Christopher Cashen, PhD
Fakultät für Mathematik
Universität Wien
OskarMorgensternPlatz 1
1090 Wien, Österreich
Email: christopher DOT cashen AT univie.ac.at
Office: 2.130
Fakultät für Mathematik
Universität Wien
OskarMorgensternPlatz 1
1090 Wien, Österreich
Email: christopher DOT cashen AT univie.ac.at
Office: 2.130
I am a postdoc in the Geometric and Analytic Group Theory group at the University of Vienna. I help organize our Seminar.
Papers
Page numbers link to the final published version on the publisher's webpage. 'Unlocked' icons link to an open access version, as permitted. You can also find my papers on the arXiv. Quasiisometries Between Tubular Groups
 Groups, Geometry and Dynamics, 4 (2010), no. 3, 473–516.
(Abstract)
We give a quasiisometry classification of graphs of virtual \(\mathbb{Z}^2\)'s amalgamated over virtually cyclic subgroups.  Line Patterns in Free Groups, w/ Nataša Macura
 Geometry & Topology,
15 (2011), no. 3,
1419–1475.
(Abstract)
Given an algebraic lamination \(\mathcal{L}\) of the free group \(\mathbb{F}\), exactly one of the following is true: \(\mathbb{F}\) splits freely or cyclically relative to \(\mathcal{L}\).
 \(\mathbb{F}\) is quasiisometrically rigid relative to \(\mathcal{L}\).
 \(\mathbb{F}\) is the fundamental group of a thricepunctured sphere and leaves of \(\mathcal{L}\) are the peripheral subgroups.
 Virtual Geometricity is Rare, w/ Jason F. Manning
 LMS Journal of Computation and Mathematics, 18
(2015), no. 1,
444–455.
(Abstract)
An element in the free group \(\mathbb{F}\) is geometric if it can be represented by an embedded curve in the boundary of a handlebody with fundamental group \(\mathbb{F}\). It is virtually geometric if it becomes geometric upon lifting to some finite index subgroup of \(\mathbb{F}\). The probability that a random element is virtually geometric decays to zero exponentially quickly with respect to the word length of the element with respect to a fixed basis. We also give experimental estimates for the rate of decay; these are obtained by our computer program that checks virtual geometricity for a given input word.  Growth Tight Actions, w/ Goulnara N. Arzhantseva and Jing Tao
 Pacific Journal of Mathematics, 278 (2015), no. 1,
1–49.
(Abstract)
A proper action of a group \(G\) on a metric space \(X\) is growth tight if for every infinite index normal subgroup \(N\) of \(G\) the exponential growth rate of \(G\) with respect to \(X\), ie, of an orbit of \(G\) in \(X\), is strictly greater than the corresponding growth rate of \(G/N\). We show that an action is growth tight if it has a strongly contracting element and if there is an orbit that is sufficiently convex. The latter condition is satisfied if there is a quasiconvex orbit, but also in other interesting cases such as the action of the mapping class group on Teichmüller space.  Mapping Tori of Free Group Automorphisms, and the BieriNeumannStrebel Invariant of Graphs of Groups, w/ Gilbert Levitt
 Journal of Group Theory, 19 (2016), no. 2, 191–216.
(Abstract)
We compute BieriNeumannStrebel (BNS) invariants for certain graphs of groups and hierarchies of groups. In particular, we show that the BNS invariant of the mapping torus of a polynomially growing free group automorphism is the complement of finitely many rationally defined subspheres.  Splitting Line Patterns in Free Groups
 Algebraic & Geometric Topology, 16
(2016), no. 2, 621–673.
(Abstract)
Techniques of Paper 2 are used to derive a canonical JSJ decomposition of a free group \(\mathbb{F}\) relative to an algebraic lamination \(\mathcal{L}\) in terms of the topology of the quotient \(\partial\mathbb{F}/\partial\mathcal{L}\). As an application, we characterize virtual geometricity (cf Paper 3) as having a relative JSJ decomposition with geometric pieces.  Growth Tight Actions of Product Groups, w/ Jing Tao
 Groups, Geometry and
Dynamics, 10 (2016), no. 2, 753–770.
