BEGIN:VCALENDAR
PRODID:calendar.unige.ch
VERSION:2.0
CALSCALE:GREGORIAN
X-WR-TIMEZONE:Europe/Zurich
BEGIN:VEVENT
DTSTART:20250925T141500Z
DTEND:20250925T141500Z
DTSTAMP:20260915T023948Z
UID:43615@calendar.unige.ch
CREATED:20250908T124935Z
LAST-MODIFIED:20250908T124935Z
SUMMARY:Fractional dimensions of critical loci (Maxim KONTSEVICH, IHES Paris)
DESCRIPTION:With a critical locus of a holomorphic function, one can associate a certain rational number that can be thought of as a "dimension." This dimension can be determined in different ways: via analysis, Hodge theory, or triangulated categories. Singularities with a dimension less than one are precisely the isolated simple ones classified by Arnold, which correspond to the simply laced Dynkin diagrams\n\n\nLe colloque sera suivi d'un apéritif
LOCATION:Salle 1-15, Colloque de mathématiques\n
TRANSP:TRANSPARENT
CATEGORIES:Colloque
CONTACT:
URL:http://agenda.unige.ch/events/view/43615
FREEBUSY;FBTYPE=FREE:
END:VEVENT
BEGIN:VEVENT
DTSTART:20251009T141500Z
DTEND:20251009T141500Z
DTSTAMP:20260915T023948Z
UID:43858@calendar.unige.ch
CREATED:20250926T102040Z
LAST-MODIFIED:20250926T104309Z
SUMMARY:Kakeya sets in R^3 (Hong WANG, IHES Paris)
DESCRIPTION:A Kakeya set is a compact subset of R^n that contains a unit line segment pointing in every direction.  Kakeya set conjecture asserts that every Kakeya set has Minkowski and  Hausdorff dimension n. We prove this conjecture in R^3 as a consequence of a more general statement about union of tubes.  \nThis is joint work with Josh Zahl.\n\nLe colloque sera suivi d'un apéritif
LOCATION:Salle 1-15, Colloque de mathématiques\n
TRANSP:TRANSPARENT
CATEGORIES:Colloque
CONTACT:
URL:http://agenda.unige.ch/events/view/43858
FREEBUSY;FBTYPE=FREE:
END:VEVENT
BEGIN:VEVENT
DTSTART:20251106T151500Z
DTEND:20251106T151500Z
DTSTAMP:20260915T023948Z
UID:44051@calendar.unige.ch
CREATED:20251014T095103Z
LAST-MODIFIED:20251014T095103Z
SUMMARY:A hunt for spectral gaps (Rostislav GRIGORCHUK, Texas A&M University)
DESCRIPTION:In  my  talk I  will  describe  an approach to studying spectra  of groups  and  graphs which originated thirty years ago during  my first visit  to Geneva  and was developed in collaboration with Laurent Bartholdi, Pierre de la Harpe, Tatiana Nagnibeda, Christophe Pittet and other colleagues. We will discuss such  topics  as graphs whose spectrum is a Cantor set,  pure point  spectrum  vs  continuous  spectrum,  the  question of Alain Valette "Can  one  hear  the  shape  of  a  group? ", Ramanujan  graphs  coming  from  automata  groups.\n\nLe colloque sera suivi d'un apéritif
LOCATION:Salle 1-15, Colloque de mathématiques\n
TRANSP:TRANSPARENT
CATEGORIES:Colloque
CONTACT:
URL:http://agenda.unige.ch/events/view/44051
FREEBUSY;FBTYPE=FREE:
END:VEVENT
BEGIN:VEVENT
DTSTART:20251218T151500Z
DTEND:20251218T151500Z
DTSTAMP:20260915T023948Z
UID:44441@calendar.unige.ch
CREATED:20251121T144553Z
LAST-MODIFIED:20251125T134609Z
SUMMARY:Non left-orderability of irreducible lattices in semi-simple Lie groups of rank at least two (Bertrand DEROIN, CNRS, Cergy Paris Université)
DESCRIPTION:I will report on the question of left-orderability of lattices in semi-simple Lie groups, and explain that when the rank is at least two, such lattices are not left-orderable, a result obtained in collaboration with Sebastian Hurtado.\n\nLe colloque sera suivi d'un apéritif
LOCATION:Salle 1-15, Colloque de mathématiques\n
TRANSP:TRANSPARENT
CATEGORIES:Colloque
CONTACT:
URL:http://agenda.unige.ch/events/view/44441
FREEBUSY;FBTYPE=FREE:
END:VEVENT
BEGIN:VEVENT
DTSTART:20260219T151500Z
DTEND:20260219T151500Z
DTSTAMP:20260915T023948Z
UID:45014@calendar.unige.ch
CREATED:20260205T084302Z
LAST-MODIFIED:20260205T084503Z
