- CV (as of September 16, 2017)
- Preprints
- Published papers

Geometric combinatorics uses geometric and/or topological techniques to study combinatorial problems. The problems that I am interested usually begin with a combinatorial object, such as a graph or partially ordered set, and attach a simplicial complex. Since a simplicial complex is just a set system that is closed under inclusion, they arise naturally in combinatorics. For example:

- In a graph
*G*, any subset of an independent set is independent. Thus, the set of all independent sets of*G*is a simplicial complex, called the*independence complex*. - In a poset
*P*, any subset of a chain (totally ordered subset) is also a chain. Thus, the set of all chains of*P*is a simplicial complex, called the*order complex*of*P*. The Euler characteristic of*P*is useful in solving inclusion-exclusion problems (via MÃ¶bius inversion).

Here's a cool example of a poset (Figure A). Its order complex can be obtained from the diagram in Figure B by an operation that is analogous to barycentric subdivision.

**Figure A:** A poset whose order complex is a torus. The order complex subdivides the cell complex shown in Figure B.

Poset diagram created with GAP and XGAP: source code.

**Figure B:** A cell complex for the torus.

Work of mine frequently involves the order complexes of lattices of subgroups or of cosets of a finite group *G*. Indeed, my thesis characterized the finite groups with a shellable or Cohen-Macaulay coset lattice. More recently, some highlighted work includes:

- In work with Jay Schweig, we've defined a new class of lattices (the
*comodernistic lattices*) which seem to encapsulate well the combinatorial niceness of subgroup lattices of solvable groups. These lattices are CL-shellable, and we exhibited many examples of them. These lattices may be to solvable groups what supersolvable lattices are to supersolvable groups. - In work with John Shareshian, we've shown that the lattice of cosets of any finite group is not contractible. We make thorough use of Smith Theory, along with the classification and certain generation theorems on finite simple groups.

**Figure C:** *L(S*_{4}*)*, the subgroup lattice of the symmetric group on 4 letters.
*L(S*_{4}*)* is an example of a comodernistic lattice.

Diagram created with GAP and XGAP.

I'm also interested in independence complexes of graphs, and more broadly of clutters. Perhaps my strongest work here shows that the minimal non-shellable independence complexes of graphs (that is, *flag complexes*) are the independence complexes of cyclic graphics of lengths other than 3 or 5. This work answered a question of Michelle Wachs. The paper has been highly-cited, partially because it played an important role in popularizing the tool of vertex-decomposability for combinatorial commutative algebraists. Indeed, there is a strong connection to commutative algebra in this work, with the essential idea being that the (non-)face ideal of a simplicial complex Δ expresses Δ as an independence complex of some clutter.

I also have some mathematical software, including my Mac OS X front-end Gap.app for the GAP computer algebra system.