Posted by: matheuscmss | October 6, 2016

## Jota-Cê …

Last September 3, Jean-Christophe Yoccoz passed away after a long battle against illness. He was an incredible man whose huge influence on this blog and, more generally, on my mathematical life, is certainly evident to some readers.

Nevertheless, as my close friends know, Jean-Christophe’s influence on me goes way beyond Mathematics, and, after spending the last month trying to cope with this terrible loss, I believe that today I’m now ready from the emotional point of view to talk a little bit about the extraordinary role model he provided to me.

Since there is no doubt that Jean-Christophe’s mathematical work will be always a recurring theme in this blog, for now I prefer to postpone all purely mathematical discussions/hommages. Instead, this post contains some souvenirs’ of my 9 years of friendship with Jean-Christophe as a way to pay a modest tribute for all fantastic and unforgettable moments (both inside and outside Mathematics) shared with him.

1. Prologue

I saw Jean-Christophe for the first time during the International Conference on Dynamical Systems held at IMPA in July 2000. Indeed, Jean-Christophe entered the auditorium Ricardo Mañé accompanied by his son (coincidently born in 1984 like me) and wife to deliver a talk about his joint work (of 217 pages) with Jacob Palis (published in 2009).

I remember that I got lost after the first 5 minutes of Jean-Christophe’s lecture, mainly because at that time my mathematical background (of a first year PhD student) was not appropriate.

Remark 1 Retrospectively, I find it funny that, even though I could not understand this talk at the time, I would join Jean-Christophe and Jacob in 2009 in the investigation of some open questions left open by their paper.Indeed, I started working with them on this subject by accident: I was present at Collège de France when the offprints of this paper of Jean-Christophe and Jacob arrived by regular mail, and we started right away to “celebrate” it by drinking a coffee while informally discussing the problems left open in this work; at some point, our informal conversation became serious because we noticed an argument allowing to improve the information on the stable sets of the so-called non-uniformly hyperbolic horseshoes.

After this initial success, Jean-Christophe laughed out while saying (as a joke) that we should ask the journal to stop printing the article so that we could add to it an appendix with our brand new result, and, in a more serious tone, he proposed to use this “low-hanging fruit” as the beginning of a separate article (still in prepartion …) on this topic.

2. My first conversation with Jean-Christophe

After finishing my PhD in 2004, I started looking for post-doctoral positions. In 2006, Jean-Christophe was visiting IMPA (and also attending ICMP 2006), and my friend Gugu (an IMO gold medalist who finished his PhD at the age of 20 years-old) strongly encouraged me to ask Jean-Christophe whether he could eventually supervise my post-doctoral sejour’ in France.

Despite Gugu’s advice, I was hesitating to approach Jean-Christophe. In fact, even though a transition from Brazil to France would be relatively smooth under Jean-Christophe’s supervision (partly because his wife was Brazilian and, thus, he spoke Portuguese fluently), I was not sure that I could handle the pressure of becoming the third Brazilian post-doctoral student of a brilliant French mathematician whose expectations about Brazilian post-doctoral students were very high: indeed, his previous experience with Brazilians was to supervise the post-doctoral sejours’ of Gugu himself and Artur Avila.

After many conversations with Gugu and my wife, I finally got sufficiently motivated to knock the door of Jean-Christophe’s temporary office at IMPA, introduce myself and ask him to be my post-doctoral supersivor: after all, at worst I would simply get a no’ as an answer.

So, I knocked the door of his office and he called me in (by saying Oui, entrez’). I told him that I was looking for a post-doctoral advisor, I asked if he could advise me, and, when I was about to give him more details about the kind of Mathematics I have done so far, he gave me a big smile and he said: “There is no need for formal presentations: Gugu talked to me about you and it would be an honor to me to be your post-doctoral advisor.”

Such a kind and humble response was certainly not what I was expecting. In particular, I was still under the shock’ when I told him that I was very happy to hear his answer. Then, I quickly left the office, and my wife (who was waiting for me) saw my puzzled face and asked me: “So, what he said? Do you think the conversation went well?”. Here, I simply replied: “I’m not sure, but I think the conversation went well: if I heard it correctly, he told that it would be an honor to him to be my advisor.”

Of course, these two traits (kindness and humbleness) of Jean-Christophe’s personality are well-known among those who met him: in particular, Gugu was not surprised by his answer to me (and this is probably why Gugu insisted that I should talk to Jean-Christophe in the first place!).

