Whitney numbers of the first and second kinds of Πn1,d\Pi_n^{1,d}, the poset of partitions of [n][n] that have each part size congruent to 11 mod dd.

Keywords

Cite

@article{arxiv.1610.05803,
  title  = {Restricted Stirling and Lah number matrices and their inverses},
  author = {John Engbers and David Galvin and Clifford Smyth},
  journal= {arXiv preprint arXiv:1610.05803},
  year   = {2018}
}

Comments

This is a substantial revision of version 1, with more extensive results and unified proofs, as well as with new connections to certain Whitney numbers