# Work of plenary speakers at ICM 2018

The next International Congress of Mathematicians (ICM) will be next year in Rio de Janeiro, Brazil. The present question is the 2018 version of similar questions from 2014 and 2010. Can you, please, for the benefit of others give a short description of the work of one of the plenary speakers?

List of plenary speakers at ICM 2018:

1. Alex Lubotzky (Israel)
2. Andrei Okounkov (Russia/USA)
3. Assaf Naor (USA)
4. Carlos Gustavo Moreira (Brazil)
5. Catherine Goldstein (France)
6. Christian Lubich (Germany)
7. Geordie Williamson (Australia/Germany)
8. Gil Kalai (Israel)
9. Greg Lawler (USA)
10. Lai-Sang Young (USA)
11. Luigi Ambrosio (Italy)
12. Michael Jordan (USA)
13. Nalini Anantharaman (France)
14. Peter Kronheimer (USA) and Tom Mrowka (USA)
15. Peter Scholze (Germany)
16. Rahul Pandharipande (Switzerland)
17. Ronald Coifman (USA)
18. Sanjeev Arora (USA)
19. Simon Donaldson (UK/USA)
20. Sylvia Serfaty (France/USA)
21. Vincent Lafforgue (France)
• Jun 16 '17 at 4:02
• Might I suggest that the answers focus on what the speakers are [probably] going to talk about? Otherwise the question seems too broad. (At the time of writing this comment, there are two answers, which both adhere to this suggestion.) Jun 17 '17 at 13:11
• Is Michael Jordan really going to attend the ICM 2018? Wow! Jun 21 '17 at 20:35
• "Might I suggest that the answers focus on what the speakers are [probably] going to talk about?" Dear Timothy, An answer might be very helpful for me :) . More seriously, I long had on my to do list to contribute two answers for the sister 2014 question. Jun 25 '17 at 14:32

Kronheimer and Mrowka have both spoken at the ICM before. Most likely, the current invitation is based on their proof that Khovanov homology detects the unknot (although they have other spectacular work since their previous ICM talks, such as the proof of Property (P)). The corresponding question for the Jones polynomial is a well-known open problem.

Kronheimer, P.B.; Mrowka, T.S., Khovanov homology is an unknot-detector, Publ. Math., Inst. Hautes Étud. Sci. 113, 97-208 (2011). ZBL1241.57017. MR2805599

The strategy of the proof is to consider (a modification of) the instanton Floer homology invariant of knots which (roughly) counts representations of the knot group to $SU(2)$ in which the meridian has trace $=0$. They show that this invariant is always non-trivial for non-trivial knots. This mimics a similar proof of non-triviality for knot Floer homology (defined by Rasmussen and Oszvath-Zsabo) by Juhasz, who showed that the highest grading of the knot Floer homology is sutured Floer homology of the complement of a minimal genus Seifert surface in the knot complement (using an adjunction inequality). Kronheimer and Mrowka had previously formulated an instanton version of sutured Floer homology so that they could mimic Juhasz's proof.

Then they show that there is a spectral sequence going from Khovanov homology to their knot instanton homology, and hence the Khovanov homology of non-trivial knots has rank at least 2. This part of the proof was modeled on a spectral sequence that Oszvath-Zsabo found from Khovanov homology to the Heegaard-Floer homology of the double branched cover. The proof of the existence of this spectral sequence is based on the TQFT-like properties of the instanton knot invariant for cobordisms between knots by surfaces in 4-manifolds, which they develop further in this paper.

Vincent Lafforgue's work span many topics and contain many striking results but the most probable recent work to be described in the spirit of the question is Chtoucas pour les groupes réductifs et paramétrisation de Langlands globale. In slogan form, this work proves the Langlands correspondence in the "automorphic to Galois" direction for reductive group over global function field of positive characteristic.

More precisely, let $F$ be the function field of a smooth, projective, geometrically irreducible curve $X$ over $\mathbb F_{p}$, let $\mathbb A$ be its adele ring and let $G$ be a connected, reductive group over $F$ assumed split for simplicity. For $N$ a finite sub-scheme of $X$, denote by $K_N$ the compact, open subgroup of $G(\mathbb A)$ equal to the kernel of $G(\mathbb O)\rightarrow G(\mathcal O_N)$ where $\mathbb O$ is the product of the unit balls of the local fields $F_v$ and $\mathcal O_N$ is the ring of functions on $N$. Denote by $Z$ the center of $G$ and fix $\Xi$ a lattice inside $Z(F)\backslash Z(\mathbb A)$. Finally fix $E$ a finite extension of $\mathbb Q_\ell$ where $\ell\nmid p$.

