BEGIN:VCALENDAR
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CALSCALE:GREGORIAN
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BEGIN:VEVENT
SUMMARY:Floris Elzinger
DTSTART:20200714T130000Z
DTEND:20200714T140000Z
DTSTAMP:20260404T111007Z
UID:GOBA/1
DESCRIPTION:Title: Free orthogonal quantum groups and strong 1-boundedness\nby Floris
Elzinger as part of Groups\, Operators\, and Banach Algebras Webinar\n\n\
nAbstract\nThe free orthogonal quantum groups\, depending on a parameter m
atrix Q\, form an accessible class of examples of compact quantum groups\,
which can be viewed as analogues of both the real orthogonal groups and c
ertain free product groups. In fact\, a particularly well-behaved subclass
\, where the parameter matrix is conjugate to either an identity $I_M$ or
standard symplectic $J_{2N}$\, shares many von Neumann algebraic propertie
s with the free groups. A natural question is then whether these objects a
re distinguishable on the von Neumann algebraic level. Recently\, Brannan
and Vergnioux managed to show that in case $Q = I_M$ these operator algebr
as satisfy a free-probabilistic property called strong 1-boundedness\, whi
ch the free group factors do not. Their proof employs techniques from the
theory of compact quantum groups\, free probability\, and the quantum anal
ogue of Cayley graphs. We will review the necessary notions and explain ho
w the techniques of Brannan and Vergnioux can be extended to also cover th
e missing case $Q = J_{2N}$.\n
LOCATION:https://stable.researchseminars.org/talk/GOBA/1/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Henrik Wirzenius
DTSTART:20200721T130000Z
DTEND:20200721T140000Z
DTSTAMP:20260404T111007Z
UID:GOBA/2
DESCRIPTION:Title: The quotient algebra of compact-by-approximable operators.\nby Hen
rik Wirzenius as part of Groups\, Operators\, and Banach Algebras Webinar\
n\n\nAbstract\nLet $K(X)$ denote the Banach algebra of compact operators a
cting on a Banach space $X$ and $A(X)$ the uniform closure of the bounded
finite rank operators. In this talk I will describe joint work with Hans-O
lav Tylli (University of Helsinki) on the quotient algebra $K(X)/A(X)$ of
compact-by-approximable operators. This is a radical Banach algebra that i
s poorly understood\, mainly because $K(X)/A(X)$ is non-trivial only withi
n the class of Banach spaces $X$ failing the approximation property. I wil
l focus on the size and algebraic structure of $K(X)/A(X)$.\n
LOCATION:https://stable.researchseminars.org/talk/GOBA/2/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Simeng Wang
DTSTART:20200728T130000Z
DTEND:20200728T140000Z
DTSTAMP:20260404T111007Z
UID:GOBA/3
DESCRIPTION:Title: Individual ergodic theorems on von Neumann algebras\nby Simeng Wan
g as part of Groups\, Operators\, and Banach Algebras Webinar\n\n\nAbstrac
t\nBirkhoff’s celebrated individual ergodic theorem asserts that for a m
easure-preserving ergodic transformation on a measure space\, the time ave
rage is equal to the space average almost everywhere. Since the theory of
von Neumann algebras is a quantum analogue of the classical measure theory
\, it is natural to study similar individual ergodic theorems in the setti
ng of von Neumann algebras. The study was exactly initiated by Lance in 19
70s\, and witnessed fruitful progress in recent decades with the help of m
odern tools from the operator space theory\, such as the noncommutative ve
ctor-valued $L^p$-spaces studied by Pisier\, Junge and Xu. This talk aims
to give a gentle introduction to the aforementioned topic\, and if time pe
rmits\, we may also present some recent results in this direction\, in par
ticular ergodic theorems for some group actions on von Neumann algebras an
d for positive contractions on $L^p$-spaces\, which is joint work with Gui
xiang Hong\, Ben Liao and Samya Ray.\n
LOCATION:https://stable.researchseminars.org/talk/GOBA/3/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Thomas Gotfredsen
DTSTART:20200804T130000Z
DTEND:20200804T140000Z
DTSTAMP:20260404T111007Z
UID:GOBA/4
DESCRIPTION:Title: The quantised interval as a quantum metric space\nby Thomas Gotfre
dsen as part of Groups\, Operators\, and Banach Algebras Webinar\n\n\nAbst
ract\nThe study of metrics on state spaces arising from semi-norms dates b
ack to Connes and was formalised as the notion of a compact quantum metric
space by Rieffel\, whose notion of quantum Gromov-Hausdorff distance on t
he class of compact quantum metric spaces\, has established a new famework
for the study of approximations of C*-algebras. \n\nIn a recent paper\, A
guilar and Kaad have shown that the standard Podleś sphere\, originally i
ntroduced as the homogeneous space for Woronowicz' quantum SU(2)\, is in f
act a compact quantum metric space\, and they posed the rather natural que
stion\, whether the standard Podleś sphere converges to the standard 2-sp
