Searcharxiv⌕ Search

arXiv subjects

Catherine Pfaff

Publications and source records attributed to Catherine Pfaff.

At least 19 recordsLinked to original sources
arXiv2026

Singularity of Cannon-Thurston maps

In a closed fibered hyperbolic 3-manifold M ,the inclusion of a fiber S, with S and M lifted to the universal covers, gives an exponentially distorted embedding of the hyperbolic plane into hyperbolic 3-space. Nevertheless, Cannon and Thurston showed that there is a map from the circle at infinity of the hyperbolic plane to the 2-sphere at infinity of hyperbolic 3-space. The Cannon-Thurston map is surjective, finite-to-one, and gives a space-filling curve. Here we use properties of geodesics to prove that many natural measures on the circle when pushed forward by the Cannon-Thurston map become singular with respect to many natural measures on the 2-sphere. The circle measures we consider are the Lebesgue measure and stationary measures that arise from fully supported random walks on the surface group. The measures on the sphere we consider are the Lebesgue measure and stationary measures that arise from geometric random walks on the 3- manifold group. We obtain the singularity of measures from the following properties of typical geodesics. We prove that a hyperbolic geodesic sampled with respect to a pushforward measure asymptotically spends a definite proportion of its time close to a fiber. On the other hand, we show that a hyperbolic geodesic sampled with respect to a natural measure on the sphere spends an asymptotically negligible proportion of its time close to a fiber. For a more restricted class of circle measures, namely the Lebesgue measure and stationary measures from geometric random walks on the surface group, we also prove an effective result for the proportion of time spent close to a fiber.

math.GT↗
arXiv2025

Quasi-geodesics in the Cannon-Thurston metric

A closed fibered 3-manifold admits a complete hyperbolic metric if and only if it has a fibration with a pseudo-Anosov monodromy. The stable and the unstable laminations associated to the pseudo-Anosov homeomorphism on the fiber surface give rise to a natural metric on the 3-manifold, the Cannon-Thurston metric, which is quasi-isometric to the hyperbolic metric. In this paper, we describe a specific family of quasi-geodesics in the Cannon-Thurston metric. We use the main results of this article in a companion paper to obtain statistics for typical geodesics with respect to various natural measures on the 2-sphere, thus giving a geometric criterion for singularity between some of these measure classes.

math.GT↗
arXiv2022

Controlling ball progression in soccer

In this paper, we examine how soccer players can use their spatial relationships to control parts of the field and safely move play up the field via chains of ``safe configurations,'' i.e. configurations of players on a team ensuring the possessor of the ball has a collection of open passing options all connected by open passing lanes. An underlying philosophy behind our work is that it is most difficult to disrupt an attacking team's progression forward (with the ball) when this attacking team has multiple ``good'' options of how to proceed at each moment in time. We provide some evidence of this. Our main construction is a directed weighted graph where the nodes encode the configurations of players, the directed edges encode transformations between these configurations, and the weights encode the relative frequencies of the transformations. We conclude with a few applications and proposed further investigations. We believe that our work can serve as a launching platform for significant further investigation into how teams can ``safely progress'' the ball up the field, strategy development, and sophisticated decision making metrics. For coaches and players, we aim to streamline the process of moving safely up the field. In particular, we aim to construct a framework for creating new successful patterns of movement and aim that our framework allows for the development of new strategy. At the same time, our work could help identify configurations on the field which often result in turnovers or that allow for a lot of strategic flexibility (also impeding defensive containment of the attacking team). *Because the contributions of the female authors to this paper were by no means less than those of the male authors, we have chosen to reverse convention and to list the authors in reverse alphabetical order to ensure that the female authors were not listed only after the male authors.

physics.soc-ph↗
arXiv2020

Taking the High-Edge Route Through Outer Space in Rank 3

Principal outer automorphisms were introduced in Algom-Kfir-Kapovich-Pfaff to emulate principal pseudo-Anosov surface homeomorphisms, i.e. those whose attracting and repelling invariant foliations have only 3-pronged singularities. It is proved in Algom-Kfir-Kapovich-Pfaff that principal fully irreducible outer automorphisms semi-mimic principal pseudo-Anosov surface homeomorphisms in important ways. We prove here which rank-3 graphs carry train track maps for principal fully irreducible outer automorphisms. As a corollary, one obtains which rank-3 Culler-Vogtmann Outer space simplices are passed through by principal axes.