3. My post-doctoral sejour’ in France

Several bureaucratic details made that I started my post-doctoral sejour’ in France in September 17, 2007, i.e., almost one year after my first conversation with Jean-Christophe.

Since the first day of my post-doctoral sejour’, Jean-Christophe was always very kind to me. For example, he offered me an office next to his own office, so that we could drink coffee, have lunch together and talk (in Portuguese) about many topics (including Mathematics) regularly.

Also, he would never hesitate to stop his research activities to help me with daily problems (e.g., openning a bank account to get my first salary): indeed, his constant support made that my first two years in France were quite smooth and, of course, this gave me the time needed to learn French.

Remark 2 Here, he gave the following precious advice: I would learn French more easily by seeing the news on TV because it is easier to absorb the information when just one person is talking calmly at a reasonably constant pace (indeed, this is what he did to learn Portuguese). Moreover, he put me in close contact with the secretaries, so that I would be forced to practice my French (because it was not possible to shift to Portuguese with them).

On the other hand, despite all his attention towards me, he left me completely free during the first year of my post-doctoral sejour’. In fact, as he told me later, he thought that the post-doctoral sejour’ was an important moment to develop our own ideas about Mathematics and, hence, it was not a good idea to impose to me any specific problem / research topic. In particular, he would limit himself to ask what I was doing recently and periodically invite me to give a talk in the Eliasson-Yoccoz seminar (so that the community would know what I was working on).

Consequently, we would start working in our first joint paper only in 2008…

4. Our first collaboration

Upon my return to Paris from a conference in Trieste (in August 2008), I talked to Jean-Christophe about the recently discovered example by Giovanni Forni and myself — later baptized Ornithorynque by Vincent Delecroix and Barak Weiss — of a translation surface with peculiar properties.

He got interested by the subject, and I started to explain to him the main features of the example: “the symmetries of this example are very particular because the Hodge structure on its cohomology …” After letting me end the description of the example, Jean-Christophe replied that, even though my explanation was mathematically correct, he was not happy because it does not allow to put your hands in the example’. In fact, his phrase (who made a profound impact on me) was: “I don’t like to work on extremely abstract theories with highly sophisticated arguments. I prefer to understand things from a concrete point of view, by working with many concrete examples before reaching the final result. In particular, my tactics’ is to cover the ground slowly via basic examples before exploring general theories.”

So, he thought that it could be a good idea to work together on a paper giving a explanation for the example in such a way that it could reveal more examples (ideally infinite families) with similar properties.

Frankly speaking, I also found that the original (Hodge-theoretical) arguments obtained with Giovanni had the drawback that they did not allow one to touch’ the example, and, for this reason, I accepted Jean-Christophe’s offer to investigate more closely the Ornithorynque.

The first meetings related to our work in the Ornithorynque were quite curious: while he would never refuse to meet anytime in the morning (9h, 8h, 7h30, …), we usually would stop our discussions by 3:30 PM or 4 PM because, as he liked to say, by this time “he had runned out of energy”.

At some point, I asked him what time he used to arrive at his office (so that I could try to maximize the span of our conversations). He simply smiled and said: “Normally I wake up around 4:30 or 5 AM, I take the first train and I arrive here around 6 AM.” (Of course, this explains why he could meet me in the morning at any time.) In fact, thanks to his metabolism, Jean-Christophe just needed to sleep about 6 hours per night.

After struggling a bit with my own metabolism, I managed to adapt myself to Jean-Christophe’s rythm and this lead us to a small competition’ to know how would arrive first at the office. Normally, I would arrive by 6:15 AM at Collège de France and, evidently, Jean-Christophe would be waiting for me with a bottle of fresh coffee (that he had prepared a few minutes ago at the kitchen of Collège), so that we would drink coffee together and talk about the latest news (on a variety of subjects: politics, chess, soccer, etc.) before starting our mathematical conversations. (In general, we would talk from 7 AM to 11:30 AM, take a break to have lunch together, and then come back to work until 3:30 PM or 4 PM.) However, it happened a couple of times that I managed somehow to arrive first in Collège: in every such occasion, Jean-Christophe would spend a couple of minutes explaining why he was late’. Indeed, this was the natural attitude to him because Jean-Christophe was someone who liked to do his best in everything regardless it was a small thing’ (e.g., arriving before me at Collège) or a big thing’ (e.g., proving theorems).