here in the quantum analogues of the Gromov-Hausdorff distance as the defo
rmation parameter tends to 1 . \nIn my talk based on joint work with Jens
Kaad and David Kyed\, I will present some new developments to the above qu
estion. In particular we have shown that the commutative C*-subalgebras ge
nerated by the self-adjoint generator of the standard Podleś sphere\, con
verge to the interval of length \\pi as one would expect if the more gener
al convergence result is true\, and that the spaces in fact vary continuou
sly. This provides some evidence that the convergence result for the Podle
s spheres may hold true as well (this is currently work in progress).\n
LOCATION:https://stable.researchseminars.org/talk/GOBA/4/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Juan Orendain
DTSTART:20200811T130000Z
DTEND:20200811T140000Z
DTSTAMP:20260404T111007Z
UID:GOBA/5
DESCRIPTION:Title: Formal Haagerup standard form on infinite index morphisms of factors\nby Juan Orendain as part of Groups\, Operators\, and Banach Algebras W
ebinar\n\n\nAbstract\nThe Haagerup $L^2$-space construction\, introduced b
y Haagerup in the 70's\, associates a standard form to every von Neumann a
lgebra\, without any reference to weights\, and is thus regarded as a coor
dinate free version of the GNS construction. The Haagerup standard form an
d the Connes fusion tensor product organize von Neumann algebras and their
representations into a bicategory. The main interest on this bicategory c
omes from the fact that it encodes weak Morita equivalence as a formal hom
otopy relation.\n\nBicategories are a specific type of categorical structu
re of second order\, corresponding to globular sets. The second order cate
gorical structures corresponding to cubical sets are double categories. Re
sults studying relations between cubical and globular categories have been
obtained continually since the 60's\, mainly in nonabelian homotopy theor
y\, but more recently in areas ranging from algebraic geometry to network
theory. I will explain results of this type regarding the existence of two
non-equivalent double categories of representations of factors\, and how
these structures relate to questions of functoriality of the Haagerup stan
dard form and the Connes fusion tensor product.\n\nThis program builds on
work of Bartels\, Douglas and Hénriques on the theory of coordinate free
conformal nets and their relation to the Stolz-Teichner program.\n
LOCATION:https://stable.researchseminars.org/talk/GOBA/5/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Aasaimani Thamizhazhagan
DTSTART:20200818T130000Z
DTEND:20200818T140000Z
DTSTAMP:20260404T111007Z
UID:GOBA/6
DESCRIPTION:Title: On the structure of invertible elements in Fourier-Stieltjes algebras<
/a>\nby Aasaimani Thamizhazhagan as part of Groups\, Operators\, and Banac
h Algebras Webinar\n\n\nAbstract\nFor a locally compact abelian group $G$\
, J. L. Taylor (1971) gave a complete characterization of invertible eleme
nts in the measure algebra $M(G)$. Using the Fourier-Stieltjes transform\,
this characterization can be carried out in the context of Fourier-Stielt
jes algebras $B(G)$. We generalise this characterization to the setting of
the Fourier-Stieltjes algebra $B(G)$ of certain classes of locally compac
t groups\, in particular\, many totally minimal groups and the $ax+b$-grou
p.\n
LOCATION:https://stable.researchseminars.org/talk/GOBA/6/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Sam Kim
DTSTART:20200825T130000Z
DTEND:20200825T140000Z
DTSTAMP:20260404T111007Z
UID:GOBA/7
DESCRIPTION:Title: Why MIP* = RE implies not-CEP and Blackadar-Kirchberg's MF problem
\nby Sam Kim as part of Groups\, Operators\, and Banach Algebras Webinar\n
\n\nAbstract\nIn this expository talk\, we explain a direct route to why t
he results of Ji\, Natarajan\, Vidick\, Wright\, and Yuen give us a II$_1$
-factor that cannot embed into a tracial ultrapower of the separable hyper
finite II$_1$-factor $\\mathcal{R}$. More specifically\, we define a class
of finitely generated unital C*-algebras $C^*(\\mathcal{G})$ for a fixed
parameter $\\mathcal{G}$ with the following property: there exist extremal
traces $\\tau$ on $C^*(\\mathcal{G})$ such that the II$_1$-factor $\\math
cal{M}_\\tau$ generated by $C^*(\\mathcal{G})$ in the GNS representation o
f $\\tau$ has the property that there cannot exist a unital *-homomorphism
from $\\mathcal{M}_\\tau$ into a tracial ultrapower of $\\mathcal{R}$. We
describe ways in which convex geometry over $\\mathbb{R}^n$ will give us
such parameters $\\mathcal{G}$ and extremal traces $\\tau$. As a consequen
ce of our construction\, we have a separable counter-example of Blackadar-
Kirchberg's MF problem\, which asks whether every stably finite C*-algebra
embeds into a norm ultrapower of the UHF algebra $\\mathcal{Q}$. Question
s related to the refinement of both the MF conjecture and the refutation o
