Some natural extensions of the parking space

We construct a family of $S_n$ modules indexed by $c\in\{1,\dots,n\}$ with the property that upon restriction to $S_{n-1}$ they recover the classical parking function representation of Haiman. The construction of these modules relies on an $S_n$-action on a set that is closely related to the set of parking functions. We compute the characters of these modules and use the resulting description to classify them up to isomorphism. In particular, we show that the number of isomorphism classes is equal to the number of divisors $d$ of $n$ satisfying $d\neq 2 \: (\!\!\!\!\mod 4)$. In the cases $c=n$ and $c=1$, we compute the number of orbits. Based on empirical evidence, we conjecture that when $c=1$, our representation is $h$-positive and is in fact the (ungraded) extension of the parking function representation constructed by Berget and Rhoades.Comment: 16 pages; comments welcom

Poset Topology: Tools and Applications

These lecture notes for the IAS/Park City Graduate Summer School in Geometric Combinatorics (July 2004) provide an overview of poset topology. These notes include introductory material, as well as recent developments and open problems. Some of the topics covered are: subspace arrangements, graph complexes, group actions on poset homology, shellability, recursive techniques, and fiber theorems.Comment: 119 pages, to appear in the volume "Geometric Combinatorics" of the IAS/Park City Mathematics serie