math.GR↗
arXiv2019

Random trees in the boundary of Outer space

We prove that for the harmonic measure associated to a random walk on Out$(F_r)$ satisfying some mild conditions, a typical tree in the boundary of Outer space is trivalent and nongeometric. This answers a question of M. Bestvina.

math.GT↗
arXiv2018

Counting conjugacy classes of fully irreducibles: double exponential growth

Inspired by results of Eskin and Mirzakhani counting closed geodesics of length $\le L$ in the moduli space of a fixed closed surface, we consider a similar question in the $Out(F_r)$ setting. The Eskin-Mirzakhani result can be equivalently stated in terms of counting the number of conjugacy classes (within the mapping class group) of pseudo-Anosovs whose dilitations have natural logarithm $\le L$. Let $\mathfrak N_r(L)$ denote the number of $Out(F_r)$-conjugacy classes of fully irreducibles satisfying that the natural logarithm of their dilatation is $\le L$. We prove for $r\ge 3$ that as $L\to\infty$, the number $\mathfrak N_r(L)$ has double exponential (in $L$) lower and upper bounds. These bounds reveal behavior not present in the surface setting or in classical hyperbolic dynamical systems.

math.GR↗
arXiv2017

Stable Strata of Geodesics in Outer Space

In this paper we propose an Outer space analogue for the principal stratum of the unit tangent bundle to the Teichmüller space $\mathcal{T}(S)$ of a closed hyperbolic surface $S$. More specifically, we focus on properties of the geodesics in Teichmüller space determined by the principal stratum. We show that the analogous Outer space "principal" periodic geodesics share certain stability properties with the principal stratum geodesics of Teichmüller space. We also show that the stratification of periodic geodesics in Outer space exhibits some new pathological phenomena not present in the Teichmüller space context.

math.GR↗
arXiv2016

Normalizers and centralizers of cyclic subgroups generated by lone axis fully irreducible outer automorphisms

We let $φ$ be an ageometric fully irreducible outer automorphism so that its Handel-Mosher axis bundle consists of a single unique axis. We show that the centralizer $Cen(\langleφ\rangle)$ of the cyclic subgroup generated by $φ$ equals the stabilizer $\text{Stab}(Λ^+_φ)$ of the attracting lamination $Λ^+_φ$ and is isomorphic to $\mathbb Z$. We further show, via an analogous result about the commensurator, that the normalizer $N(\langleφ\rangle)$ of $\langle φ\rangle$ is isomorphic to either $\mathbb Z$ or $\mathbb Z_2 * \mathbb Z_2$.

math.GR↗
arXiv2016

A dense geodesic ray in the $Out(F_r)$-quotient of reduced Outer Space

In 1981 Masur proved the existence of a dense geodesic in the moduli space for a Teichmüller space. We prove an analogue theorem for reduced Outer Space endowed with the Lipschitz metric. We also prove two results possibly of independent interest: we show Brun's unordered algorithm weakly converges and from this prove that the set of Perron-Frobenius eigenvectors of positive integer $m \times m$ matrices is dense in the positive cone $\mathbb{R}^m_+$ (these matrices will in fact be the transition matrices of positive automorphisms). We give a proof in the appendix that not every point in the boundary of Outer Space is the limit of a flow line.

math.GR↗
arXiv2015

Ideal Whitehead Graphs in $Out(F_r)$ IV: Building ideal Whitehead graphs in higher ranks and ideal Whitehead graphs with cut vertices

We provide an example in each rank of an ageometric fully irreducible outer automorphism whose ideal Whitehead graph has a cut vertex. Consequently, we show that there exist examples in each rank of Handel-Mosher axis bundles that are not just a single axis, as well as of "nongeneric" behavior in the sense of the "train track directed" random walk of Kapovich-Pfaff. The tools developed here also allow one to construct a whole new array of ideal Whitehead graphs achieved by ageometric fully irreducible outer automorphisms in all ranks.

math.GR↗
arXiv2015

Lone Axes in Outer Space

Handel and Mosher define the axis bundle for a fully irreducible outer automorphism in "Axes in Outer Space." In this paper we give a necessary and sufficient condition for the axis bundle to consist of a unique periodic fold line. As a consequence, we give a setting, and means for identifying in this setting, when two elements of an outer automorphism group $Out(F_r)$ have conjugate powers.