f CEP are raised at the end.\n
LOCATION:https://stable.researchseminars.org/talk/GOBA/7/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Mario Klisse
DTSTART:20201116T150000Z
DTEND:20201116T160000Z
DTSTAMP:20260404T111007Z
UID:GOBA/8
DESCRIPTION:Title: Topological boundaries of connected graphs and Coxeter groups\nby
Mario Klisse as part of Groups\, Operators\, and Banach Algebras Webinar\n
\n\nAbstract\nIn this talk we will present a method which allows to associ
ate certain topological spaces with connected rooted graphs. These spaces
reflect combinatorial and order theoretic properties of the underlying gra
ph and are particularly tractable in the case of Cayley graphs of finite r
ank Coxeter groups. In that context we speak of the compactification and t
he boundary of the Coxeter group. They have some desirable properties and
nicely relate to various other important constructions such as Gromov's hy
perbolic compactification\, the Higson compactification and Furstenberg bo
undaries of Coxeter groups.\n\nThe study of (certain) compactifications an
d boundaries of groups has lots of interesting operator algebraic applicat
ions. For instance\, they play a role in the rigidity theory of von Neuman
n algebras and are crucial in Kalantar-Kennedy's solution of the simplicit
y question for group C*-algebras. Our construction turns out to be closely
related to Hecke C*-/ and Hecke von Neumann algebras. These are operator
algebras associated with (Iwahori) Hecke algebras. We will discuss some im
plications of this connection.\n
LOCATION:https://stable.researchseminars.org/talk/GOBA/8/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Sanaz Pooya
DTSTART:20201123T150000Z
DTEND:20201123T160000Z
DTSTAMP:20260404T111007Z
UID:GOBA/9
DESCRIPTION:Title: Higher Kazhdan projections and Baum-Connes conjectures\nby Sanaz P
ooya as part of Groups\, Operators\, and Banach Algebras Webinar\n\n\nAbst
ract\nThe Baum-Connes conjecture\, if it holds for a certain group\, provi
des topological tools to compute the K-theory of its reduced group C*-alge
bra. This conjecture has been confirmed for large classes of groups\, such
as amenable groups\, but also for some Kazhdan's property (T) groups. Pro
perty (T) and its strengthening are driving forces in the search for poten
tial counterexamples to the conjecture. Having property (T) for a group is
characterised by the existence of a certain projection in the universal g
roup C*-algebra of the group\, known as the Kazhdan projection. It is this
projection and its analogues in other completions of the group ring\, whi
ch obstruct known methods of proof for the Baum-Connes conjecture. In this
talk\, I will introduce a generalisation of Kazhdan projections. Employin
g these projections we provide a link between surjectivity of the Baum-Con
nes map and the l²-Betti numbers of the group. A similar relation can be
obtained in the context of the coarse Baum-Connes conjecture. This is base
d on joint work with Kang Li and Piotr Nowak.\n
LOCATION:https://stable.researchseminars.org/talk/GOBA/9/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Chris Bruce (Queen Mary University of London and the University of
Glasgow)
DTSTART:20201130T160000Z
DTEND:20201130T170000Z
DTSTAMP:20260404T111007Z
UID:GOBA/10
DESCRIPTION:Title: C*-algebras from actions of congruence monoids\nby Chris Bruce (Q
ueen Mary University of London and the University of Glasgow) as part of G
roups\, Operators\, and Banach Algebras Webinar\n\n\nAbstract\nI will give
an overview of recent results for semigroup C*-algebras associated with n
umber fields. These results are already interesting in the case where the
field is the rational numbers\, and I will focus mostly on this case to ma
ke everything more explicit and accessible.\nC*-algebras of full ax+b-semi
groups over rings of algebraic integers were first studied by Cuntz\, Deni
nger\, and Laca\; their construction has since been generalized by conside
ring actions of congruence monoids. Semigroup C*-algebras obtained this wa
y provide an example class of unital\, separable\, nuclear\, strongly pure
ly infinite C*-algebras which\, in many cases\, completely characterize th
e initial number-theoretic data. They also carry canonical time evolutions
\, and the associated C*-dynamical systems exhibit intriguing phenomena. F
or instance\, the striking similarity between the K-theory formula and the
parameterization space for the low temperature KMS states\, observed by C
untz in the case of the full ax+b-semigroup\, persists in the more general
setting.\nPart of this work is joint with Xin Li\, and part is joint with
Marcelo Laca and Takuya Takeishi.\n
LOCATION:https://stable.researchseminars.org/talk/GOBA/10/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Maria Stella Adamo (Mathematical Research Institute of Oberwolfach
)
DTSTART:20201207T150000Z
