Allcock\, building on the work of Greenburg\, pro ved that for any countable group \\(G\\)\, there is a a complete hyperboli c surface whose isometry group is exactly \\(G\\). When \\(G\\) is finite\ , Allcock’s construction yields a closed surface. Otherwise\, the const ruction gives an infinite-genus surface. \n\n

In this talk\, we discuss a related question. We fix any infinite-genus surface \\(S\\) and characte rise all groups that can arise as the isometry group for a complete hyperb olic structure on \\(S\\). In the process\, we give a classification type theorem for infinite-genus surfaces and\, if time allows\, two application s of the main result. \n\n

This talk is based on joint work with T. Aoug ab and N. Vlamis.\n LOCATION:https://researchseminars.org/talk/GaTO/4/ END:VEVENT BEGIN:VEVENT SUMMARY:Neil Hoffman (Oklahoma SU) DTSTART;VALUE=DATE-TIME:20200512T150000Z DTEND;VALUE=DATE-TIME:20200512T153000Z DTSTAMP;VALUE=DATE-TIME:20211209T080340Z UID:GaTO/5 DESCRIPTION:Title: Hig h crossing knot complements with few tetrahedra\nby Neil Hoffman (Okla homa SU) as part of Geometry and topology online\n\nLecture held in N/A.\n \nAbstract\n

It is well known that given a diagram of a knot \\(K\\) wit h \\(n\\) crossings\, one can construct a\ntriangulation of \\(S^3 - K\\) with at most \\(4n\\) tetrahedra. A natural question is then: given a tri angulation of a knot complement with \\(t\\) tetrahedra\, is the minimum c rossing number (for a diagram) of K bounded by a linear or polynomial func tion in \\(t\\)? We will answer the question in the negative by construct ing a family of hyperbolic knot complements where for each knot \\(K_n\\) in \\(S^3\\) whose the minimum crossing number goes as a function of \\(O( b^n)\\) for \\(b > 1\\)\, but the minimum number of tetrahedra in a triang ulation of \\(S^3 - K_n\\) is bounded above by \\(O(n)\\). Similar constr uctions exist for torus and satellite knot complements.\n\n