Our work on the Ornithorynque project’ was going well: the concrete approach of Jean-Christophe (computing tons of particular cases [including multiplying many 4×4 matrices by hand] at an extremely fast pace [which always forced me to be extremely concentrated to be able to follow him up…] for several hours in a row, always keeping an eye for symmetries’ to reduce the sizes of calculations, etc.) introduced me to a whole new way of doing Mathematics. In particular, there is no doubt that my own vision of Mathematics completely changed by seeing so closely how the mind of a brilliant mathematician works. Moreover, despite the enormous differences in our mathematical skills, Jean-Christophe was always open to hear my ideas and suggestions (and I will always be grateful to him for such an humble attitude towards me).

The rapid and steady progress in dissecting the Ornithorynque’ made that I did not want to completely stop working on it during the summer vacations in August 2009. So, I had no doubts that I should accept Jean-Christophe’s invitation to spend 10 days in his vacations house in Loctudy (in French Brittany). Since the vacations are sacred in France (it is usually very hard to contact French friends in August …), Jean-Christophe said that the amount of mathematical work in these 10 days would be decided by chance’, or, more accurately, by the weather. More precisely, we did the following agreement: we would start our day around 5h30 and work until 8h30; if the weather was nice (i.e., not raining too much) outside, we would stop talking about Mathematics and we would go for a walking in the forest or a boat trip in nearby islands, etc.; otherwise, we stay at home working on the Ornithorynque.

In principle, this agreement meant that we would work about ${50\%}$ of the time because the weather in French Brittany is very unstable: one might see several seasons’ within a given day …. However, as Jean-Christophe pointed out later, I was extremely lucky that it rained only in three days, so that we had plenty of leisure: we played pétanque, visited Île-de-Sein, etc.

The photos below illustrate such moments (during my second visit [in 2011] to Loctudy): the first picture displays Jean-Christophe plotting a strategy to improve the performance of our participation in a pétanque tournement in Loctudy (but my bad skills in this game made that we were kicked out of the tournement after three matches) and the next two photos show him in Île-de-Sein.

Remark 3 In Loctudy’s pétanque club, almost everyone knew that the well-known joke that you can give mathematicians a hard time by asking them to split a bill in a restaurant’ did not apply to Jean-Christophe. In fact, the standard procedure’ in the end of the tournement was to go to the bar, ask for a drink and wait for Jean-Christophe to compute (within 2 or 3 minutes) the rankings for the 42 or so participants, sizes of prizes, etc. Also, I overheard a conversation between a newcomer asking an organizer of the tournement: the newcomer was puzzled why everybody went take a drink instead to doing laborious calculations for setting rankings of participants, and the organizer smiled and said: “There is no need to worry. That guy over there will take care of all calculations in 2 minutes, and he knows what he is doing: indeed, he is a sort of Nobel prize’ in Mathematics…”

5. Some anecdotes of Jean-Christophe

Closing this post, I would like to share some anecdotes about Jean-Christophe: in fact, instead of describing more details of our subsequent collaborations, I believe that the reader might get a better idea of Jean-Christophe’s personality via a few anecdotes about other aspects of his life.

Jean-Christophe participated twice in Samba school parades in Rio de Janeiro, and he always laughed at the fact he had all kinds of experiences in these parades: in his first participation, his Samba school won the parade, but in his second participation, his Samba school got the last place…

Jean-Christophe liked to laugh at the fact that if Internet existed before, then the failed attempts of many countries to name the bird Turkey’ after its country of origin could have been avoided: indeed, the bird is called Turkey‘ in English, ‘Dinde‘ (a variant of D’Inde’, i.e., from India’) in French, Peru‘ in Portuguese, but the bird is originally Mexican.

Jean-Christophe was a strong chess player: his ELO rating was 2200+ at some point and we followed together many chess tournements via Chessbomb Arena (with particular attention to TCEC, a tournement for chess engines) during some breaks in our mathematical conversations, and he has many interesting anecdotes involving chess.

For example, he wrote here that his PhD advisor (Michel Herman) was so afraid that Jean-Christophe was spending too much time with Chess instead of Mathematics that he called Jean-Christophe’s parents for a conversation about his worries.

