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From Higher Invariants

Regensburg low-dimensional geometry and topology seminar

The seminar takes place on Zoom (see information below) every Tuesday at 16:00 CET (that is 7am on the US west coast, 10am on the US east coast, 10pm in China and 11pm in Korea and Japan).

Talks are split into a 40 minute and a 20 minute part, with a 15 minute tea break in between (you have to brew your own tea).


Date Speaker Title Abstract
May 5 Peter Kronheimer (Harvard) Genus versus double-points for immersed surfaces Video of the talk If X is a simply-connected closed 4-manifold containing an oriented embedded surface S of genus g, is there always an immersed sphere S' which represents the same homology class and has only g transverse double-points? Colloquially, can we "trade handles for double points"? This is an open question, though a "relative" version of the question (concerning surfaces in the 4-ball bounding a given knot in the 3-sphere) is known to have a negative answer. For closed surfaces in closed 4-manifolds, a particularly interesting class of examples comes from algebraic geometry, and includes the question of whether two smooth quintic surfaces can intersect in a singular rational curve. We will explore whether gauge theory might be a tool that can be used to explore these questions.
May 12 Danny Calegari (University of Chicago) Taut foliations leafwise branch cover the 2-sphere A co-oriented foliation of an oriented 3-manifold is taut if and only if there is a map from the 3-manifold to the 2-sphere which is a branched cover when restricted to each leaf. I shall give two proofs of this theorem and explain its relation to theorems of Ghys and Donaldson.
May 19 Richard Webb (University of Manchester) Quasimorphisms on diffeomorphism groups I will explain how to construct an unbounded quasimorphism on the group of isotopically-trivial diffeomorphisms of a surface of positive genus. As a corollary the commutator length and fragmentation norm are both (stably) unbounded, which solves a problem of Burago--Ivanov--Polterovich. The proof uses a new hyperbolic graph on which these groups act by isometries, which is inspired by techniques from mapping class groups. This is joint work with Jonathan Bowden and Sebastian Hensel.
May 26 Zhenkun Li (MIT) tba tba
June 2 no talk (pentecoste)
June 9 Boyu Zhang (Princeton) tba tba
June 16 Gordana Matic (Univ. of Georgia/MPIM Bonn) tba tba
June 23 NN tba tba
June 30 Kathryn Mann (Cornell) tba tba
July 7 Mikhail Khovanov (Columbia University) Introduction to foam evaluation and its uses tba
July 14 Claudius Zibrowius (UBC Vancouver) tba tba

The program is also contained in the RLGTS Google Calendar. To receive announcements by email, sign up to our mailing list.


Zoom information

Zoom Meeting ID: 924 2829 6353

The password is the answer to the following riddle (lower case, singular, no article): As a topologist, what do you typically glue together from charts? (Hint: It's not an orbifold).

We encourage you to enable your video, for a more interactive atmosphere. The talks are recorded and made available on this webpage, including audio and video of questions from the audience.


Jonathan Bowden, Lukas Lewark, Raphael Zentner.