This is join
t work with Robert Haraway.\n
LOCATION:https://researchseminars.org/talk/GaTO/5/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Martin Scharlemann (UC Santa Barbara)
DTSTART;VALUE=DATE-TIME:20200512T153000Z
DTEND;VALUE=DATE-TIME:20200512T160000Z
DTSTAMP;VALUE=DATE-TIME:20211209T080340Z
UID:GaTO/6
DESCRIPTION:Title: A s
trong Haken's theorem\nby Martin Scharlemann (UC Santa Barbara) as par
t of Geometry and topology online\n\nLecture held in N/A.\n\nAbstract\nSup
pose that \\(T\\) is a Heegaard splitting\nsurface for a compact orientabl
e three-manifold \\(M\\)\; suppose\nthat \\(S\\) is a reducing sphere for
\\(M\\). In 1968 Haken\nshowed that there is then also a reducing sphere
\\(S^*\\) for\nthe Heegaard splitting. That is\, \\(S^*\\) is a reducing s
phere\nfor \\(M\\) and the surfaces \\(T\\) and \\(S^*\\) intersect in a\n
single circle. In 1987 Casson and Gordon extended the result\nto boundary
-reducing disks in \\(M\\) and noted that in both\ncases \\(S^*\\) is obta
ined from \\(S\\) by a sequence of\noperations called one-surgeries. Here
we show that in fact\none may take \\(S^* = S\\)\, at least in the case w
here \\(M\\)\ncontains no \\(S^1 \\times S^2\\) summands.\n
LOCATION:https://researchseminars.org/talk/GaTO/6/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Henry Segerman (Oklahoma SU)
DTSTART;VALUE=DATE-TIME:20200519T150000Z
DTEND;VALUE=DATE-TIME:20200519T153000Z
DTSTAMP;VALUE=DATE-TIME:20211209T080340Z
UID:GaTO/7
DESCRIPTION:Title: Fro
m veering triangulations to Cannon-Thurston maps\nby Henry Segerman (O
klahoma SU) as part of Geometry and topology online\n\nLecture held in NA.
\n\nAbstract\nAgol introduced veering triangulations of\nmapping tori\, wh
ose combinatorics are canonically associated\nto the pseudo-Anosov monodro
my. In previous work\, Hodgson\,\nRubinstein\, Tillmann and I found examp
les of veering\ntriangulations that are not layered and therefore do not c
ome\nfrom Agol's construction.\n\n However\, non-layered veering tr
iangulations retain many of the\n good properties enjoyed by mappin
g tori. For example\,\n Schleimer and I constructed a canonical ci
rcular ordering of\n the cusps of the universal cover of a veering
triangulation.\n Its order completion gives the *veering circle\;\n collapsing a pair of canonically defined laminations gives a
\n surjection onto the veering sphere.\n\n In work in
progress\, Manning\, Schleimer\, and I prove that the\n veering sp
here is the Bowditch boundary of the manifold's\n fundamental group
(with respect to its cusp groups). As an\n application we produce
Cannon-Thurston maps for all veering\n triangulations. This gives
the first examples of\n Cannon-Thurston maps that do not come\, ev
en virtually\, from\n surface subgroups.\n
LOCATION:https://researchseminars.org/talk/GaTO/7/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Baris Coskunuzer (UT Dallas)
DTSTART;VALUE=DATE-TIME:20200519T153000Z
DTEND;VALUE=DATE-TIME:20200519T160000Z
DTSTAMP;VALUE=DATE-TIME:20211209T080340Z
UID:GaTO/8
DESCRIPTION:Title: Min
imal surfaces in hyperbolic three-manifolds\nby Baris Coskunuzer (UT D
allas) as part of Geometry and topology online\n\n\nAbstract\nThe existenc
e of minimal surfaces in three-manifolds is a classical problem in both ge
ometric analysis and geometric topology. In the past years\, this question
has been settled for closed\, and also for finite volume\, riemannian thr
ee-manifolds. In this talk\, we will prove the existence of smoothly embed
ded\, closed\, minimal surfaces in any infinite volume hyperbolic three-ma
nifold\, barring a few special cases.\n
LOCATION:https://researchseminars.org/talk/GaTO/8/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Daniel Allcock (UT Austin)
DTSTART;VALUE=DATE-TIME:20200526T150000Z
DTEND;VALUE=DATE-TIME:20200526T153000Z
DTSTAMP;VALUE=DATE-TIME:20211209T080340Z
UID:GaTO/9
DESCRIPTION:Title: Big
mapping class groups fail the Tits alternative\nby Daniel Allcock (UT
Austin) as part of Geometry and topology online\n\n\nAbstract\nLet \\(S\\
) be a surface with infinitely many\npunctures\, or infinitely many handle
s\, or containing a disk\nminus Cantor set. (This accounts for almost all
infinite-type\nsurfaces.) Then the mapping class group of S fails to sat
isfy\nthe Tits alternative. Namely\, we construct a finitely\ngenerated s
ubgroup which is not virtually solvable and\ncontains no free group of ran
k greater than one. The\nGrigorchuk group is a key element in the constru
ction.\n
LOCATION:https://researchseminars.org/talk/GaTO/9/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Talia Fernos (UNC Greensboro)
DTSTART;VALUE=DATE-TIME:20200526T153000Z
DTEND;VALUE=DATE-TIME:20200526T160000Z
DTSTAMP;VALUE=DATE-TIME:20211209T080340Z
UID:GaTO/10
DESCRIPTION:Title: Bo
undaries and CAT(0) cube complexes\nby Talia Fernos (UNC Greensboro) a
s part of Geometry and topology online\n\n\nAbstract\nThe universe of \\(\
\CAT(0)\\) cube complexes\nis rich and diverse thanks to the ease by which
they can be\nconstructed and the many of natural metrics they admit. As
a\nconsequence\, there are several associated boundaries\, such as\nthe vi
sual boundary and the Roller boundary. In this talk we\nwill discuss some
relationships between these boundaries\,\ntogether with the Furstenberg-P
oisson boundary of a "nicely"\nacting group.\n
LOCATION:https://researchseminars.org/talk/GaTO/10/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Daniel Woodhouse (Oxford)
DTSTART;VALUE=DATE-TIME:20200602T150000Z
DTEND;VALUE=DATE-TIME:20200602T153000Z
DTSTAMP;VALUE=DATE-TIME:20211209T080340Z
UID:GaTO/11
DESCRIPTION:Title: Qu
asi-isometric rigidity of graphs of free groups with cyclic edge groups\nby Daniel Woodhouse (Oxford) as part of Geometry and topology online\n\
n\nAbstract\nLet \\(F\\) be a finitely generated free group.\nLet \\(w_1\\
) and \\(w_2\\) be suitably random/generic elements in\n\\(F\\). Consider
the HNN extension \\( G = \\langle F\, t \\\,{\\mid}\\\, t w_1\nt^{-1} =
w_2 \\rangle\\). It is already known that \\(G\\) will be\none-ended and
hyperbolic. What we have shown is that \\(G\\) is\nquasi-isometrically
rigid. That is\, if a finitely\ngenerated group \\(H\\) is quasi-iso
metric to \\(G\\)\, then \\(G\\)\nand \\(H\\) are virtually isomorphic. T
he main argument\ninvolves applying a new proof of Leighton's graph coveri
ng\ntheorem.\n\nOur full result is for finite graphs of groups with virtua
lly\nfree vertex groups and and two-ended edge groups. However the\nstate
ment here is more technical\; in particular\, not all such\ngroups are qua
si-isometrically rigid.\n\nThis is joint work with Sam Shepherd.\n
LOCATION:https://researchseminars.org/talk/GaTO/11/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Rylee Lyman (Tufts)
DTSTART;VALUE=DATE-TIME:20200602T153000Z
DTEND;VALUE=DATE-TIME:20200602T160000Z
DTSTAMP;VALUE=DATE-TIME:20211209T080340Z
UID:GaTO/12
DESCRIPTION:Title: Ou
ter automorphisms of free Coxeter groups\nby Rylee Lyman (Tufts) as pa
rt of Geometry and topology online\n\n\nAbstract\nA famous theorem of Birm
an and Hilden\nprovides a close link between the mapping class group of a\
npunctured sphere and the centraliser\, in the mapping class\ngroup of a c
losed surface\, of a hyperelliptic involution.\nThere is a group theory an
alogue of this in Out(\\(F_n\\))\, the\nouter automorphism group of a free
group. Namely\, the outer\nautomorphism of a free Coxeter group i
s linked to the\ncentraliser\, in Out(\\(F_n\\))\, of a hyperelliptic invo
lution.\nIn this talk we will meet the outer automorphism group of a\nfree
Coxeter group\, try to understand the analogy with mapping\nclass groups\