Also, once I invited Jean-Christophe, Gugu and Artur Avila for a barbecue at the vacations house of my wife’s family in Vargem Grande, Rio de Janeiro. At some point, we decided to play chess and, as usual, Gugu wanted to play against Jean-Christophe. For some reason, we thought that it could be a good idea to ask Jean-Christophe to play blindfold (with Artur and I moving the pieces in his place), while Gugu would play normally. After 25 moves or so, Gugu announces his move. Jean-Christophe takes a deep breath, spend a couple of minutes thinking, and then asks: Gugu, are you sure about your move?’. Gugu says yes’ and Jean-Christophe announces a checkmate in a couple of moves. Gugu is puzzled by Jean-Christophe’s claim, so he looks at the board and he comes up with the genius phrase in response to his blindfold friend: Is it already checkmate? I could not see it!’ Of course, Jean-Christophe was right and, after laughing for a couple of minutes at Gugu’s phrase, we passed to the next match (Jean-Christophe versus me…).

Jean-Christophe liked sports in general: during his youth, he played rugby and he sailed together with his friends from École Normale Supérieure, and he followed on TV and newspapers many tournements (soccer, rugby, golf, snooker, …). In particular, we saw together several finals of the UEFA Champions League while drinking his favorite beer (namely, Guinness) and he followed on the hospital’s TV the Rio 2016 Olympic Games (which was especially interesting to him because he knew Rio’s Zona Sul’ [where rowing and cycling took place]).

Finally, Jean-Christophe was an avid reader and an erudite person: he read (in French, English and Portuguese) many books and novels per month, he saw many movies, and he liked to go to museums. For example:

• he was happy when he learned that I read The Murders in the Rue Morgue (partly because Edgar Allan Poe was one of his favorite writers),
• he borrowed me Crimes Cèlebres by Alexandre Dumas and Ne le dis à personne (French version of a thriller novel) by Harlan Corben, so that I could improve my French,
• besides the classical in Brazilian literature (by Machado de Assis, José de Alencar, etc.), Jean-Christophe liked Agosto’ by Rubem Fonseca. In fact, Jean-Christophe told me that he read the original book in Portuguese because he was not sure that the French translation was very good: indeed, the title of Agosto’ — literally August’ in English — was translated as Un été brésilien‘ — a Brazilian summer’ — reveals that the editors probably did a job of questionable quality since it is winter (and not summer) in Brazil in August …
• he was a big fan of Alfred Hitchock (among many other film directors): in particular, I will always remember with affection a Hitchcock session’ in Jean-Christophe’s vacations house in Loctudy.
• after trying to read (unsuccessfully) À la recherche du temps perdu‘ during his youth, Jean-Christophe spent the last months of his life (doing Mathematics and) re-reading all seven volumes of Proust’s novel …

6. Epilogue

Since a picture is worth more than a thousand words, I’m sharing below a couple of photos of Jean-Christophe (holding my daughter, next to my wife [during vacations in French Brittany], working with me at Mittag-Leffler institute on our joint paper with Martin Moeller, and together with Gugu, Artur, Fernando Codá Marques, Susan Schommer and myself in Artur’s appartment, respectively).

Posted by: matheuscmss | August 3, 2016

## Yoccoz proof of Jakobson theorem II

Last time, we introduced the notion of regular parameter ${c\in[-2,0)}$ of the quadratic family ${P_c(x) = x^2+c}$ and we saw that the orbits of ${P_c}$ have a nice statistical description when ${c}$ is regular. In particular, this reduced our initial goal (of proving Jakobson’s theorem) to show that regular parameters are abundant near ${-2}$, i.e.,

$\displaystyle \lim\limits_{\varepsilon\rightarrow 0} \frac{\textrm{Leb}(\{c\in[-2,-2+\varepsilon]; c \textrm{ is regular}\})}{\varepsilon} = 1 \ \ \ \ \ (1)$

As it turns out, Yoccoz’s proof of (1) is indirect: first, he introduces the notion of strongly regular parameter and he proves that strongly regular parameters are a special case of regular parameters; secondly, he exploits the nice features of strong regularity to transfer some key properties from the phase space ${x\in\mathbb{R}}$ to the parameter space ${c\in[-2,0)}$ in order to prove that

$\displaystyle \lim\limits_{\varepsilon\rightarrow 0} \frac{\textrm{Leb}(\{c\in[-2,-2+\varepsilon]; c \textrm{ is strongly regular}\})}{\varepsilon} = 1 \ \ \ \ \ (2)$

Today, we shall define strong regularity and prove the regularity of such parameters (while leaving the proof of (2) for the final post of this series).