DTEND:20201207T160000Z
DTSTAMP:20260404T111007Z
UID:GOBA/11
DESCRIPTION:Title: The problem of continuity for representable functionals on Banach qua
si *-algebras\nby Maria Stella Adamo (Mathematical Research Institute
of Oberwolfach) as part of Groups\, Operators\, and Banach Algebras Webina
r\n\n\nAbstract\nA way to study problems concerning quantum statistical me
chanics is to consider locally convex quasi *-algebras\, for which Banach
quasi *-algebras constitute a special class. For example\, Banach quasi *-
algebras can be obtained by taking the completion of a *-algebra $A_0$ wit
h respect to a norm $|| \\cdot ||$ for which the multiplication is (only)
separately continuous.\nIn the (locally convex) quasi *-algebras setting\,
a relevant role is played by representable functionals. Roughly speaking\
, a linear functional will be called representable if it allows a GNS-like
construction.\n\nIn this talk\, we discuss the problem of continuity for
these functionals and some related results. We begin our discussion by loo
king at the properties of representable (and continuous) functionals\, esp
ecially in the simplest case of Hilbert quasi *-algebras. This discussion
leads naturally to look at the problem of continuity for these functionals
. Hence\, we examine the approaches to study this problem. If time permits
\, we will discuss some applications.\nThe first part of the talk is joint
work with C. Trapani. The second part is joint work with M. Fragoulopoulo
u.\n
LOCATION:https://stable.researchseminars.org/talk/GOBA/11/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Max Arnott (University of Lancaster)
DTSTART:20201214T150000Z
DTEND:20201214T160000Z
DTSTAMP:20260404T111007Z
UID:GOBA/12
DESCRIPTION:Title: Kernels of bounded operators on transfinite Banach sequence spaces\nby Max Arnott (University of Lancaster) as part of Groups\, Operators\,
and Banach Algebras Webinar\n\n\nAbstract\nFor a Banach space $E$\, we as
k the following question: "Is it true that for every closed subspace $Y$ o
f $E$\, there exists some bounded linear operator $T : E \\to E$ for which
$Y= \\ker T$?"\n\nIn a recent paper by Niels Laustsen and Jared White\, i
t was proved that every separable Banach space answers the question in the
positive\, and that there exists a reflexive Banach space which answers t
he question in the negative.\n\nLet $\\Gamma$ be an uncountable cardinal.
In this talk we will investigate the above question for the transfinite Ba
nach sequence spaces $\\ell_p(\\Gamma)$ for $1\\leq p <\\infty$\, and $c_0
(\\Gamma)$. The question is answered in the negative for $\\ell_1(\\Gamma)
$\, and in the positive for $\\ell_p(\\Gamma)$ for $1 < p <\\infty$ and $c
_0(\\Gamma)$.\n
LOCATION:https://stable.researchseminars.org/talk/GOBA/12/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Eusebio Gardella
DTSTART:20210111T150000Z
DTEND:20210111T160000Z
DTSTAMP:20260404T111007Z
UID:GOBA/15
DESCRIPTION:Title: Group representations on $L^p$-spaces\nby Eusebio Gardella as par
t of Groups\, Operators\, and Banach Algebras Webinar\n\n\nAbstract\nFor a
given locally compact group\, we study group representations\n(by inverti
ble isometries) on $L^p$-spaces\, for $p \\in [1\,\\infty)$\, and the asso
ciated \nBanach algebras. For example\, the algebra associated to the left
regular \nrepresentation was first studied by Herz in the 70's\, and has
received renewed \nattention in the past two decades. There is also a "uni
versal" $L^p$-operator group\nalgebra. For $p=2$ one obtains the group C*-
algebras\, and the behaviour of these \nobjects for other values of p tend
to exhibit a mixture between the case p=2 and \nthe much more rigid case
of $L^1(G)$. I will give an overview of what is known and \nwhat questions
remain open.\n
LOCATION:https://stable.researchseminars.org/talk/GOBA/15/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Eirik Berge
DTSTART:20210118T150000Z
DTEND:20210118T160000Z
DTSTAMP:20260404T111007Z
UID:GOBA/16
DESCRIPTION:Title: Quantization on the affine group\nby Eirik Berge as part of Group
s\, Operators\, and Banach Algebras Webinar\n\n\nAbstract\nThe classical n
otion of quantization in Euclidean space -- using time and frequency shift
s -- has traditionally been of interest to people in classical analysis an
d theoretical physics. However\, it has become more apparent in recent yea
rs that understanding geometric and algebraic aspects of quantization is e
ssential. In this talk\, I will review (a version of) classical quantizati
on before moving on to the more recent phenomenon of quantization with tim
e shifts and dilations. As probably suspected\, this will naturally involv
e the affine group. I will show that the theory here is heavily influenced
by the non-unimodularity of the affine group. If time permits\, I will ta
lk about some recent work and open problems that are left in this setting.