, and survey some recent results and interesting\nquestions.\n
LOCATION:https://researchseminars.org/talk/GaTO/12/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Kathryn Mann (Cornell)
DTSTART;VALUE=DATE-TIME:20200609T150000Z
DTEND;VALUE=DATE-TIME:20200609T153000Z
DTSTAMP;VALUE=DATE-TIME:20211209T080340Z
UID:GaTO/13
DESCRIPTION:Title: La
rge-scale geometry of big mapping class groups\nby Kathryn Mann (Corne
ll) as part of Geometry and topology online\n\n\nAbstract\nMapping class g
roups of infinite type surfaces are not finitely generated\; they are not
even locally compact. Nonetheless\, in many cases it is still meaningful t
o discuss their large scale geometry. We will explore which mapping class
groups have nontrivial coarse geometry.\n\nThis is joint work with Kasra R
afi.\n
LOCATION:https://researchseminars.org/talk/GaTO/13/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Eric Samperson (UIUC)
DTSTART;VALUE=DATE-TIME:20200609T153000Z
DTEND;VALUE=DATE-TIME:20200609T160000Z
DTSTAMP;VALUE=DATE-TIME:20211209T080340Z
UID:GaTO/14
DESCRIPTION:Title: Ho
w helpful is hyperbolic geometry?\nby Eric Samperson (UIUC) as part of
Geometry and topology online\n\n\nAbstract\nHyperbolic geometry serves du
al roles at the intersection of group theory and three-manifold topology.
It plays the hero of group theory — rescuing the field from a morass of
uncomputability — but the anti-hero of low-dimensional topology—seemin
gly responsible for much of the complexity of three-manifolds. Where do th
ese roles overlap?\n\nI’ll give examples of group-theoretic invariants o
f three-manifolds (or knots) that are NP-hard to compute\, even for three-
manifolds (or knots) that are promised to be hyperbolic. The basic idea is
to show that the right-angled Artin semigroups of reversible circuits (a
kind of combinatorial abstraction of particularly simple computer programs
) can be quasi-isometrically embedded inside mapping class groups. Recent
uniformity results concerning the coarse geometry of curve complexes play
a key role.\n\nThis is joint work with Chris Leininger that builds on prev
ious work with Greg Kuperberg.\n
LOCATION:https://researchseminars.org/talk/GaTO/14/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Corey Bregman (Brandeis)
DTSTART;VALUE=DATE-TIME:20200616T150000Z
DTEND;VALUE=DATE-TIME:20200616T153000Z
DTSTAMP;VALUE=DATE-TIME:20211209T080340Z
UID:GaTO/15
DESCRIPTION:Title: Is
otopy and equivalence of knots in three-manifolds\nby Corey Bregman (B
randeis) as part of Geometry and topology online\n\n\nAbstract\nIt is a we
ll-known fact that the notions of\n(ambient) isotopy and equival
ence coincide for\nknots in \\(S^3\\). This is because all orientatio
n-preserving\nhomeomorphisms of \\(S^3\\) are isotopic to the identity. I
n\nthis talk\, we compare the notions of equivalence and isotopy\nfor knot
s in more general three-manifolds.\n\nWe show that the mapping class group
of a three-manifold\n"sees" all the isotopy classes of knots\; that is\,
if an\norientation-preserving homeomorphism fixes every isotopy\nclass\, t
hen it is isotopic to the identity. In the case of\n\\(S^1 \\times S^2\\)
we give infinitely many examples of knots\nwhose isotopy classes are chan
ged by the Gluck twist. Along\nthe way we prove that every three-manifold
group satisfies\nGrossman's Property A.\n\nThis is joint work with Paolo
Aceto\, Christopher Davis\,\nJungHwan Park\, and Arunima Ray.\n
LOCATION:https://researchseminars.org/talk/GaTO/15/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Caroline Series (Warwick)
DTSTART;VALUE=DATE-TIME:20200623T150000Z
DTEND;VALUE=DATE-TIME:20200623T153000Z
DTSTAMP;VALUE=DATE-TIME:20211209T080340Z
UID:GaTO/16
DESCRIPTION:Title: Ge
ometry in non-discrete groups of hyperbolic isometries: Primitive stabilit
y and the Bowditch Q-conditions are equivalent.\nby Caroline Series (W
arwick) as part of Geometry and topology online\n\n\nAbstract\nThere are g
eometrical conditions on a group of hyperbolic isometries which are of int
erest even when the group is not discrete. We explain two such conditions\
; these are stated in terms of the images of primitive elements of the fre
e group \\(F_2\\) under an \\(\\textrm{SL}(2\,\\mathbb{C})\\) representati
on. One is Minsky’s condition of primitive stability\; the other
is the so-called BQ-conditions introduced by Bowditch and generalis
ed by Tan\, Wong\, and Zhang.\n\nThese two conditions have been shown to b
e equivalent by Jaijeong Lee and Binbin Xu (Trans AMS 2020) and independen
tly by the speaker (arxiv 2019). We will explain the ideas using an combin
ation of both methods. If time permits\, we also explain another\, closely
related\, condition which constrains the axes of palindromic primitive el
ements.\n
LOCATION:https://researchseminars.org/talk/GaTO/16/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Yulan Qing (Toronto)
DTSTART;VALUE=DATE-TIME:20200616T153000Z
DTEND;VALUE=DATE-TIME:20200616T160000Z
DTSTAMP;VALUE=DATE-TIME:20211209T080340Z
UID:GaTO/17
DESCRIPTION:Title: Th
e sub-linearly Morse boundary\nby Yulan Qing (Toronto) as part of Geom
etry and topology online\n\n\nAbstract\nThe Gromov boundary\, of a hyperbo
lic metric\nspace\, plays a central role in many aspects of geometric grou
p\ntheory. In this talk\, we introduce a generalization of the\nGromov bo
undary that also applies to non-hyperbolic\nspaces. For a given proper geo
desic metric space and a given\nsublinear function \\(\\kappa\\)\, we defi
ne the \\(\\kappa\\)-Morse\nboundary to be the space of all \\(\\kappa\\)-
sublinearly-Morse\nquasi-geodesics rays starting at a given base point.\n\
nWe show that\, equipped with a coarse version of the cone\ntopology\, the
\\(\\kappa\\)-boundary is metrizable and is a\nQI-invariant. For some gr
oups\, we show that their Poisson\nboundaries can be realized on the \\(\\
kappa\\)-boundary of their\nCayley graphs. These groups include all \\(\\
CAT(0)\\) groups\,\nmapping class groups\, Teichmü\;ller spaces\, hier
archically\nhyperbolic groups\, and relatively hyperbolic groups.\n\nThis
talk is based on joint projects with Ilya Gekhtmann\,\nKasra Rafi\, and Gi
ulio Tiozzo.\n
LOCATION:https://researchseminars.org/talk/GaTO/17/
END:VEVENT
BEGIN:VEVENT
SUMMARY:William Worden (Rice)
DTSTART;VALUE=DATE-TIME:20200623T153000Z
DTEND;VALUE=DATE-TIME:20200623T160000Z
DTSTAMP;VALUE=DATE-TIME:20211209T080340Z
UID:GaTO/18
DESCRIPTION:Title: De
hn filling and knot complements that do not irregularly cover\nby Will
iam Worden (Rice) as part of Geometry and topology online\n\n\nAbstract\nI
t is a longstanding conjecture of Neumann\nand Reid that exactly three kno
t complements can irregularly\ncover a hyperbolic orbifold -- the figure-e
ight knot and the two\nAitchison--Rubinstein dodecahedral knots. This con
jecture\,\nwhen combined with work of Boileau--Boyer--Walsh\, implies a\nm
ore recent conjecture of Reid and Walsh\, which states that\nthere are at
most three knot complements in the commensurability\nclass of any hyperbol
ic knot. We give a Dehn filling criterion\nthat is useful for producing l
arge families of knot\ncomplements that satisfy both conjectures.\n\nThe w
ork we will discuss is partially joint with Hoffman and\nMillichap and als
o partially joint with Chesebro\, Deblois\,\nHoffman\, Millichap\, and Mon
dal.\n
LOCATION:https://researchseminars.org/talk/GaTO/18/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Michael Landry (WUSTL)
DTSTART;VALUE=DATE-TIME:20200721T150000Z
DTEND;VALUE=DATE-TIME:20200721T153000Z
DTSTAMP;VALUE=DATE-TIME:20211209T080340Z
UID:GaTO/19
DESCRIPTION:Title: Fa
ces of the Thurston norm ball up to isotopy\nby Michael Landry (WUSTL)
as part of Geometry and topology online\n\n\nAbstract\n*