SUMMARY:Building surfaces from equilateral triangles (Lasse REMPE, University of Manchester)
DESCRIPTION:The construction of conformal maps of the earth, i.e. those that faithfully depict angles and orientations, has a long history. The most famous examples are given by stereographic projection (used since antiquity) and Mercator projection (constructed in 1569 and still used for internet maps today). One may also map a tetrahedron conformally to the sphere, taking care to specify what this means at the vertices. In 1965, cartographer L. P. Lee proposed using this classical fact as the basis for a map projection with less dramatic area distortion than the classical projections.\n\nWhile this has not caught on, it suggests the following mathematical question: Which orientable surfaces can be conformally represented on a collection of equilateral triangles? Equivalently, which such surfaces can be built (up to a conformal change of coordinate) by glueing together a finite or infinite collection of copies of a closed equilateral triangle? Such surfaces are called *equilaterally triangulable*. \n\nThe answer in the compact case is given by a famous classical theorem of Belyi, which states that a compact Riemann surface is equilaterally triangulable if and only if it is defined over a number field. These *Belyi surfaces* - and their associated “dessins d’enfants” - have found applications across many fields of mathematics, including mathematical physics.\n\nIn joint work with Chris Bishop, we give a complete answer of the same question for the case of infinitely many triangles (i.e., for non-compact Riemann surfaces). The talk should be accessible to a general mathematical audience, including postgraduate students.\n\nLe colloque sera suivi d'un apéritif
LOCATION:Salle 1-15, Colloque de mathématiques\n
TRANSP:TRANSPARENT
CATEGORIES:Colloque
CONTACT:
URL:http://agenda.unige.ch/events/view/45014
FREEBUSY;FBTYPE=FREE:
END:VEVENT
BEGIN:VEVENT
DTSTART:20260319T151500Z
DTEND:20260319T151500Z
DTSTAMP:20260915T023948Z
UID:45236@calendar.unige.ch
CREATED:20260220T081904Z
LAST-MODIFIED:20260310T101528Z
SUMMARY:Critical long-range percolation (Tom HUTCHCROFT, Caltech)
DESCRIPTION:It is conjectured that many models of statistical mechanics have a rich, fractal-like behaviour at and near their points of phase transition, with power-law scaling governed by critical exponents that are expected to depend on the dimension but not on the small-scale details of the model such as the choice of lattice. This is now reasonably well understood in two dimensions and in high dimensions, but remains poorly understood in intermediate dimensions (e.g. d=3). I will overview the conjectures around this area and describe recent progress on related problems for models with long-range interactions.\n\nLe colloque sera suivi d'un apéritif
LOCATION:Salle 1-15, Colloque de mathématiques\n
TRANSP:TRANSPARENT
CATEGORIES:Colloque
CONTACT:
URL:http://agenda.unige.ch/events/view/45236
FREEBUSY;FBTYPE=FREE:
END:VEVENT
BEGIN:VEVENT
DTSTART:20260423T141500Z
DTEND:20260423T141500Z
DTSTAMP:20260915T023948Z
UID:45239@calendar.unige.ch
CREATED:20260220T090009Z
LAST-MODIFIED:20260402T144426Z
SUMMARY:Inapproximability through undecidability (Thomas VIDICK, EPFL)