math.GR↗
arXiv2015

A train track directed random walk on $Out(F_r)$

Several known results, by Rivin, Calegari-Maher and Sisto, show that an element $ϕ_n\in Out(F_r)$, obtained after $n$ steps of a simple random walk on $Out(F_r)$, is fully irreducible with probability tending to 1 as $n\to\infty$. In this paper we construct a natural "train-track directed" random walk $\mathcal W$ on $Out(F_r)$ (where $r\ge 3$). We show that, for the element $ϕ_n\in Out(F_r)$, obtained after $n$ steps of this random walk, with asymptotically positive probability the element $ϕ_n$ has the following properties: $ϕ_n$ is an ageometric fully irreducible, which admits a train-track representative with no periodic Nielsen paths and exactly one nondegenerate illegal turn, that $ϕ_n$ has "rotationless index" $\frac{3}{2}-r$ (so that the geometric index of the attracting tree $T_{ϕ_n}$ of $ϕ_n$ is $2r-3$), has index list $\{\frac{3}{2}-r\}$ and the ideal Whitehead graph being the complete graph on $2r-1$ vertices, and that the axis bundle of $ϕ_n$ in the Outer space $CV_r$ consists of a single axis.

math.GR↗
arXiv2014

Ideal Whitehead Graphs in Out(F_r) II: The Complete Graph in Each Rank

We show how to construct, for each $r \geq 3$, an ageometric, fully irreducible $ϕ\in Out(F_r)$ whose ideal Whitehead graph is the complete graph on $2r-1$ vertices. This paper is the second in a series of three where we show that precisely eighteen of the twenty-one connected, simplicial, five-vertex graphs are ideal Whitehead graphs of fully irreducible $ϕ\in Out(F_3)$. The result is a first step to an $Out(F_r)$ version of the Masur-Smillie theorem proving precisely which index lists arise from singular measured foliations for pseudo-Anosov mapping classes. In this paper we additionally give a method for finding periodic Nielsen paths and prove a criterion for identifying representatives of ageometric, fully irreducible $ϕ\in Out(F_r)$

math.GR↗
arXiv2014

Ideal Whitehead Graphs in Out(F_r) III: Achieved Graphs in Rank 3

By proving precisely which singularity index lists arise from the pair of invariant foliations for a pseudo-Anosov surface homeomorphism, Masur and Smillie determined a Teichmüller flow invariant stratification of the space of quadratic differentials. In this final paper of a three-paper series, we give a first step to an $Out(F_r)$ analog of the Masur-Smillie theorem. Since the ideal Whitehead graphs defined by Handel and Mosher give a strictly finer invariant in the analogous $Out(F_r)$ setting, we determine which of the twenty-one connected, simplicial, five-vertex graphs are ideal Whitehead graphs of fully irreducible outer automorphisms in $Out(F_3)$.

math.GR↗
arXiv2014

$Out(F_3)$ Index Realization

By proving precisely which singularity index lists arise from the pair of invariant foliations for a pseudo-Anosov surface homeomorphism, Masur and Smillie determined a Teichmueller flow invariant stratification of the space of quadratic differentials. In this paper we determine an analog to the theorem for $Out(F_3)$. That is, we determine which index lists permitted by the Gaboriau-Jaeger-Levitt-Lustig index sum inequality are achieved by fully irreducible outer automorphisms of the rank-$3$ free group.

math.GR↗
arXiv2012

Ideal Whitehead Graphs in Out(F_r) I: Some Unachieved Graphs

H. Masur and J. Smillie proved precisely which singularity index lists arise from pseudo-Anosov mapping classes. In search of an analogous theorem for outer automorphisms of free groups, Handel and Mosher ask: Is each connected, simplicial, (2r-1)-vertex graph the ideal Whitehead graph of a fully irreducible outer automorphism in Out(F_r)? We answer this question in the negative by exhibiting, for each r, examples of connected (2r-1)-vertex graphs that are not the ideal Whitehead graph of any fully irreducible outer automorphism in Out(F_r)? In the course of our proof we also develop machinery used in "Constructing and Classifying Fully Irreducible Outer Automorphisms of Free Groups" to fully answer the question in the rank-three case.

math.GR↗