(Abstract)
If \(G=\prod_{i=1}^nG_i\) is a product of finitely generated groups and for each \(i\) we have a proper, cocompact action of \(G_i\) on a metric space \(X_i\) with a strongly contracting element then the product action of \(G\) on \(X=\prod_{i=1}^nX_i\) is growth tight if \(X\) is given the \(L^p\) metric for \(p>1\) and not growth tight if \(X\) is given the \(L^1\) metric. In particular, if the \(X_i\) are Cayley graphs of the \(G_i\) then the \(L^1\) metric and the \(L^\infty\) metric both correspond to word metrics on \(G\). This provides the first construction of a group such that the action on one of its Cayley graphs is growth tight and the action on another of its Cayley graphs is not.  Quasiisometries Need Not Induce Homeomorphisms of Contracting Boundaries with the Gromov Product Topology
 Analysis and
Geometry in Metric Spaces, 4 (2016), no. 1, 278–281.
(Abstract)
We consider a 'contracting boundary' of a proper geodesic metric space consisting of equivalence classes of geodesic rays that behave like geodesics in a hyperbolic space. We topologize this set via the Gromov product, in analogy to the topology of the boundary of a hyperbolic space. We show that when the space is not hyperbolic, quasiisometries do not necessarily give homeomorphisms of this boundary. Continuity can fail even when the spaces are required to be CAT(0). We show this by constructing an explicit example.  Quasiisometries Between Groups with TwoEnded Splittings, w/ Alexandre Martin
 Mathematical Proceedings of the Cambridge Philosophical Society, in
press, 43pp.
(Abstract)
We construct invariants for boundary homeomorphism and quasiisometry of hyperbolic groups that split over twoended subgroups in terms of the respective homeomorphism/quasiisometry types of the vertex groups relative to the edge groups. For boundary homeomorphism we get a complete invariant. For quasiisometry we get a complete invariant when the vertex groups are rigid relative to their incident edge groups.  A Geometric Proof of the Structure Theorem for Cyclic Splittings of Free Groups
 Topology
Proceedings, in press, 14pp. preprint
(Abstract)
Application of the techniques of Papers 2 and 6 giving an alternate proof that an amalgam of two free groups over a cyclic subgroup is free if and only if the edge group is a free factor of one of the vertex groups. Applies also to virtually free groups amalgamated over a virtually cyclic subgroup.  Characterizations of Morse Quasigeodesics via Superlinear Divergence and Sublinear Contraction, w/ Goulnara N. Arzhantseva, Dominik Gruber, and David Hume
 submitted, 29pp. preprint
(Abstract)
We initiate a systematic study of contracting projections. We show that sublinear contraction is equivalent to the wellknown Morse property and to the property of having completely superlinear divergence. We prove sublinear analogues of several theorem about strongly contracting geodesics.  Contracting Geodesics in Graphical Small Cancellation Groups, w/ Goulnara N. Arzhantseva, Dominik Gruber, and David Hume
 submitted, 41pp. preprint
(Abstract)
We give necessary and sufficient conditions for a geodesic in a graphical small cancellation group to be contracting. Some consequences are: Many \(Gr'(1/6)\) graphical small cancellation group contains a strongly contracting element.
 A geodesic \(\alpha\) in a classical \(C'(1/6)\) small cancellation group is Morse if and only if there is a sublinearly growing function \(f\) such that for every relator \(\Pi\) we have \(\alpha\cap\Pi\leq f(\Pi)\).
Notes on the arXiv
 Computing the Maximum Slope Invariant in Tubular Groups (2009).

(Abstract)
Addendum to Paper 1. Gives an example of two tubular groups that are distinguished by the ‘maximum slope invariant’ introduced there but not by Dehn function or other obvious quasiisometry invariants.