1. Some preliminaries

1.1. Quick review of the regularity property

For ${c\in [-2,0)}$, ${P_c(x):=x^2+c}$ has two fixed points ${\alpha=\alpha(c)}$ and ${\beta=\beta(c)}$ with ${-\beta<\alpha<0}$. Note that the critical value ${c=P_c(0)}$ belongs to ${[-\beta,\alpha)}$.

In a certain sense, the key idea is to study the dynamics of ${P_c}$ via certain intervals (“Yoccoz puzzle pieces”) bounded by points in the pre-orbits of ${\pm\alpha}$.

For example, the notion of regular parameter was defined with the aid of the intervals ${A:=[\alpha,-\alpha]}$ and ${\widehat{A}:=[\alpha^{(1)},-\alpha^{(1)}]}$ where ${\alpha^{(1)}=\alpha^{(1)}(c)\in (-\beta,\alpha)}$ is given by ${P_c(\alpha^{(1)}) = -\alpha}$. Indeed, ${c\in [-2,0)}$ is regular if there are ${C>0}$ and ${\theta>0}$ such that

$\displaystyle \textrm{Leb}(\{x\in A: x \textrm{ is not } n\textrm{-regular}\})\leq C e^{-\theta n}$

for all ${n\in\mathbb{N}}$. Here, ${x}$ is called ${n}$regular if there are ${0 and an interval ${\widehat{J}\ni x}$ such that ${P_c^m}$ sends ${\widehat{J}}$ diffeomorphically onto ${\widehat{A}}$ in such a way that ${P_c^m(x)\in A}$. For later use, we denote by ${g_J}$ the inverse branch of ${P_c^m}$ restricted to ${\widehat{J}}$.

In general, any ${n}$-regular point belongs to a regular interval of order ${m\leq n}$, that is, an interval ${J\subset [-\beta,\beta]}$ possessing an open neighborhood ${\widehat{J}\supset J}$ such that ${P_c^m}$ sends ${\widehat{J}}$ diffeomorphically onto ${\widehat{A}}$ in such a way that ${P_c^m(J)=A}$. In other words, the set of ${n}$-regular points is the union of regular intervals of orders ${\leq n}$.

It is easy to describe regular intervals (“Yoccoz puzzle pieces”) in terms of the pre-orbits of ${\pm\alpha}$. In fact, denote by ${\Delta_n=\Delta_n(c):=P_c^{-n-1}(\{\alpha\})}$ (so that ${\Delta_0=\{\alpha,-\alpha\}}$ and ${\Delta_1=\{\pm\alpha,\pm\alpha^{(1)}\}}$). It is not difficult to check that if ${J=[\gamma^-,\gamma^+]}$ is a regular interval of order ${m}$ and ${\widehat{J}=(\widehat{\gamma}^-,\widehat{\gamma}^+)}$ is the associated neighborhood, then ${\gamma^-<\gamma^+}$ are consecutive points of ${\Delta_m}$ and ${\widehat{\gamma}^-<\gamma^-<\gamma^+}$ are consecutive points of ${\Delta_{m+1}}$.

1.2. Dynamically meaningful partition of the parameter space

For later use, we organize the parameter space ${[-2,0)}$ as follows. For each ${M\in\mathbb{N}}$, we consider a maximal open interval ${(c^{(M)}, c^{(M-1)})}$ such that ${P_c^M(0)}$ is the first return of ${0}$ to ${A}$ under ${P_c}$ for all ${c\in(c^{(M)}, c^{(M-1)})}$.

In analytical terms, we can describe the sequence ${c^{(m)}}$ as follows. For ${c\in [-2,0)}$, let ${\alpha^{(0)} = \alpha^{(0)}(c)}$ be ${\alpha^{(0)}:=\alpha}$ and, for ${m>0}$, define recursively ${\alpha^{(m)}=\alpha^{(m)}(c)}$ as

$\displaystyle P_c(\alpha^{(m)})=-\alpha^{(m-1)}, \quad \alpha^{(m)} < 0$

In these terms, ${c^{(m)}}$ is the solution of the equation ${c=P_c(0)=\alpha^{(m-1)}(c)}$.

