If you are interested in our seminar you might also be interested in
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July 16, 2020: Stationary measure and orbit closure classification for random walks on surfaces
[abstract]
We study the problem of classifying
stationary measures and orbit closures for nonabelian action on
surfaces. Using a result of Brown and Rodriguez Hertz, we show that
under a certain average growth condition, the orbit closures are
either finite or dense. Moreover, every infinite orbit equidistributes
on the surface. This is analogous to the results of BenoistQuint and
EskinLindenstrauss in the homogeneous setting, and the result of
EskinMirzakhani in the setting of moduli spaces of translation
surfaces.
We then consider the problem of verifying this growth condition in
concrete settings. In particular, we apply the theorem to two
settings, namely discrete perturbations of the standard map and the
\Out(F_2)action on a certain character variety. We verify the growth
condition analytically in the former setting, and verify numerically
in the latter setting.

July 2, 2020: Quantitative weak mixing for random substitution tilings
[abstract]
"Quantitative weak mixing" is the term
used to bound the dimensions of spectral measures of a
measurepreserving system. This type of study has gained popularity
over the last decade, led by a series of results of Bufetov and
Solomyak for a large class of flows which include general
onedimensional tiling spaces as well as translation flows on flat
surfaces, as well as results on quantitative weak mixing by Forni.
In this talk I will present results which extend the results for
flows to higher rank parabolic actions, focusing on quantitative
results for a broad class of tilings in any dimension. The talk
won't assume familiarity with almost anything, so I will define all
objects in consideration.
(talk postponed from June 25)

June 18, 2020: Counting social interactions for discrete subsets of the plane
[abstract]
Given a discrete subset V in the plane,
how many points would you expect there to be in a ball of radius 100?
What if the radius is 10,000? Due to the results of Fairchild and
forthcoming work with Burrin, when V arises as orbits of nonuniform
lattice subgroups of SL(2,R), we can understand asymptotic growth rate
with error terms of the number of points in V for a broad family of
sets. A crucial aspect of these arguments and similar arguments is
understanding how to count pairs of saddle connections with certain
properties determining the interactions between them, like having a
fixed determinant or having another point in V nearby. We will spend
the first 40 minutes discussing how these sets arise and counting
results arise from the study of concrete translation surfaces.
The following 40 minutes will be spent highlighting the proof strategy
used to obtain these results, and advertising the generality and
strength of this argument that arises from the computation of all
higher moments of the SiegelVeech transform over quotients of
SL(2,R) by nonuniform lattices.

June 11, 2020: There exists a weakly mixing billiard in a polygon
[abstract]
This main result of this talk is that
there exists a billiard flow in a polygon that is weakly mixing with
respect to Lebesgue measure on the unit tangent bundle to the
billiard. This strengthens Kerckhoff, Masur and Smillie's result that
there exists ergodic billiard flows in polygons. The existence of a
weakly mixing billiard follows, via a Baire category argument, from
showing that for any translation surface the product of the flows in
almost every pair of directions is ergodic with respect to Lebesgue
measure. This in turn is proven by showing that for every translation
surface the flows in almost every pair of directions do not share
nontrivial common eigenvalues. This talk will explain the problem,
related results, and approach. The talk will not assume familiarity
with translation surfaces.
This is joint work with Giovanni Forni.

June 4, 2020, 13:00 PDT: Arithmetic and geometric properties of planar selfsimilar sets
[abstract]
Furstenberg's conjecture on the dimension
of the intersection of x2,x3invariant Cantor sets can be restated as
a bound on the dimension of linear slices of the product of
x2,x3Cantor sets, which is a selfaffine set in the plane. I will
discuss some older and newer variants of this, where the selfaffine
set is replaced by a selfsimilar set such as the Sierpinski triangle,
Sierpinski carpet or (support of) a complex Bernoulli convolution.
Among other things, I will show that the intersection of the
Sierpinski carpet with circles has small dimension, but on the other
hand the Sierpinski carpet can be covered very efficiently by linear
tubes (neighborhoods of lines). The latter fact is a recent result
joint with A. Pyörälä, V. Suomala and M. Wu.