* Let \\(M\\) be
a three-manifold with\n nondegenerate Thurston norm \\(x\\) on its
second homology.\n There is a partial dictionary between the co
mbinatorics\n of the polyhedral unit ball of \\(x\\) and\n
the topological features of \\(M\\). This dictionary is\n
quite incomplete\, but its existing entries are tantalizing.\n *

\n Currently\, most of the entries of this dictionary co ncern\n fibered faces of the unit ball. Thurston proved that these \n organize all fibrations of \\(M\\) over the circle. Fried and\n Mosher tell us more: for each fibered face \\(F\\) there is a\n (canonical) pseudo-Anosov flow whose Euler class computes the\n norm \\(x\\) in the cone over \\(F\\). Furthermore\, the flow\n "sees" certain least-complexity surfaces. Further work of\n Mosher shows that\, under certain conditions\, pseudo-Anosov\n flows can n aturally specify nonfibered faces of the unit ball.\n

\n\n After giving some of this background I will discuss results\n from my recent preprint (see link). I\n show that Agol's veer ing triangulations can be used to\n determine faces of Thurston nor m balls\, to compute the\n Thurston norm over those faces\, and to collate all isotopy\n classes of least-complexity surfaces over tho se faces. This\n analysis includes nonfibered faces.\n

\n \nNo password is required.\n LOCATION:https://researchseminars.org/talk/GaTO/19/ END:VEVENT BEGIN:VEVENT SUMMARY:Rich Schwartz (Brown) DTSTART;VALUE=DATE-TIME:20200728T150000Z DTEND;VALUE=DATE-TIME:20200728T153000Z DTSTAMP;VALUE=DATE-TIME:20211209T080340Z UID:GaTO/20 DESCRIPTION:Title: Th e spheres of Sol\nby Rich Schwartz (Brown) as part of Geometry and top ology online\n\n\nAbstract\nWe give a complete characterization of the cut locus of the identity in Sol\, one of the strangest of the eight Thurston geometries. As a corollary we prove that the metric spheres in Sol are in fact topological spheres.\n\nThis is joint work with Matei Coiculescu\n LOCATION:https://researchseminars.org/talk/GaTO/20/ END:VEVENT BEGIN:VEVENT SUMMARY:Scott Taylor (Colby) DTSTART;VALUE=DATE-TIME:20200728T153000Z DTEND;VALUE=DATE-TIME:20200728T160000Z DTSTAMP;VALUE=DATE-TIME:20211209T080340Z UID:GaTO/21 DESCRIPTION:Title: Eq uivariant Heegaard genus of reducible three-manifolds\nby Scott Taylor (Colby) as part of Geometry and topology online\n\n\nAbstract\n\n
Suppose that \\(M\\) is a closed\, connected\,\n oriented three-
manifold which comes with a group action by a\n finite group of (or
ientation preserving) diffeomorphisms.\n The *equivariant Heegaar
d genus* of \\(M\\) is then the\n minimal genus of an equivarian
t Heegaard surface. The\n equivariant sphere theorem\, together wi
th recent work of\n Scharlemann\, suggests that equivariant Heegaar
d genus might be\n additive under equivariant connected sum\, while
analogies with\n tunnel number suggest it should not be.\n \n

\n I will describe some examples showing that equivari ant\n Heegaard genus can be sub-additive\, additive\, or\n s uper-additive. Building on recent work with Tomova\, I’ll\n sket ch machinery that gives rise both to sharp bounds on the\n addivity of equivariant Heegaard genus and to a closely\n related invariant that is in fact additive.\n

\n LOCATION:https://researchseminars.org/talk/GaTO/21/ END:VEVENT BEGIN:VEVENT SUMMARY:Marissa Loving (Georgia Tech) DTSTART;VALUE=DATE-TIME:20200804T150000Z DTEND;VALUE=DATE-TIME:20200804T153000Z DTSTAMP;VALUE=DATE-TIME:20211209T080340Z UID:GaTO/22 DESCRIPTION:Title: Co vers and curves\nby Marissa Loving (Georgia Tech) as part of Geometry and topology online\n\n\nAbstract\n\n It is a celebrated result of Scott that every\n closed curve on a hyperbolic surface \\(S\\) lifts to a simple\n closed curve on some finite cover. In the spir it of this work\n we pose the following question: "What information about two\n covers \\(X\\) and \\(Y\\) of \\(S\\) can be derived b y\n understanding how curves on \\(S\\) lift simply to \\(X\\) and\ n \\(Y\\)?" In this talk\, we will explore the answer to this\n question for regular finite covers of a closed hyperbolic\n su rface.\n

\n\n This is joint work with Tarik Aouga b\, Max Lahn\, and Yang\n (Sunny) Xiao.\n

\n LOCATION:https://researchseminars.org/talk/GaTO/22/ END:VEVENT BEGIN:VEVENT SUMMARY:Sarah Dean Rasmussen (Cambridge) DTSTART;VALUE=DATE-TIME:20200804T153000Z DTEND;VALUE=DATE-TIME:20200804T160000Z DTSTAMP;VALUE=DATE-TIME:20211209T080340Z UID:GaTO/23 DESCRIPTION:Title: Ta ut foliations from left orders\, in Heegaard genus two\nby Sarah Dean Rasmussen (Cambridge) as part of Geometry and topology online\n\n\nAbstrac t\n\n Suppose that \\(M\\) is a closed\, connected\,\n or iented three-manifold which is not graph. All previously\n known c onstructions of taut foliations on such \\(M\\) used\n branched sur faces. These branched surfaces come from sutured\n manifold hierar chies\, following Gabai\, come from spanning\n surfaces of knot ext eriors\, following Roberts\, or come from\n one-vertex triangulatio ns with foliar orientations\, following\n Dunfield.\n

\n\n In this talk\, we give a new construction that does not u se\n branched surfaces. Instead\, we build a taut foliation from\n the data of a Heegaard diagram for \\(M\\) and a left order on\n the fundamental group \\(\\pi_1(M)\\). We glue an\n \\(\\mat hbb{R}\\)-transverse foliation (over a thickened Heegaard\n surface ) to a pair of handlebody foliations\; we then suitably\n cancel an y singularities. For Heegaard diagrams satisfying\n mild condition s\, this can be done reliably in Heegaard genus\n two. In some cas es this construction can be extended to\n higher Heegaard genus. T his helps explain numerical results\n of Dunfield: (i) tens of thou sands of Heegaard-genus two\n hyperbolic L-spaces certifiably fail to admit fundamental\n group left orders and (ii) no hyperbolic L-s pace is known to\n admit a fundamental group left order.\n

\n LOCATION:https://researchseminars.org/talk/GaTO/23/ END:VEVENT BEGIN:VEVENT SUMMARY:Kasia Jankiewicz (Chicago) DTSTART;VALUE=DATE-TIME:20200818T150000Z DTEND;VALUE=DATE-TIME:20200818T153000Z DTSTAMP;VALUE=DATE-TIME:20211209T080340Z UID:GaTO/24 DESCRIPTION:Title: Ge neralized Tits conjecture for Artin groups\nby Kasia Jankiewicz (Chica go) as part of Geometry and topology online\n\n\nAbstract\n\n Th e Tits conjecture\, proved by Crisp and\n Paris\, states that the s ubgroup of an Artin group generated by\n powers of the standard gen erators is the "obvious"\n right-angled Artin group (RAAG). We aim to generalize this: the\n subgroup generated by a collection of na turally distinguished\n elements\, specifically powers of the Garsi de elements\, is a\n RAAG. I will discuss our partial results\, fo r certain families\n of Artin groups.\n