The material presented is based on joint work with Stine M. Berge\, Franz
Luef\, and Eirik Skrettingland.\n
LOCATION:https://stable.researchseminars.org/talk/GOBA/16/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Priyanga Ganesan
DTSTART:20210125T150000Z
DTEND:20210125T160000Z
DTSTAMP:20260404T111007Z
UID:GOBA/17
DESCRIPTION:Title: Quantum graphs\nby Priyanga Ganesan as part of Groups\, Operators
\, and Banach Algebras Webinar\n\nAbstract: TBA\n
LOCATION:https://stable.researchseminars.org/talk/GOBA/17/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Samya Kumar Ray
DTSTART:20210201T150000Z
DTEND:20210201T160000Z
DTSTAMP:20260404T111007Z
UID:GOBA/18
DESCRIPTION:Title: Isometries between Schatten-$p$ classes\nby Samya Kumar Ray as pa
rt of Groups\, Operators\, and Banach Algebras Webinar\n\n\nAbstract\nIsom
etries between commutative and non-commutative $L_p$-spaces have a long hi
story which starts from the seminal work of Banach himself. However\, desp
ite many remarkable results and characterization theorems not much is know
n when finite dimensional Schatten-$p$ classes embed between each other. I
n this direction\, together with my collaborators\, I have some rigidity r
esults about the isometric embeddability of finite dimensional Schatten-$p
$ class. For example\, if $2 < p<\\infty$ and $T:\\ell_p^2\\to B(\\ell_2)$
is an isometry\, then $T (e_1)\,T(e_2) \\in B(\\ell_2) K(\\ell_2)$. We al
so have applications in the direction of operator spaces. Interestingly\,
our methods are completely new and use various concepts such as Birkhoff-J
ames orthogonality\, norm parallelism\, multiple operator integral and Kat
o-Rellich theorem in the perturbation of a linear operator.\n
LOCATION:https://stable.researchseminars.org/talk/GOBA/18/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Krishnendu Khan
DTSTART:20210208T150000Z
DTEND:20210208T160000Z
DTSTAMP:20260404T111007Z
UID:GOBA/19
DESCRIPTION:Title: Fundamental group of certain property (T) factors\nby Krishnendu
Khan as part of Groups\, Operators\, and Banach Algebras Webinar\n\n\nAbst
ract\nCalculation of fundamental group of type $II_1$ factor is\, in gener
al\, an extremely hard and central problem in the field of von Neumann alg
ebras. In this direction\, a conjecture due to A. Connes states that the f
undamental group of the group von Neumann algebra associated to any icc pr
operty (T) group is trivial. Up to now there was no single example of prop
erty (T) factor satisfying the conjecture. In this talk\, I shall provide
the first examples of property (T) group factors (arising from group theor
etic constructions) with trivial fundamental group. This talk is based on
a joint work with Ionut Chifan\, Sayan Das and Cyril Houdayer.\n
LOCATION:https://stable.researchseminars.org/talk/GOBA/19/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Dimitrios Gerontogiannis
DTSTART:20210215T150000Z
DTEND:20210215T160000Z
DTSTAMP:20260404T111007Z
UID:GOBA/20
DESCRIPTION:Title: Smooth algebras associated to Smale spaces and extensions by Schatten
ideals\nby Dimitrios Gerontogiannis as part of Groups\, Operators\, a
nd Banach Algebras Webinar\n\n\nAbstract\nIn the 1980's\, Douglas initiate
d the study of smooth extensions of C*-algebras\; C*-algebraic extensions
by the ideal of compact operators that on certain dense *-subalgebras (in
cases Banach) reduce to algebraic extensions by Schatten ideals. Douglas s
tudied smooth extensions of C(X)\, for X being a finite complex. Shortly a
fter\, Douglas and Voiculescu studied the case of sphere extensions. In th
e noncommutative setting\, examples of C*-algebras with a pervading presen
ce of smooth extensions include the Cuntz-Krieger algebras (Goffeng-Meslan
d)\, and crossed product C*-algebras formed by Gromov hyperbolic groups ac
ting on their boundary (Emerson-Nica). In this talk I will present the not
ion of smoothness in C*-algebras and that the smooth extensions of Ruelle
algebras (higher dimensional analogues of Cuntz-Krieger algebras) associat
ed to Smale spaces\, are generic in some sense. The smoothness of Ruelle a
lgebras has interesting connections with the Hausdorff dimension of the un
