My research is in topology and geometry—specifically, surgery theory and geometric group theory. A few of my favorite things include: aspherical manifolds, rational homotopy types of manifolds, group actions on manifolds, quantified versions of classical invariants.

You may also be interested in seeing a list of my publications or reading my research statement.

## Works in progress

#### L-class computations

Computation of the total $L$-class would also solve some manifold recognition problems, such as recognizing whether a particular combinatorial 15-vertex triangulations of an 8-manifold is the quaternionic projective plane $\mathbb{H}P^8$. One of these examples $X^8$ is especially symmetric, and likely PL homeomorphic to $\mathbb{H}P^8$.

#### The no-three-in-line problem on a torusJoint work with Andrew Groot, Deven Pandya, Bart Snapp

For a group $G$, let $T(G)$ denote the cardinality of the largest subset $S \subset G$ so that no three elements of $S$ are in the same coset of a cyclic subgroup. Undergraduates Andrew Groot and Deven Pandya, advised by myself and my colleague Bart Snapp, considered the case $G = \mathbb{Z}/m\mathbb{Z} \times \mathbb{Z}/n\mathbb{Z}$, and showed that $T(\Z_p \times \Z_{p^2}) = 2p$ and $T(\Z_p \times \Z_{pq}) = p+1$.

#### Projective planesJoint work with Zhixu Su

The Hirzebruch $L$-polynomial is one place where number theory very strongly interacts with high-dimensional topology \cite{MR339202}. Recall that the Hirzebruch signature theorem relates the signature of a smooth closed manifold $M^{4k}$ to $\sum_I L_I p_I(M)$. Unfortunately, the na\"ive method to compute coefficients $L_I$ of the Hirzebruch $L$-polynomial is much too slow for applications; Zhixu Su and I have discovered a recursive method which is fast enough to compute many coefficients. Solutions to some Diophantine equations related to these $L$-polynomials give rise to manifolds having a truncated polynomial algebra as their rational cohomology ring; such manifolds may exist even when the corresponding truncated polynomial algebra over $\Z$ is not the cohomology ring of any space. For instance, there is a manifold having the rational cohomology that $\mathbb{O}P^4$ would be expected to have, if $\mathbb{O}P^4$ existed.

## Previous projects

#### Bounded homotopy theoryJoint work with Crichton Ogle

Given a bounding class ${\mathcal B}$, we construct a bounded refinement ${\mathcal{B}K}(-)$ of Quillen's $K$-theory functor from rings to spaces. As defined, ${\mathcal{B}K}(-)$ is a functor from weighted rings to spaces, and is equipped with a comparison map $BK \to K$ induced by “forgetting control.” In contrast to the situation with $\mathcal{B}$-bounded cohomology, there is a functorial splitting ${\mathcal{B}K}(-) \simeq K(-) \times {\mathcal{B}K}^{rel}(-)$ where ${\mathcal{B}K}^{rel}(-)$ is the homotopy fiber of the comparison map.

#### Cleanliness of geodesics in hyperbolic 3-manifoldsJoint work with Colin Adams, Adam Colestock, William Gillam, Eric Katerman

Imagine you are inside a cusped hyperbolic 3-manifold, and you have situated yourself at infinity, and now you are looking down the cusp. You see a pattern of horoballs within the fundamental parallelogram for the cusp subgroup fixing $\infty$. Imagining that horoballs are opaque, how many are visible? Do finitely many horoballs suffice to cover the fundamental parallelogram? This question was first asked by Darren Long, and this paper answers it.

Specifically, we derive conditions guaranteeing the existence of geodesics avoiding the cusps and use these geodesics to show that in “almost all” finite volume hyperbolic 3-manifolds, infinitely many horoballs in the universal cover corresponding to a cusp are visible in a fundamental domain of the cusp when viewed from infinity.

#### Cusp size bounds from singular surfaces in hyperbolic 3-manifoldsJoint work with Colin Adams, Adam Colestock, William Gillam, Eric Katerman

Singular maps of surfaces into a hyperbolic 3-manifold are utilized to find upper bounds on meridian length, $\ell$-curve length and maximal cusp volume for the manifold. This allows a proof of the fact that there exist hyperbolic knots with arbitrarily small cusp density and that every closed orientable 3-manifold contains a knot whose complement is hyperbolic with maximal cusp volume less than or equal to 9.

We also find particular upper bounds on meridian length, $\ell$-curve length and maximal cusp volume for hyperbolic knots in $S^3$ depending on crossing number. Particular improved bounds are obtained for alternating knots.

#### Finiteness properties

A combination of Bestvina–Brady Morse theory and an acyclic reflection group trick produces a torsion-free finitely presented $\mathbb{Q}$-Poincaré duality group which is not the fundamental group of an aspherical closed ANR $\mathbb{Q}$-homology manifold.

The acyclic construction suggests asking which $\mathbb{Q}$-Poincaré duality groups act freely on $\mathbb{Q}$-acyclic spaces (i.e., which groups are $\mbox{FH}(\mathbb{Q})$). For example, the orbifold fundamental group $\Gamma$ of a good orbifold satisfies $\mathbb{Q}$-Poincaré duality, and we show $\Gamma$ is $\mbox{FH}(\mathbb{Q})$ if the Euler characteristics of certain fixed sets vanish.

#### Aspherical manifolds that cannot be triangulatedJoint work with Michael W. Davis, Jean-François Lafont

Kirby and Siebenmann showed that there are manifolds that do not admit PL structures, and yet the possibility remained that all manifolds could be triangulated, meaning that for every manifold $M$, there is a simplicial complex $K$ so that the geometric realization of $K$ is homeomorphic to $M$, but of course the simplicial complex $K$ is not a PL triangulation, meaning the links are not spheres. Freedman showed that there are 4-manifolds that cannot be triangulated. Davis and Januszkiewicz applied a hyperbolization procedure to Freedman's 4-manifolds to get closed aspherical 4-manifolds that cannot be triangulated. What about higher dimensions?'

We say that a group $G$ is $\mathbb{Q}$-$\mathrm{PD}$ if it satisfies Poincare duality with rational coefficients (i.e., if its classifying space $BG$ does). Examples include the fundamental groups of aspherical manifolds. But there are other geometric examples: if a group $G$ acts freely on a rationally-acyclic, rational homology manifold, then $G$ is $\mathbb{Q}$-$\mathrm{PD}$. Does every $\mathbb{Q}$-$\mathrm{PD}$ arise in this way—does every $\mathbb{Q}$-$\mathrm{PD}$ group act on such an object? The answer is no: lattices with torsion in semisimple Lie groups are counterexamples.