\n\n This is joint work with Kevin Schreve.\n

\n LOCATION:https://researchseminars.org/talk/GaTO/24/ END:VEVENT BEGIN:VEVENT SUMMARY:Andras Stipsicz (Renyi) DTSTART;VALUE=DATE-TIME:20200811T150000Z DTEND;VALUE=DATE-TIME:20200811T153000Z DTSTAMP;VALUE=DATE-TIME:20211209T080340Z UID:GaTO/25 DESCRIPTION:Title: Co nnected Floer homology of covering involutions\nby Andras Stipsicz (Re nyi) as part of Geometry and topology online\n\n\nAbstract\n\n W e use the covering involution of double\n branched covers of knots to define a knot invariant inspired\n by connected Heegaard Floer h omology. Using this\, we obtain\n novel concordance results.\n

\n\n This is joint work with Antonio Alfieri and Sun gkyung Kang.\n

\n LOCATION:https://researchseminars.org/talk/GaTO/25/ END:VEVENT BEGIN:VEVENT SUMMARY:Jing Tao (Oklahoma) DTSTART;VALUE=DATE-TIME:20200818T153000Z DTEND;VALUE=DATE-TIME:20200818T160000Z DTSTAMP;VALUE=DATE-TIME:20211209T080340Z UID:GaTO/26 DESCRIPTION:Title: Th e Nielsen-Thurston classification\, revisited\nby Jing Tao (Oklahoma) as part of Geometry and topology online\n\n\nAbstract\n\n I will explain a new proof of the\n Nielsen-Thurston classification of ma pping classes\, using the\n Thurston metric on Teichmuller space.\n

\n\n This is joint work with Camille Horbez.\n

\n LOCATION:https://researchseminars.org/talk/GaTO/26/ END:VEVENT BEGIN:VEVENT SUMMARY:Dawid Kielak (Oxford) DTSTART;VALUE=DATE-TIME:20200825T153000Z DTEND;VALUE=DATE-TIME:20200825T160000Z DTSTAMP;VALUE=DATE-TIME:20211209T080340Z UID:GaTO/27 DESCRIPTION:Title: Po incaré\; duality groups\nby Dawid Kielak (Oxford) as part of Geo metry and topology online\n\n\nAbstract\n\n It is a classical fa ct that a Poincaré\;\n duality group\, in dimension two\, is a surface group. In this\n talk I will discuss a relatively short new proof of this.\n

\n\n This is joint work with Peter Kropholler.\n

\n LOCATION:https://researchseminars.org/talk/GaTO/27/ END:VEVENT BEGIN:VEVENT SUMMARY:Mehdi Yazdi (Oxford) DTSTART;VALUE=DATE-TIME:20201008T140000Z DTEND;VALUE=DATE-TIME:20201008T143000Z DTSTAMP;VALUE=DATE-TIME:20211209T080340Z UID:GaTO/28 DESCRIPTION:Title: Th e complexity of determining knot genus in a fixed three-manifold\nby M ehdi Yazdi (Oxford) as part of Geometry and topology online\n\n\nAbstract\ n\n The *genus* of a knot in a three-manifold is\n d
efined to be the minimum genus of a compact\, orientable\n surface
bounding that knot\, if such a surface exists. In\n particular a k
not can be untangled if and only if it has genus\n zero. We consid
er the computational complexity of determining\n knot genus. Such
problems have been studied by several\n mathematicians\; among them
are the works of\n Hass-Lagarias-Pippenger\, Agol-Hass-Thurston\,
Agol and\n Lackenby. For a fixed three-manifold the knot genus pro
blem asks\,\n given a knot \\(K\\) and an integer \\(g\\)\, whether
the genus of \\(K\\) is\n equal to \\(g\\). Marc Lackenby proved
that the knot genus problem\n for the three-sphere lies in NP. In
joint work with Lackenby\, we\n prove that this can be generalised
to any fixed\, compact\,\n orientable three-manifold\, answering a
question of\n Agol-Hass-Thurston from 2002.\n

\n In his seminal 19 76 paper Bill Thurston\n observed that a closed leaf \\(S\\) of a f oliation has Euler\n characteristic equal\, up to sign\, to the Eul er class of the\n foliation evaluated on \\([S]\\)\, the homology c lass represented\n by \\(S\\). We give a converse for taut foliati ons: if the\n underlying manifold is hyperbolic and if the Euler cl ass of a\n taut foliation \\(F\\) evaluated on \\([S]\\) equals\, u p to sign\,\n the Euler characteristic of \\(S\\)\, then there exis ts another\n taut foliation \\(F'\\) such that \\(S\\) is homologou s to a union\n of leaves and such that the plane field of \\(F'\\) is homotopic\n to that of \\(F\\). In particular\, \\(F\\) and \\( F'\\) have the\n same Euler class.\n

\n\n In the same paper Thurston proved that taut foliations on\n closed hyperbolic three-manifolds have Euler class of norm at\n most one\, and conjectured that\, conversely\, any integral\n cohomology clas s with norm equal to one is the Euler class of\n a taut foliation. Work of Yazdi\, together with our main\n result\, give a negative answer to Thurston's conjecture.\n

\n\n This is j oint work with Mehdi Yazdi.\n

\n LOCATION:https://researchseminars.org/talk/GaTO/29/ END:VEVENT BEGIN:VEVENT SUMMARY:Esmee te Winkel (Warwick) DTSTART;VALUE=DATE-TIME:20201015T140000Z DTEND;VALUE=DATE-TIME:20201015T143000Z DTSTAMP;VALUE=DATE-TIME:20211209T080340Z UID:GaTO/30 DESCRIPTION:Title: Kn ots in the curve graph\nby Esmee te Winkel (Warwick) as part of Geomet ry and topology online\n\n\nAbstract\n\n By a famous theorem of Thurston the space\n \\(\\PML\\) of projective (measured) laminatio ns on a five-times\n punctured sphere is a three-sphere. An element ary example of a\n projective lamination is a simple closed geodesi c with the\n counting measure. This defines a map from the set of c urves to\n \\(\\PML\\)\, which extends to an injective map from the curve\n graph to \\(\\PML\\). The topology of the image of the cur ve\n graph in \\(\\PML\\) and its complement were previously studie d\n by Gabai.\n

\nIn this talk we will introduce certain finite subgraphs of\n the curve graph of the five-times pu nctured sphere and\n determine whether their image in \\(\\PML\\) i s knotted.\n

\n LOCATION:https://researchseminars.org/talk/GaTO/30/ END:VEVENT BEGIN:VEVENT SUMMARY:Rudradip Biswas (Manchester) DTSTART;VALUE=DATE-TIME:20201015T143000Z DTEND;VALUE=DATE-TIME:20201015T150000Z DTSTAMP;VALUE=DATE-TIME:20211209T080340Z UID:GaTO/31 DESCRIPTION:Title: Ge neration of unbounded derived categories of modules over groups in Krophol ler's hierarchy\nby Rudradip Biswas (Manchester) as part of Geometry a nd topology online\n\n\nAbstract\n\n For a group $G$ in Kropholler's h ierarchy and\n a commutative ring $R$\, we will go through some recently\ n discovered generation properties of $D(\\rm{Mod}(RG))$ in terms of\n l ocalising and colocalising subcategories. If time permits\, we\n will try to include a few comments on how these generation\n properties shed some light on some deep properties of\n $D(\\rm{Mod}(RG))$ as a triangulated category.\n

