arXiv subjects

Kasra Rafi

Publications and source records attributed to Kasra Rafi.

At least 19 recordsLinked to original sources

arXiv2026

Large genus asymptotics for frequency of non-simple curves

We give an expression for the frequency of non-simple curves in closed surfaces and exploit it to study relative frequencies of such curves in large genus. This extend to the case of non-simple curves Mirzakhani's expressions of frequencies in terms of Konsevitch polynomials and Delecroix-Goujard-Zograf-Zorich large genus asymptotics for those frequencies. In particular, with K fixed, we identify which types of curves with K intersections are most common.

math.GT↗

arXiv2026

The non-peripheral curve graph and divergence in big mapping class groups

We introduce a numerical invariant $\zeta(\Sigma)$ measuring the end-complexity of $\Sigma$ and use it to organize coarse-geometric features of Map($\Sigma$). Our main tool is the \emph{non-peripheral curve graph} $C_{\rm np}(\Sigma)$, whose vertices are those essential simple closed curves that cannot be pushed out of every compact subsurface, with edges given by disjointness. Assuming Map($\Sigma$) is CB-generated and $\zeta(\Sigma)\ge 5$, we prove that $C_{\rm np}(\Sigma)$ is connected, has infinite diameter, is Gromov hyperbolic, and that the Map($\Sigma$)-action has unbounded orbits. As applications, we show that if $\zeta(\Sigma)\ge 4$ then Map($\Sigma$) has infinite coarse rank, and if $\zeta(\Sigma)\ge 5$ then Map($\Sigma$) has at most quadratic divergence, hence is one-ended.

math.GT↗

arXiv2025

Benjamini-Schramm limits of high genus translation surfaces: research announcement

We prove that the sequence of Masur-Smillie-Veech (MSV) distributed random translation surfaces, with area equal to genus, Benjamini-Schramm converges as genus tends to infinity. This means that for any fixed radius $r>0$, if $X_g$ is an MSV-distributed random translation surface with area $g$ and genus $g$, and $o$ is a uniformly random point in $X_g$, then the radius-$r$ neighborhood of $o$ in $X_g$, as a pointed measured metric space, converges in distribution to the radius $r$ neighborhood of the root in a Poisson translation plane, which is a random pointed surface we introduce here. Along the way, we obtain bounds on statistical local geometric properties of translation surfaces, such as the probability that the random point $o$ has injectivity radius at most $r$, which may be of independent interest.

math.GT↗

arXiv2024

Lengths of saddle connections on random translation surfaces of large genus

We determine the distribution of the number of saddle connections on a random translation surface of large genus. More specifically, for genus $g$ tending to infinity, the number of saddle connections with lengths in a given interval $[\frac{a}{g}, \frac{b}{g}]$ converges in distribution to a Poisson distributed random variable. Furthermore, the numbers of saddle connections associated to disjoint intervals of lengths are independent.

math.GT↗

arXiv2024

Classification of Stable Surfaces with respect to Automatic Continuity

We provide a complete classification of when the homeomorphism group of a stable surface, $\Sigma$, has the automatic continuity property: Any homomorphism from Homeo$(\Sigma)$ to a separable group is necessarily continuous. This result descends to a classification of when the mapping class group of $\Sigma$ has the automatic continuity property. Towards this classification, we provide a general framework for proving automatic continuity for groups of homeomorphisms. Applying this framework, we also show that the homeomorphism group of any stable second countable Stone space has the automatic continuity property. Under the presence of stability this answers two questions of Mann.

math.GT↗

arXiv2024

Thurston geodesics: no backtracking and active intervals

We develop the notion of the active interval for a subsurface along a geodesic in the Thurston metric on Teichmuller space of a surface S. That is, for any geodesic in the Thurston metric and any subsurface R of S, we find an interval of times where the length of the boundary of R is uniformly bounded and the restriction of the geodesic to the subsurface R resembles a geodesic in the Teichmuller space of R. In particular, the set of short curves in R during the active interval represents a reparametrized quasi-geodesic in the curve graph of R (no backtracking) and the amount of movement in the curve graph of R outside of the active interval is uniformly bounded which justifies the name active interval. These intervals provide an analogue of the active intervals introduced by the third author in the setting of Teichmuller space equipped with the Teichmuller metric.