May 28, 2020: AlmostPrime Times in Horospherical Flows
[abstract]
There is a rich connection between
homogeneous dynamics and number theory. Often in such applications
it is desirable for dynamical results to be effective (i.e. the
rate of convergence for dynamical phenomena are known). In the first
part of this talk, I will provide the necessary background and
relevant history to state an effective equidistribution result for
horospherical flows on the space of unimodular lattices in R^n.
I will then describe an application to studying the distribution of
almostprime times (integer times having fewer than a fixed number of
prime factors) in horospherical orbits and discuss connections of this
work to Sarnak’s Mobius disjointness conjecture. In the second part
of the talk I will describe some of the ingredients and key steps that
go into proving these results.

May 21, 2020: A BratteliVershik model for Z^2 actions, or how cohomology can help us make dynamical systems
[abstract]
The BratteliVershik model is a method of
producing minimal actions of the integers on a Cantor set. It was
given by myself, Rich Herman and Chris Skau, building on seminal ideas
of Anatoly Vershik, over 30 years ago. Rather disappointingly and
surprisingly, there isn't a good version for Z^2 actions. I'll report
on a new outlook on the problem and recent progress with Thierry
Giordano (Ottawa) and Christian Skau (Trondheim). The new outlook
focuses on the model as an answer to the question: which cohomological
invariants can arise from such actions? I will not assume any
familiarity with either the original model or the cohomology. The
first half of the talk will be a gentle introduction to the Zcase
and the second half will deal with how to adapt the question to get
an answer for Z^2.

May 14, 2020: Gaps of saddle connection directions for some branched covers of tori
[abstract]
Translation surfaces given by gluing
two identical tori along a slit have genus two and two conetype
singularities of angle $4\pi$. There is a distinguished set of
trajectories called saddle connections that are the straight lines
trajectories between cone points. We can associate a
holonomy vector in the plane to each saddle
connection whose components are the horizontal and vertical
displacement of the saddle connection. How random is the planar set of
holonomy of saddle connections? We study this question by computing
the gap distribution for slopes of saddle
connections for these and other related classes of translation surfaces.

May 7, 2020: Factors of Gibbs measures on subshifts
[abstract]
Classical results of Dobrushin and
LanfordRuelle establish, in rough terms, that for a local energy
function on a subshift without too much longrange order, the
translationinvariant Gibbs measures are precisely the equilibrium
measures. There are multiple definitions of a Gibbs measure in
the literature, which do not always coincide. We will discuss two
of these definitions, one introduced by Capocaccia and the other used
by DobrushinLanfordRuelle, and outline a proof (available at
arxiv.org/abs/2003.05532)
that they are equivalent.
We will also discuss forthcoming work, in which we show that
Gibbsianness is preserved by pushforward through a certain kind of
almost invertible factor map. As an application in one dimension,
we show that for a sufficiently regular potential, any equilibrium
measure on an irreducible sofic shift is Gibbs. As far as we know,
this is the first reasonably general result of the LanfordRuelle
type for a class of subshifts without the topological Markov property.
Joint work with Luísa Borsato, with extensive advice from Brian Marcus
and Tom Meyerovitch.

April 30, 2020: Quantum Unique Ergodicity
[abstract]
In the first half I'll give a
colloquiumstyle introduction to the
equidistribution problem for Laplace eigenfucntions on Riemannian
manifolds, with emphasis on the locally symmetric spaces. I will
introduce positive results for exact eigenfunctions (with and
without reference to the numbertheoretic symmetries of the manifold),
and negative results for approximate eigenfunctions. I will present
results (independenlty) joint with A. Venkatesh, N. Anantharaman,
and S. Eswarathasan. In the second half I'll answer questions and
provide details as requested by the audience.

April 23, 2020: Effective equidistribution of horospherical flows in infinite volume
[abstract]
The horospherical flow on finitevolume hyperbolic
surfaces is wellunderstood. In particular, effective equidistribution
of nonclosed horospherical orbits is known. New difficulties arise
when studying the infinitevolume setting. We will discuss the setting
in finite and infinitevolume manifolds, and the measures that play
a crucial role in the latter. This is joint work with Jacqueline
Warren.