\n LOCATION:https://researchseminars.org/talk/GaTO/31/ END:VEVENT BEGIN:VEVENT SUMMARY:Yair Minsky (Yale) DTSTART;VALUE=DATE-TIME:20201022T140000Z DTEND;VALUE=DATE-TIME:20201022T143000Z DTSTAMP;VALUE=DATE-TIME:20211209T080340Z UID:GaTO/32 DESCRIPTION:Title: Ve ering triangulations and their polynomials\nby Yair Minsky (Yale) as p art of Geometry and topology online\n\n\nAbstract\n\n This is an introduction to Sam's talk.\n McMullen introduced certain polynomi als associated to fibered\n three-manifolds\, which package togethe r the dynamical data\n associated to all the fibrations in a given fibered face of\n Thurston's norm ball. Agol's veering triangulati ons provide a\n good setting in which similar invariants can be def ined. I\n will review this background\, explain the definition of the\n "veering Polynomial" and the "taut Polynomial"\, the\n relationship between them\, and how they recover McMullen's\n poly nomial in the fibered face.\n

\n\n This is joint work with Michael Landry and Sam Taylor.\n

\n LOCATION:https://researchseminars.org/talk/GaTO/32/ END:VEVENT BEGIN:VEVENT SUMMARY:Sam Taylor (Temple) DTSTART;VALUE=DATE-TIME:20201022T143000Z DTEND;VALUE=DATE-TIME:20201022T150000Z DTSTAMP;VALUE=DATE-TIME:20211209T080340Z UID:GaTO/33 DESCRIPTION:Title: Th e veering polynomial\, the flow graph\, and the Thurston norm\nby Sam Taylor (Temple) as part of Geometry and topology online\n\n\nAbstract\n
\n This is a continuation of Yair’s talk on the\n veering
polynomial. Here we show how the veering polynomial\n can be const
ructed as the Perron polynomial of a certain\n combinatorially defi
ned directed graph\, which we call\n the *flow graph*. This pe
rspective will allows us to\n relate our polynomial to a face \\(F\
\) of the Thurston norm ball\n and to see that the cone over \\(F\\
) is spanned by surfaces that\n are "carried" by the veering triang
ulation. We’ll also discuss\n criteria for when the face \\(F\\)
is fibered.\n

\n This is joint work with Micha el Landry and Yair Minsky.\n

\n LOCATION:https://researchseminars.org/talk/GaTO/33/ END:VEVENT BEGIN:VEVENT SUMMARY:Mark Bell (Independent) DTSTART;VALUE=DATE-TIME:20201029T150000Z DTEND;VALUE=DATE-TIME:20201029T153000Z DTSTAMP;VALUE=DATE-TIME:20211209T080340Z UID:GaTO/34 DESCRIPTION:Title: Co mputations in big mapping class groups\nby Mark Bell (Independent) as part of Geometry and topology online\n\n\nAbstract\n\n We will t ake a brief look at some of the\n computations that are possible in big mapping class groups. In\n particular we will discuss the impl ementation\n of Bigger\n - a Python package which allows you to study and manipulat e\n laminations and mapping classes on infinite-type surfaces.\n

\n LOCATION:https://researchseminars.org/talk/GaTO/34/ END:VEVENT BEGIN:VEVENT SUMMARY:Chenxi Wu (Rutgers) DTSTART;VALUE=DATE-TIME:20201105T150000Z DTEND;VALUE=DATE-TIME:20201105T153000Z DTSTAMP;VALUE=DATE-TIME:20211209T080340Z UID:GaTO/35 DESCRIPTION:Title: Bo unds on asymptotic translation length on free factor and free splitting co mplexes\nby Chenxi Wu (Rutgers) as part of Geometry and topology onlin e\n\n\nAbstract\n\n The free factor and free splitting complexes \n are analogies for the curve complex on surfaces. We found some\n upper bound on the asymptotic translation length on these\n complexes when the train track maps have homotopic mapping\n tori\ , analogous to an upper bound we found earlier in the\n setting of curve complexes.\n

\n\n This is joint work with H yungryul Baik and Dongryul Kim.\n

\n LOCATION:https://researchseminars.org/talk/GaTO/35/ END:VEVENT BEGIN:VEVENT SUMMARY:Ivan Dynnikov (Steklov) DTSTART;VALUE=DATE-TIME:20201105T153000Z DTEND;VALUE=DATE-TIME:20201105T160000Z DTSTAMP;VALUE=DATE-TIME:20211209T080340Z UID:GaTO/36 DESCRIPTION:Title: An algorithm to compare Legendrian knots\nby Ivan Dynnikov (Steklov) as part of Geometry and topology online\n\n\nAbstract\n\n We have w orked out a general method to decide\n whether two given Legendrian knots are Legendrian\n equivalent. The method yields a formal algo rithmic solution to\n the problem (with very high algorithmic compl exity) and\, in\n certain circumstances\, allows one to distinguish Legendrian\n knots practically\, including some cases in which the \n computation of any known algebraic invariant except for the\n two classical ones (Thurston--Bennequin's and Maslov's) is\n i nfeasible. We use this\, in particular\, to provide an example\n of an annulus embedded in the three-sphere and tangent to the\n conta ct structure along the whole boundary\, such that the two\n connect ed components of the boundary are not equivalent as\n Legendrian kn ots.\n

\n\n The talk is based on joint works with Maxim Prasolov and Vladimir Shastin.\n

\n LOCATION:https://researchseminars.org/talk/GaTO/36/ END:VEVENT BEGIN:VEVENT SUMMARY:Tara Brendle (Glasgow) DTSTART;VALUE=DATE-TIME:20201203T150000Z DTEND;VALUE=DATE-TIME:20201203T153000Z DTSTAMP;VALUE=DATE-TIME:20211209T080340Z UID:GaTO/37 DESCRIPTION:Title: Th e mapping class group of connect sums of \\(S^2 \\times S^1\\)\nby Tar a Brendle (Glasgow) as part of Geometry and topology online\n\n\nAbstract\ n\n Let \\(M_n\\) denote the connect sum of \\(n\\)\n cop ies of \\(S^2 \\times S^1\\). Laudenbach showed that the\n mapping class group \\(\\Mod(M_n)\\) is an extension of the group\n \\(\\O ut(F_n)\\) by \\((\\ZZ/2)^n\\)\, where the latter group is the\n "s phere twist" subgroup of \\(\\Mod(M_n)\\).\n

\n\n We prove that this extension splits. In this talk\, I will\n desc ribe the splitting and discuss some simplifications of\n Laudenbach 's original proof that arise from our techniques.\n

