# Two Posets of Noncrossing Partitions Coming From Undesired Parking Spaces

## TL;DR

This paper investigates specific subposets of noncrossing partitions related to parking spaces, proving their supersolvability, shellability, and confirming a conjecture about their order complex's contractibility.

## Contribution

It introduces new subposets of noncrossing partitions, proves their supersolvability and shellability, and confirms a conjecture about their order complex's contractibility.

## Key findings

- The subposets are supersolvable lattices.
- The subposets are lexicographically shellable.
- The conjecture on contractibility of the order complex is proven.

## Abstract

Consider the noncrossing set partitions of an $n$-element set which either do not contain the block $\{n-1,n\}$, or which do not contain the singleton block $\{n\}$ whenever $1$ and $n-1$ are in the same block. In this article we study the subposet of the noncrossing partition lattice induced by these elements, and show that it is a supersolvable lattice, and therefore lexicographically shellable. We give a combinatorial model for the NBB bases of this lattice and derive an explicit formula for the value of its M\"obius function between least and greatest element. This work is motivated by a recent article by M. Bruce, M. Dougherty, M. Hlavacek, R. Kudo, and I. Nicolas, in which they introduce a subposet of the noncrossing partition lattice that is determined by parking functions with certain forbidden entries. In particular, they conjecture that the resulting poset always has a contractible order complex. We prove this conjecture by embedding their poset into ours, and showing that it inherits the lexicographic shellability.