derlying Smale space. This research is part of my PhD thesis.\n
LOCATION:https://stable.researchseminars.org/talk/GOBA/20/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Sophie Emma Mikkelsen
DTSTART:20210222T150000Z
DTEND:20210222T160000Z
DTSTAMP:20260404T111007Z
UID:GOBA/21
DESCRIPTION:Title: On the quantum twistor bundle\nby Sophie Emma Mikkelsen as part o
f Groups\, Operators\, and Banach Algebras Webinar\n\n\nAbstract\nThe conc
ept of a quantum principal bundle is well established by now. However\, ge
neral (locally trivial) fiber bundles are much less understood in the nonc
ommutative setting.\nWe investigate a noncommutative sphere bundle from w
hat we call the quantum twistor bundle.It is constructed as a quotient of
the quantum instanton bundle of Bonechi\, Ciccoli and Tarlini $SU_q(2)\\to
S_q^7\\to S_q^4$ for a suitable circle action on the Vaksmann-Soibelman
quantum sphere $S_q^7$. It is an example of a locally trivial noncommutati
ve bundle fulfilling conditions of the framework proposed by Brzeziski and
Szymanski.\n
LOCATION:https://stable.researchseminars.org/talk/GOBA/21/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Ali Raad
DTSTART:20210301T150000Z
DTEND:20210301T160000Z
DTSTAMP:20260404T111007Z
UID:GOBA/22
DESCRIPTION:Title: Existence and Uniqueness of Canonical Cartan Subalgebras in Inductive
Limit C*-algebras\nby Ali Raad as part of Groups\, Operators\, and Ba
nach Algebras Webinar\n\n\nAbstract\nCartan subalgebras of C*-algebras hav
e witnessed major breakthroughs recently\, becoming the cornerstone of how
to build a bridge between C*-algebras on the one hand\, and geometric gro
up theory and topological dynamics on the other. As such\, existence and u
niqueness questions become crucial. \n\nIn this talk I will introduce the
notion of a Cartan subalgebra and discuss the question of existence and un
iqueness in the setting of inductive limit C*-algebras. Indeed Stratila an
d Voiculescu show in 1975 that AF-algebras admit a canonical Cartan subalg
ebra. I will provide a novel uniqueness result for the uniqueness of these
subalgebras in AF-algebras\, and also completely settle the question of e
xistence and uniqueness of canonical Cartan subalgebras in AI-algebras and
AT-algebras. If time permits\, I will generalize a theorem of Renault's t
hat gives a correspondence between Cartan pairs and C*-algebras of twisted
étale groupoids.\n
LOCATION:https://stable.researchseminars.org/talk/GOBA/22/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Gabriel Favre (University of Stockholm)
DTSTART:20210315T150000Z
DTEND:20210315T160000Z
DTSTAMP:20260404T111007Z
UID:GOBA/23
DESCRIPTION:Title: An Algebraic Characterization of the Type I Property for Ample Groupo
ids\nby Gabriel Favre (University of Stockholm) as part of Groups\, Op
erators\, and Banach Algebras Webinar\n\n\nAbstract\nI will discuss the ty
pe I property for second countable locally compact Hausdorff ample groupoi
ds. Loosely speaking\, the type I property says that the von Neumann algeb
ras generated by unitary representations are the simplest possible kind of
von Neumann algebras to understand.\nAfter developing a feel for this pro
perty\, the discussion will shift towards the noncommutative Stone duality
between ample groupoids and Boolean inverse semigroups. This duality is u
sed in a new characterization of the type I property for groupoids\, that
we obtained. This characterization will appear as the semigroup counterpar
t to a result of van Wyk. If time permits\, I will apply our result to alg
ebraically characterize discrete inverse semigroups of type I.\nThis is jo
int work with S. Raum.\n
LOCATION:https://stable.researchseminars.org/talk/GOBA/23/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Blake Green (University of Lancaster)
DTSTART:20210322T150000Z
DTEND:20210322T160000Z
DTSTAMP:20260404T111007Z
UID:GOBA/24
DESCRIPTION:by Blake Green (University of Lancaster) as part of Groups\, O
perators\, and Banach Algebras Webinar\n\nAbstract: TBA\n
LOCATION:https://stable.researchseminars.org/talk/GOBA/24/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Jacek Krajczok (IMPAN\, Warsaw)
DTSTART:20210426T140000Z
DTEND:20210426T150000Z
DTSTAMP:20260404T111007Z