\n\n This is joint work with N. Broaddus and A. Putman.\n

\n LOCATION:https://researchseminars.org/talk/GaTO/37/ END:VEVENT BEGIN:VEVENT SUMMARY:Ying Hu (UNO) DTSTART;VALUE=DATE-TIME:20201203T153000Z DTEND;VALUE=DATE-TIME:20201203T160000Z DTSTAMP;VALUE=DATE-TIME:20211209T080340Z UID:GaTO/38 DESCRIPTION:Title: Eu ler class of taut foliations on Q-homology spheres and Dehn fillings\n by Ying Hu (UNO) as part of Geometry and topology online\n\n\nAbstract\n\n The Euler class of an oriented plane field\n over a thre e-manifold is a second cohomology class\, which\n determines the pl ane field up to isomorphism. In this talk\,\n we will discuss the Euler class of taut foliations on a\n \\(\\QQ\\)-homology sphere. W e view \\(\\QQ\\)-homology spheres as\n Dehn fillings on knot manif olds and give necessary and\n sufficient conditions for the Euler c lass of taut foliations\n on such manifolds to vanish. We will also apply these results\n to study the orderability of three-manifold groups.\n

\n LOCATION:https://researchseminars.org/talk/GaTO/38/ END:VEVENT BEGIN:VEVENT SUMMARY:Ruth Charney (Brandeis) DTSTART;VALUE=DATE-TIME:20201210T150000Z DTEND;VALUE=DATE-TIME:20201210T153000Z DTSTAMP;VALUE=DATE-TIME:20211209T080340Z UID:GaTO/39 DESCRIPTION:Title: Ou ter space for right-angled Artin groups\nby Ruth Charney (Brandeis) as part of Geometry and topology online\n\n\nAbstract\n\n Right-an gled Artin groups (RAAGs) span a\n range of groups from free groups to free abelian groups.\n Thus\, their (outer) automorphism groups range from \\(\\Out(F_n)\\) to\n \\(\\GL(n\,\\ZZ)\\). Automorphism group s of RAAGs have been well-studied\n over the past decade from a pur ely algebraic viewpoint. To\n allow for a more geometric approach\ , one needs to construct a\n contractible space with a proper actio n of the group.\n

\n\n In this pair of talks we w ill construct such a space\, namely an\n analogue of Culler-Vogtman n’s Outer Space for arbitrary RAAGs.\n

\n\n Thi s is joint work with Corey Bregman and Karen Vogtmann.\n

\n LOCATION:https://researchseminars.org/talk/GaTO/39/ END:VEVENT BEGIN:VEVENT SUMMARY:Corey Bregman (Brandeis) DTSTART;VALUE=DATE-TIME:20201210T153000Z DTEND;VALUE=DATE-TIME:20201210T160000Z DTSTAMP;VALUE=DATE-TIME:20211209T080340Z UID:GaTO/40 DESCRIPTION:Title: Ou ter space for right-angled Artin groups\nby Corey Bregman (Brandeis) a s part of Geometry and topology online\n\n\nAbstract\n\n Right-a ngled Artin groups (RAAGs) span a\n range of groups from free group s to free abelian groups.\n Thus\, their (outer) automorphism groups range from \\(\\Out(F_n)\\) to\n \\(\\GL(n\,\\ZZ)\\). Automorphism grou ps of RAAGs have been well-studied\n over the past decade from a pu rely algebraic viewpoint. To\n allow for a more geometric approach \, one needs to construct a\n contractible space with a proper acti on of the group.\n

\n\n In this pair of talks we will construct such a space\, namely an\n analogue of Culler-Vogtma nn’s Outer Space for arbitrary RAAGs.\n

\n\n Th is is joint work with Ruth Charney and Karen Vogtmann.\n

\n LOCATION:https://researchseminars.org/talk/GaTO/40/ END:VEVENT BEGIN:VEVENT SUMMARY:Robert Kropholler (University of Warwick) DTSTART;VALUE=DATE-TIME:20211007T140500Z DTEND;VALUE=DATE-TIME:20211007T145500Z DTSTAMP;VALUE=DATE-TIME:20211209T080340Z UID:GaTO/41 DESCRIPTION:Title: Co arse embeddings and homological filling functions\nby Robert Kropholle r (University of Warwick) as part of Geometry and topology online\n\n\nAbs tract\n\n The homological filling function of a\n finitel y presented group \\(G\\) measures the difficulty of\n filling loop s with surfaces in a classifying space. The\n behaviour of this fun ction when passing to finitely presented\n subgroups is rather wild . If one adds assumptions on the\n dimension of \\(G\\)\, then one can bound the homological filling\n function of the subgroup by th at of \\(G\\). I will discuss how\n to generalise these results fr om subgroups to coarse\n embeddings and also to higher dimensional filling functions.\n

\n\n This is joint work with Mark Pengitore.\n

\n\nWe start five minutes after the hour.\n LOCATION:https://researchseminars.org/talk/GaTO/41/ END:VEVENT BEGIN:VEVENT SUMMARY:Ian Leary (Southampton) DTSTART;VALUE=DATE-TIME:20211014T140500Z DTEND;VALUE=DATE-TIME:20211014T145500Z DTSTAMP;VALUE=DATE-TIME:20211209T080340Z UID:GaTO/42 DESCRIPTION:Title: Gr aphical small cancellation and groups of type \\(\\mathrm{FP}\\)\nby I an Leary (Southampton) as part of Geometry and topology online\n\n\nAbstra ct\n\n Graphical small cancellation was introduced\n by G
romov to embed an expanding family inside the Cayley graph\n of a f
initely generated group. We use this technique to\n construct a la
rge family of groups of type \\(\\mathrm{FP}\\)\, most of\n which a
re not finitely presented. This is the first time\n non-finitely p
resented groups of type \\(\\mathrm{FP}\\) have been\n constructed
*without* using Bestvina-Brady Morse theory.\n I will give an
idea of how graphical small cancellation works\n and how we use it.
\n