math.GT↗

arXiv2024

The quasi-redirecting Boundary

We generalize the notion of Gromov boundary to a larger class of metric spaces beyond Gromov hyperbolic spaces. Points in this boundary are classes of quasi-geodesic rays and the space is equipped with a topology that is naturally invariant under quasi-isometries. It turns out that this boundary is compatible with other notions of boundary in many ways; it contains the sublinearly Morse boundary as a topological subspace and it matches the Bowditch boundary of relative hyperbolic spaces when the peripheral subgroups have no intrinsic hyperbolicity. We also give a complete description of the boundary of the Croke-Kleiner group where the quasi-redirecting boundary reveals a new class of QI-invariant, Morse-like quasi-geodesics.

math.GR↗

arXiv2023

Random walks on groups and superlinear divergent geodesics

In this paper, we study random walks on groups that contain superlinear divergent geodesics, in the line of thoughts of Goldsborough-Sisto. The existence of a superlinear divergent geodesic is a quasi-isometry invariant which allows us to execute Gouëzel's pivoting technique. We develop the theory of superlinear divergence and establish a central limit theorem for random walks on these groups.

math.GT↗

arXiv2022

Fibered 3-manifolds and Veech groups

We study Veech groups associated to the pseudo-Anosov monodromies of fibers and foliations of a fixed hyperbolic 3-manifold. Assuming Lehmer's Conjecture, we prove that the Veech groups associated to fibers generically contain no parabolic elements. For foliations, we prove that the Veech groups are always elementary.

math.GT↗

arXiv2022

Genericity of sublinearly Morse directions in CAT(0) spaces and the Teichmüller space

We show that the sublinearly Morse directions in the visual boundary of a rank-1 CAT(0) space with a geometric group action are generic in several commonly studied senses of the word, namely with respect to Patterson-Sullivan measures and stationary measures for random walks. We deduce that the sublinearly Morse boundary is a model of the Poisson boundary for finitely supported random walks on groups acting geometrically on rank-1 CAT (0) spaces. We prove an analogous result for mapping class group actions on Teichmüller space. Our main technical tool is a criterion, valid in any unique geodesic metric space, that says that any geodesic ray with sufficiently many (in a statistical sense) strongly contracting segments is sublinearly contracting.

math.GR↗

arXiv2022

Sublinearly Morse Boundary I: CAT(0) Spaces

To every Gromov hyperbolic space X one can associate a space at infinity called the Gromov boundary of X. Gromov showed that quasi-isometries of hyperbolic metric spaces induce homeomorphisms on their boundaries, thus giving rise to a well-defined notion of the boundary of a hyperbolic group. Croke and Kleiner showed that the visual boundary of non-positively curved (CAT(0)) groups is not well-defined, since quasi-isometric CAT(0) spaces can have non-homeomorphic boundaries. For any sublinear function $κ$, we consider a subset of the visual boundary called the $κ$-Morse boundary and show that it is QI-invariant and metrizable. This is to say, the $κ$-Morse boundary of a CAT(0) group is well-defined. In the case of Right-angled Artin groups, it is shown in the Appendix that the Poisson boundary of random walks is naturally identified with the $\sqrt{t \log t}$--boundary.

math.GT↗

arXiv2022

Two results on end spaces of infinite type surfaces

We answer two questions about the topology of end spaces of infinite type surfaces and the action of the mapping class group that have appeared in the literature. First, we give examples of infinite type surfaces with end spaces that are not self-similar, but a unique maximal type of end, either a singleton or Cantor set. Secondly, we use an argument of Tsankov to show that the "local complexity" relation $\preccurlyeq$ on end types gives an equivalence relation that agrees with the notion of being locally homeomorphic.