UID:GOBA/25
DESCRIPTION:Title: On the von Neumann algebra of class functions on a compact quantum gr
oup\nby Jacek Krajczok (IMPAN\, Warsaw) as part of Groups\, Operators\
, and Banach Algebras Webinar\n\n\nAbstract\nA famous result of Pytlik sta
tes that the radial subalgebra in the group von Neumann algebra of a free
group on n>=2 generators is maximal abelian (MASA). One can study an analo
gue of this subalgebra - the von Neumann algebra generated by characters -
in a more general context of discrete quantum groups. By a result of Fres
lon and Vergnioux\, it is known that this algebra is MASA for the discrete
quantum group dual to the Kac-type orthogonal quantum group. I will show
that the situation is quite different when our compact quantum group is no
t of Kac type (equivalently\, the discrete dual is non-unimodular). The cr
ucial notion for our work is that of quasi-split inclusions.\nThis is a jo
int work with Mateusz Wasilewski.\n
LOCATION:https://stable.researchseminars.org/talk/GOBA/25/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Nick Manor (University of Waterloo)
DTSTART:20210503T140000Z
DTEND:20210503T150000Z
DTSTAMP:20260404T111007Z
UID:GOBA/26
DESCRIPTION:Title: Nonunital operator systems and noncommutative convexity\nby Nick
Manor (University of Waterloo) as part of Groups\, Operators\, and Banach
Algebras Webinar\n\n\nAbstract\nThe recent work on nc convex sets of David
son-Kennedy and Kennedy-Shamovich show that there is a rich interplay betw
een the category of operator systems and the category of compact nc convex
sets\, leading to new insights even in the case of C*-algebras. The categ
ory of nc convex sets are a generalization of the usual notion of a compac
t convex set that provides meaningful connections between convex theoretic
notions and notions in operator system theory. In this talk\, we present
a related duality theorem for norm closed self-adjoint subspaces of $B(H)$
. Using this duality\, we will describe various C*-algebraic and operator
system theoretic notions\, as well as a rich class of examples arising as
duals of well-understood operator systems. This is joint work with Matthew
Kennedy and Se-Jin Kim.\n
LOCATION:https://stable.researchseminars.org/talk/GOBA/26/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Alex Frei
DTSTART:20210517T140000Z
DTEND:20210517T150000Z
DTSTAMP:20260404T111007Z
UID:GOBA/27
DESCRIPTION:Title: Relative Cuntz-Pimsner algebras: a complete description of their latt
ice of gauge-invariant ideals\nby Alex Frei as part of Groups\, Operat
ors\, and Banach Algebras Webinar\n\n\nAbstract\nWe give a new\, systemati
c approach to the gauge-invariant uniqueness theorem describing all relati
ve Cuntz-Pimsner algebras\,\nand whence revealing a complete description o
f their gauge-invariant ideal lattice.\n\nFor this we start with a swift i
ntroduction to C*-correspondences\, in particular drawing a comparison to
Fell bundles.\n\nContinuing\, we provide a slightly deeper analysis of cov
ariances as well as their relation to kernels and quotients. With these ob
servations at hand\, we introduce the relevant reduction leading us to a s
uitable parametrization of relative Cuntz-Pimsner algebras\, and so reveal
ing a complete description of their gauge-invariant ideal lattice.\nOur pa
rametrization is a heuristic analog of Katsura's originally obtained one.\
n\nWith this at hand\, we arrive at the gauge-invariant uniqueness theorem
\, for all arbitrary gauge-equivariant representations.\n\nFrom here we mo
ve on to the analysis part of the program. We compute the covariances in t
he case of the Fock representation and its quotients. As a result\, we der
ive that the parametrization of relative Cuntz-Pimsner algebras introduced
above is also classifying. In other words\, we obtain a complete and intr
insic picture of the lattice of quotients\, and equivalently of their latt
ice of gauge-invariant ideals.\n\nIf time permits\, we finish off with the
next chapter on their induced Fell bundles and dilations\, as already inv
estigated by Schweizer.\n
LOCATION:https://stable.researchseminars.org/talk/GOBA/27/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Jordy van Velthoven