\n This is joint work with Tom Brown.\n

\n LOCATION:https://researchseminars.org/talk/GaTO/42/ END:VEVENT BEGIN:VEVENT SUMMARY:Giles Gardam (Münster) DTSTART;VALUE=DATE-TIME:20211125T150500Z DTEND;VALUE=DATE-TIME:20211125T155500Z DTSTAMP;VALUE=DATE-TIME:20211209T080340Z UID:GaTO/43 DESCRIPTION:Title: Th e Kaplansky conjectures\nby Giles Gardam (Münster) as part of Geometr y and topology online\n\n\nAbstract\nThree conjectures on group rings of t orsion-free groups are commonly attributed to Kaplansky\, namely the unit\ , zero divisor and idempotent conjectures. For example\, the zero divisor conjecture predicts that if $K$ is a field and $G$ is a torsion-free group \, then the group ring $K[G]$ has no zero divisors. I will discuss these c onjectures and their relationship to other conjectures and properties of g roups. I will then explain how modern solvers for Boolean satisfiability c an be applied to them\, producing the first counterexample to the unit con jecture.\n LOCATION:https://researchseminars.org/talk/GaTO/43/ END:VEVENT BEGIN:VEVENT SUMMARY:Sam Kim (KIAS) DTSTART;VALUE=DATE-TIME:20211118T150500Z DTEND;VALUE=DATE-TIME:20211118T155500Z DTSTAMP;VALUE=DATE-TIME:20211209T080340Z UID:GaTO/44 DESCRIPTION:Title: Op timal regularity of mapping class group actions on the circle\nby Sam Kim (KIAS) as part of Geometry and topology online\n\n\nAbstract\nWe prove that for each finite index subgroup $H$ of the mapping class group of a c losed hyperbolic surface\, and for each real number $r>1$ there does not e xist a faithful $C^r$-action (in Hölder's sense) of $H$ on a circle. For this\, we partially determine the optimal regularity of faithful actions b y right-angled Artin groups on a circle. (Joint with Thomas Koberda and Cr istobal Rivas)\n LOCATION:https://researchseminars.org/talk/GaTO/44/ END:VEVENT BEGIN:VEVENT SUMMARY:Camille Horbez (Orsay) DTSTART;VALUE=DATE-TIME:20211021T140500Z DTEND;VALUE=DATE-TIME:20211021T145500Z DTSTAMP;VALUE=DATE-TIME:20211209T080340Z UID:GaTO/45 DESCRIPTION:Title: Or bit equivalence rigidity of irreducible actions of right-angled Artin grou ps\nby Camille Horbez (Orsay) as part of Geometry and topology online\ n\n\nAbstract\nA central goal in measured group theory is to classify f ree\, ergodic\, measure-preserving actions of countable groups on probabil ity spaces up to orbit equivalence: that is\, up to the existence of a mea sure space isomorphism sending orbits to orbits. Rigidity occurs when orbi t equivalence of two actions forces them to be conjugate through a group i somorphism. In this talk\, I will present orbit equivalence rigidity pheno mena for actions of (centerless\, one-ended) right-angled Artin groups\, u pon imposing that every standard generator acts ergodically on the space.\ n\n

This is joint work with Jingyin Huang.\n LOCATION:https://researchseminars.org/talk/GaTO/45/ END:VEVENT BEGIN:VEVENT SUMMARY:Rachel Skipper (OSU) DTSTART;VALUE=DATE-TIME:20211104T150500Z DTEND;VALUE=DATE-TIME:20211104T155500Z DTSTAMP;VALUE=DATE-TIME:20211209T080340Z UID:GaTO/46 DESCRIPTION:Title: Br aiding groups of homeomorphisms of Cantor sets\nby Rachel Skipper (OSU ) as part of Geometry and topology online\n\n\nAbstract\n

\n We w ill discuss some ways in which one can\n braid some classical subgr oups of the homeomorphism group of\n the Cantor set. This includes Higman-Thompson groups and\n self-similar groups\, as well as the topological finiteness\n properties of the resulting groups.\n

\n\n The talk will include some joint work with Xiaol ei Wu and\n Matthew Zaremsky.\n

\n LOCATION:https://researchseminars.org/talk/GaTO/46/ END:VEVENT BEGIN:VEVENT SUMMARY:Emily Stark (Wesleyan University) DTSTART;VALUE=DATE-TIME:20211202T150500Z DTEND;VALUE=DATE-TIME:20211202T155500Z DTSTAMP;VALUE=DATE-TIME:20211209T080340Z UID:GaTO/47 DESCRIPTION:Title: Gr aphically discrete groups and rigidity\nby Emily Stark (Wesleyan Unive rsity) as part of Geometry and topology online\n\n\nAbstract\nRigidity the orems prove that a group's geometry determines its algebra\, typically up to virtual isomorphism. Motivated by rigidity problems\, we study graphica lly discrete groups\, which impose a discreteness criterion on the automor phism group of any graph the group acts on geometrically. Classic examples of graphically discrete groups include virtually nilpotent groups and fun damental groups of closed hyperbolic manifolds. We will present new exampl es\, proving this property is not a quasi-isometry invariant. We will disc uss action rigidity for free products of residually finite graphically dis crete groups. This is joint work with Alex Margolis\, Sam Shepherd\, and D aniel Woodhouse.\n LOCATION:https://researchseminars.org/talk/GaTO/47/ END:VEVENT BEGIN:VEVENT SUMMARY:Arnaud de Mesmay (Laboratoire d'Informatique Gaspard-Monge) DTSTART;VALUE=DATE-TIME:20211209T150500Z DTEND;VALUE=DATE-TIME:20211209T155500Z DTSTAMP;VALUE=DATE-TIME:20211209T080340Z UID:GaTO/48 DESCRIPTION:Title: Sh ort canonical decompositions of non-orientable surfaces\nby Arnaud de Mesmay (Laboratoire d'Informatique Gaspard-Monge) as part of Geometry and topology online\n\nInteractive livestream: https://wwu.zoom.us/j/687603239 60\n\nAbstract\nAn important primitive for graphs embedded on surfaces is to cut the surface into a disk\, for applied purposes\, algorithm design o r even just to represent the graph. \nFor orientable surfaces\, it has lon g been known how to compute a canonical system of loops that is short: tha t is\, such that each loop uses each edge of the graph a constant number o f times. In this talk we will survey such cutting problems. We will then explain how to obtain such a short canonical system of loops for non-orien table surfaces. \n\nThis is joint work with Niloufar Fuladi and Alfredo Hu bard.\n LOCATION:https://researchseminars.org/talk/GaTO/48/ URL:https://wwu.zoom.us/j/68760323960 END:VEVENT BEGIN:VEVENT SUMMARY:Benjamin Ward (BGSU) DTSTART;VALUE=DATE-TIME:20211111T150500Z DTEND;VALUE=DATE-TIME:20211111T155500Z DTSTAMP;VALUE=DATE-TIME:20211209T080340Z UID:GaTO/49 DESCRIPTION:Title: Ma ssey Products for Graph Homology.\nby Benjamin Ward (BGSU) as part of Geometry and topology online\n\n\nAbstract\nThis talk is about graph compl exes and their homology. A graph complex can be thought of as a generaliz ation of a dg associative algebra\, but with more sophisticated compositio n operations allowing for particles to collide along any graph\, not just along a line. Is every graph complex quasi-isomorphic to its homology? C ontinuing the analogy with associative algebras the answer is no\, but we will see how an A-infinity analog of graph complexes can be used to rectif y this situation. We will then discuss what these higher operations can t ell us in the particular cases of Lie and commutative graph homology.\n LOCATION:https://researchseminars.org/talk/GaTO/49/ END:VEVENT END:VCALENDAR