math.GT↗

arXiv2021

Asymptotic Dimension of Big Mapping Class Groups

Even though big mapping class groups are not countably generated, certain big mapping class groups can be generated by a coarsely bounded set and have a well defined quasi-isometry type. We show that the big mapping class group of a stable surface of infinite type with a coarsely bounded generating set that contains an essential shift has infinite asymptotic dimension. This is in contrast with the mapping class groups of surfaces of finite type where the asymptotic dimension is always finite. We also give a topological characterization of essential shifts.

math.GT↗

arXiv2021

Benjamini-Schramm convergence of periodic orbits

We prove a criterion for Benjamini-Schramm convergence of periodic orbits of Lie groups. This general observation is then applied to homogeneous spaces and the space of translation surfaces.

math.DS↗

arXiv2020

Quasi-geodesics in Out(F_n) and their shadows in sub-factors

We study the behaviour of quasi-geodesics in Out(F_n). Given an element f in Out(F_n) there are several natural paths connecting the origin to f in Out(F_n); for example, paths associated to sequences of Stallings folds and paths induced by the shadow of greedy folding paths in Outer Space. We show that none of these paths is, in general, a quasi-geodesic in Out(F_n). In fact, in contrast with the mapping class group setting, we construct examples where any quasi-geodesic in Out(F_n) connecting f to the origin will have to back-track in some free factor of F_n.

math.GR↗

arXiv2020

Sublinearly Morse Boundary II: Proper geodesic spaces

We build an analogue of the Gromov boundary for any proper geodesic metric space, hence for any finitely generated group. More precisely, for any proper geodesic metric space $X$ and any sublinear function $\kappa$, we construct a boundary for $X$, denoted $\mathcal{\partial}_{\kappa} X$, that is quasi-isometrically invariant and metrizable. As an application, we show that when $G$ is the mapping class group of a finite type surface, or a relatively hyperbolic group, then with minimal assumptions the Poisson boundary of $G$ can be realized on the $\kappa$-Morse boundary of $G$ equipped the word metric associated to any finite generating set.

math.GT↗

arXiv2020

Big mapping class groups with hyperbolic actions: classification and applications

We address the question of determining which mapping class groups of infinite-type surfaces admit nonelementary continuous actions on hyperbolic spaces. More precisely, let $Σ$ be a connected, orientable surface of infinite type with tame endspace whose mapping class group is generated by a coarsely bounded subset. We prove that $\mathrm{Map}(Σ)$ admits a continuous nonelementary action on a hyperbolic space if and only if $Σ$ contains a finite-type subsurface which intersects all its homeomorphic translates. When $Σ$ contains such a nondisplaceable subsurface $K$ of finite type, the hyperbolic space we build is constructed from the curve graphs of $K$ and its homeomorphic translates via a construction of Bestvina, Bromberg and Fujiwara. Our construction has several applications: first, the second bounded cohomology of $\mathrm{Map}(Σ)$ contains an embedded $\ell^1$; second, using work of Dahmani, Guirardel and Osin, we deduce that $\mathrm{Map}(Σ)$ contains nontrivial normal free subgroups (while it does not if $Σ$ has no nondisplaceable subsurface of finite type), has uncountably many quotients and is SQ-universal.

math.GT↗

arXiv2020

Expected covering radius of a translation surface

A translation structure equips a Riemann surface with a singular flat metric. Not much is known about the shape of a random translation surface. We compute an upper bound on the expected value of the covering radius of a translation surface in any stratum H_1(kappa). The covering radius of a translation surface is the largest radius of an immersed disk. In the case of the stratum H_1(2g-2) of translation surfaces of genus g with one singularity, the covering radius is comparable to the diameter. We show that the expected covering radius of a surface is bounded above by a uniform multiple of ((log g)/g)^(1/2), independent of the stratum. This is smaller than what one would expect by analogy from the result of Mirzakhani about the expected diameter of a hyperbolic metric on a Riemann surface. To prove our result, we need an estimate for the volume of the thin part of H_1(kappa) which is given in the appendix.

math.GT↗