Then the finite-dimensional $E$ vector space $$C^{\operatorname{cusp}}_c(G(F)\backslash G(\mathbb A)/K_N\Xi,E)$$ of cuspidal functions admits a direct-sum decomposition indexed by global Langlands parameter $$\sigma:\operatorname{Gal}(\bar{F}/F)\longrightarrow \widehat{G}(E)$$ with values in the Langlands dual group. This decomposition is compatible with the Satake isomorphism.

In addition to the result itself, the method he introduced (the so-called excursion operators) looks very promising even in the characteristic zero case.

Let's say something about what this method entails. The standard strategy, going back at least to Eichler-Shimura, for proving statements of this type is to study the cohomology groups of some space (in the classical case, a modular curve or Shimura variety, and in the function field case, a moduli space of Shtukas) and show that it admits a Galois action and an action of Hecke operators, and then show that the Hecke eigenspace is a Galois representation arising from a Langlands parameter $\sigma:\operatorname{Gal}(\bar{F}/F)\longrightarrow \widehat{G}(E)$ composed with some fixed representation of $\widehat{G}(E)$.

This works quite well when $G = GL_n$, and was used by Vincent's brother Laurent Lafforgue to great effect in that case. However, there are some difficulties for other groups. If we pick out the subspace of the cohomology corresponding to a particular automorphic form, it may be difficult to show that the Galois action factors through the group $\hat{G}$, or it might factor through the group $\hat{G}$ in multiple indistinguishable ways.

V. Lafforgue solves this by working simultaneously with the cohomology of a huge array of spaces, in which Langlands parameters are expected to appear via different representations. He defines several different maps between the cohomology of these different spaces. Composing these maps appropriately, he defines excursion operators on the original space $C^{cusp}_c$ of automorphic forms. These operators include the Hecke operators, but are not limited to them. They satisfy some relations, forming a ring, whose characters he checks by abstract group theory correspond to Langlands parameters. The compatibility with Satake can be established by comparing these operators to the classical Hecke operators.

• @SylvainJULIEN: It seems pretty presumptuous to assume that Vincent learned the subject from his brother, and kind of insane to say that he is not an expert on it. Jun 16 '17 at 17:16
• Actually, it seems that he pretty much learned the subject by himself : he explains in this interview to the CIRM that as he was working at CNRS, he had no obligation to teach or to ask for funding, which allowed him to switch from non-commutative geometry to working on the Langlands program. His brother getting the MF does not imply that he is a lesser mathematician. Here is the interview : youtube.com/watch?v=xuhIjDi5_LAprogramm. Jun 16 '17 at 17:32
• This message describes the Langlands facet of V.Lafforgue's work, but the previous facet, notably the works on Baum-Connes conjecture and on the strengthened Property T, is equally recognized (by an essentially disjoint community).
– YCor
Jun 17 '17 at 0:36
• Sylvain JULIEN's comment was no doubt intended as a compliment, but its unfortunate phrasing illustrates one of the barriers facing anyone who wants to switch fields (see these MO questions: mathoverflow.net/questions/69937/… and mathoverflow.net/questions/12684/switching-research-fields ). The moral seems to be: (1) it is definitely possible to switch fields and do research of the highest caliber in your new field and (2) to the extent possible, you should ignore any sociological labels and assumptions that people attach to you. Jun 17 '17 at 14:05
• Thank you Timothy. My phrasing was probably infortunate indeed, as my English is far from being perfect. I indeed meant that Vincent Lafforgue was not known to be an expert in Langlands program related issues though there is no doubt he's now one. Jun 17 '17 at 21:37

Nalini Anantharaman is a french mathematician working in the fields of dynamical systems, partial differential equations and mathematical physics.

Her early works deal with the counting of closed geodesics on hyperbolic surfaces in a given homology class. She gave a full asymptotic expansion for the counting function following dynamical methods introduced by D. Dolgopyat, when the homology class lies in the interior of the set of winding cycles of the invariant measures of the geodesic flow. She also gave estimates when the homology class is in the boundary of this set. In that case, the problem is connected to the zero temperature limit in the theory of Markov processes and maximizing measures in the Aubry-Mather theory of Lagrangian systems.

She then got interested in semi-classical analysis and used entropy methods to study the weak limits of the sequence of probability measures $$|\psi_k|^2 d\hbox{vol}$$ where $\psi_k$ are the eigenfunctions of the Laplacian defined on a negatively curved compact manifold. The quantum unique ergodicity conjecture asserts that the sequence should converge to the Liouville measure after a suitable lift of the measures to the unit tangent bundle of the manifold. She showed that any cluster points of the sequence must have positive entropy, thus ruling out a convergence to the Dirac mass on some closed orbit. So this is a remarkable application of ergodic theory to the study of the linear wave equation and Schrodinger equation. See a survey of P. Sarnak for additional details.

More recently, she studied related problems on billiards and regular graphs. Interesting pictures of cardioid billiards can be found in her joint work with Arnd Backer. Her webpage contains a few of her lectures in video format.