DTSTART:20210524T140000Z
DTEND:20210524T150000Z
DTSTAMP:20260404T111007Z
UID:GOBA/28
DESCRIPTION:Title: Completeness of coherent systems associated to lattices\nby Jordy
van Velthoven as part of Groups\, Operators\, and Banach Algebras Webinar
\n\n\nAbstract\nThe talk considers the relation between the spanning prope
rties of a lattice orbit of a square-integrable projective representation
and the associated lattice co-volume. Under a suitable compatibility condi
tion between the cocycle and the lattice\, the density theorem provides a
trichotomy that precisely describes the spanning properties of a given lat
tice orbit. For classes of Lie groups\, the interplay between the density
theorem and Perelomov’s completeness problem will be considered.\n
LOCATION:https://stable.researchseminars.org/talk/GOBA/28/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Mitch Haslehurst
DTSTART:20210607T140000Z
DTEND:20210607T150000Z
DTSTAMP:20260404T111007Z
UID:GOBA/29
DESCRIPTION:Title: Relative K-theory for C*-algebras and factor groupoids\nby Mitch
Haslehurst as part of Groups\, Operators\, and Banach Algebras Webinar\n\n
\nAbstract\nIt is a reasonable question to ask\, given some K-theory data\
, whether or not there is a groupoid whose C*-algebra has this K-theory da
ta. There has been substantial recent progress on this question\, notably
Xin Li's construction of Cartan subalgebras in classifiable C*-algebras as
well as work of Robin Deeley\, Ian Putnam\, and Karen Strung via minimal
dynamical systems. In this talk I will discuss an approach to this problem
using factor groupoids. I will begin with an overview of relative K-theor
y\, which is essential for computational purposes\, and proceed to describ
e some constructions of factor groupoids which provide a degree of control
over the K-theory of their C*-algebras.\n
LOCATION:https://stable.researchseminars.org/talk/GOBA/29/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Przemyslaw Ohrysko
DTSTART:20210531T140000Z
DTEND:20210531T150000Z
DTSTAMP:20260404T111007Z
UID:GOBA/30
DESCRIPTION:by Przemyslaw Ohrysko as part of Groups\, Operators\, and Bana
ch Algebras Webinar\n\nAbstract: TBA\n
LOCATION:https://stable.researchseminars.org/talk/GOBA/30/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Przemyslaw Ohrysko (University of Warsaw)
DTSTART:20210614T140000Z
DTEND:20210614T150000Z
DTSTAMP:20260404T111007Z
UID:GOBA/31
DESCRIPTION:Title: Inversion problem in measure and Fourier-Stieltjes algebras\nby P
rzemyslaw Ohrysko (University of Warsaw) as part of Groups\, Operators\, a
nd Banach Algebras Webinar\n\n\nAbstract\nLet $G$ be a locally compact Abe
lian group with its dual $\\widehat{G}$ and let $M(G)$ denote the\nBanach
algebra of complex-valued measures on $G$. The classical Wiener-Pitt pheno
menon\nasserts that the spectrum of a measure may be strictly larger than
the closure of the range\nof its Fourier-Stieltjes transform. In particula
r\, if $G$ is non-discrete\, there exists µ ∈ $M(G)$\nsuch that |$\\wid
ehat{\\mu}$(γ)| > c > 0 for every γ ∈ $\\widehat{G}$ but µ is not inv
ertible. In the paper [N]\, N.\nNikolski suggested the following problem.\
n\nProblem 1. Let µ ∈ $M(G)$ satisfy $\\|\\mu\\|$ ≤ 1 and |$\\widehat
{\\mu}$(γ)| ≥ δ for every γ ∈ $\\widehat{G}$. What is\nthe minimal
value of $\\delta_0$ assuring the invertibility of µ for every δ > $\\de
lta_0$? What can be said\nabout the inverse (in terms of δ)?\n\nIn my tal
k I show that $\\delta_0 =1/2$\nis the optimal value for the first questio
n (for non-discrete\n$G$). Also\, I will present a partial solution for th
e quantitative variant of the problem\n(second question): if all elements
of G (except the unit) are of infinite order then we can\ncontrol the norm
of the inverse for every δ > $\\frac{-1+\\sqrt{33}}{8}$. This improves t
he original\nresult of Nikolski: δ > $\\frac{1}{\\sqrt{2}}$.\nIf time per
mits I will present some generalizations of the aformentioned results for
FourierStieltjes algebras built on non-commutative groups.\nThe talk is ba
sed on a paper [OW] written in collaboration with Mateusz Wasilewski.\n
LOCATION:https://stable.researchseminars.org/talk/GOBA/31/
END:VEVENT
END:VCALENDAR