DESCRIPTION:Infinite combinatorial, algebraic or other structures, such as a graph, a group, or a probability space, can sometimes be approximated (in a suitable sense) by finite structures, and sometimes not. In the former case, finite approximability can provide a convenient proxy for studying the infinite structure; in the latter, inapproximability underscores a genuinely distinct behavior from the finite case. While the existence of finite approximations can follow from suitable approximation techniques, their in-existence often requires deeper conceptual observations. \n \nRecently we were able to resolve some long-standing inapproximability questions, in algebra and in ergodic theory, by showing that an associated computational problem is undecidable. Associating a measure of complexity to the infinite structure allows us to make use of methods from complexity theory and demonstrate the in-existence of finite approximations in an unexpected way. \n \nIn the talk we will discuss this "method" and show how we used it to answer the Connes Embedding Problem, about the existence of inapproximable classes of (type II_1) von Neumann algebras, and the Aldous-Lyons conjecture, about the existence of inapproximable (unimodular) infinite graphs. \n \nBased on joint works with Ji, Natarajan, Yuen and Wright, and Bowen, Chapman and Lubotzky. \n\nLe colloque sera suivi d'un apéritif
LOCATION:Salle 1-15, Colloque de mathématiques\n
TRANSP:TRANSPARENT
CATEGORIES:Colloque
CONTACT:
URL:http://agenda.unige.ch/events/view/45239
FREEBUSY;FBTYPE=FREE:
END:VEVENT
BEGIN:VEVENT
DTSTART:20260521T141500Z
DTEND:20260521T141500Z
DTSTAMP:20260915T023948Z
UID:45241@calendar.unige.ch
CREATED:20260220T092032Z
LAST-MODIFIED:20260507T085925Z
SUMMARY:A panorama of L^2-invariants (Wolfgang LÜCK, University of Bonn)
DESCRIPTION:Betti numbers of closed manifolds or finite simplicial complexes are classical invariants in algebraic  topology. Atiyah proposed an L^2-version obtained from the universal covering and the action of the fundamental group using von Neumann algebras.  \nWe will present the basic properties of these L^^2-Betti numbers without going into technical details and discuss interesting and striking applications to  topology, algebra, group theory, and geometry which are both interesting for and easy to explain to a general audience. If time allows, we will also introduce L^2-torsion which is the analogue of the classical Reidemeister torsion.\n\nLe colloque sera suivi d'un apéritif
LOCATION:Salle 1-15, Colloque de mathématiques\n
TRANSP:TRANSPARENT
CATEGORIES:Colloque
CONTACT:
URL:http://agenda.unige.ch/events/view/45241
FREEBUSY;FBTYPE=FREE:
END:VEVENT
BEGIN:VEVENT
DTSTART:20261015T141500Z
DTEND:20261015T141500Z
DTSTAMP:20260915T023948Z
UID:46615@calendar.unige.ch
CREATED:20260904T091659Z
LAST-MODIFIED:20260904T091659Z
SUMMARY:Algorithmic Problems for Recurrence Sequences (Ben WORELL, University of Oxford)
DESCRIPTION:Recurrence sequences arise across mathematics and theoretical computer science and some of their most basic properties lead to surprisingly difficult algorithmic questions.  For example: given a sequence of rational numbers defined by a linear recurrence and initial conditions, can we determine whether the sequence has a zero and/or whether all terms are nonnegative?\n\nIn this talk, we will survey these and related decision problems, and explain how they are related to topics in program analysis, dynamical systems, and combinatorics.  We start with the classical Skolem–Mahler–Lech theorem, which gives a structural description of the zeros of linear recurrence sequences but leaves open the question of effectively determining the zeros.\n\nI will discuss the current state of the art on the decidability of such problems, highlighting what is known and salient open questions.\nAlong the way, we  will explain how central results from Diophantine geometry and transcendence theory, such as the Subspace Theorem and the analytic subgroup theorem, enter the algorithmic theory of recurrence sequences.\n\nLe colloque sera suivi d'un apéritif
LOCATION:Salle 1-15, Colloque de mathématiques\n
TRANSP:TRANSPARENT
CATEGORIES:Colloque
CONTACT:
URL:http://agenda.unige.ch/events/view/46615
FREEBUSY;FBTYPE=FREE:
END:VEVENT
END:VCALENDAR