Remark 1 From this analytical definition of ${c^{(m)}}$, one can show inductively that ${P_c^M(0)\in A}$ for ${c\in (c^{(M)},c^{(M-1)})}$ along the following lines.By definition, ${\frac{\partial\alpha^{(m)}}{\partial c} = -\frac{1}{2\alpha^{(m)}} (1+\frac{\partial\alpha^{(m-1)}}{\partial c})}$. This inductive relation can be exploited to give that ${1/3\leq \frac{\partial\alpha^{(m)}(c)}{\partial c}\leq 1/2}$ for all ${m\in\mathbb{N}}$ and ${c\in [-2,-3/2]}$.

This estimate allows us to show that the function ${c\mapsto P_c(0) - \alpha^{(m-1)}(c)}$ has derivative between ${1/2}$ and ${2/3}$ for ${c\in[-2,-3/2]}$. Since this function takes a negative value ${-2-\alpha^{(m-1)}(-2)<0}$ at ${c=-2}$ and a positive value ${\alpha^{(m-2)}(c^{(m-1)})-\alpha^{(m-1)}(c^{(m-1)})>0}$ at ${c=c^{(m-1)}}$, we see that this function has a unique simple zero ${c^{(m)}\in (-2, c^{(m-1)})}$ such that ${P_c(0)\in [-2,\alpha^{(m-1)}(c)]}$ for ${c\in [-2,c^{(m)}]}$, as desired.

Remark 2 Note that ${c^{(m)}}$ is a decreasing sequence such that ${\frac{1}{C 4^m}\leq c^{(m)}+2\leq \frac{C}{4^m}}$ for some universal constant ${C>0}$. Indeed, the function ${c\mapsto P_c(0)-\alpha^{(m-1)}(c)}$ takes the value ${-2-\alpha^{(m-1)}(-2)=-4\sin^2\frac{\pi}{3 2^m}}$ at ${c=-2}$ (cf. Subsection 4.2 of the previous post), it vanishes at ${c=c^{(m)}}$, and it has derivative between ${1/2}$ and ${2/3}$, so that ${1/1000\leq 4^m(c^{(m)}+2)\leq 1000}$.

From now on, we think of ${c\in (c^{(M)}, c^{(M-1)})}$ where ${M}$ is a large integer.

2. Strong regularity

Given ${c\in (c^{(M)}, c^{(M-1)})}$, let ${\mathcal{J}}$ be the collection of maximal regular intervals of positive order contained in ${A}$ and consider

$\displaystyle W := \bigcup\limits_{J\in\mathcal{J}} \textrm{int}(J),$

${N:W\rightarrow\mathbb{N}}$ the function ${N(x)=\textrm{order}(J)}$ for ${x\in J}$, and ${T:W\rightarrow A}$ the map ${T(x)=P_c^{N(x)}(x)}$ (${=P_c^{\textrm{order of } J}(x)}$ for ${x\in J}$): cf. Subsection 4.3.3 of this post here.

Remark 3 Even though ${0}$ is not contained in any element of ${\mathcal{J}}$, we set ${N(0)=M}$ and ${T(0)=P_c^M(0)}$ for ${c\in (c^{(M)}, c^{(M-1)})}$.

The elements of ${\mathcal{J}}$ of “small” orders are not hard to determine. Given ${1\leq n < M}$, define ${\widetilde{\alpha}^{(n)}\in\Delta_n}$ by:

$\displaystyle P_c(\widetilde{\alpha}^{(n)}) = \alpha^{(n-1)}, \quad \widetilde{\alpha}^{(n)} < 0$

It is not difficult to check that the sole elements of ${\mathcal{J}}$ of order ${1 are the intervals

$\displaystyle C_n^+ := [\widetilde{\alpha}^{(n-1)}, \widetilde{\alpha}^{(n)}] \quad \textrm{and} \quad C_n^- = [-\widetilde{\alpha}^{(n)}, -\widetilde{\alpha}^{(n-1)}]$

and, furthermore, any other element of ${\mathcal{J}}$ has order ${>M}$.

The intervals ${C_n^{\pm}}$, ${1, are called simple regular intervals: this terminology reflects the fact that they are the most “basic” type of regular intervals.

In this setting, a parameter ${c}$ is strongly regular if “most” of the returns of ${\{P_c^j(0)\}_{j\in\mathbb{N}}}$ to ${W}$ occur on simple regular intervals:

Definition 1 We say that ${c\in (c^{(M)}, c^{(M-1)})}$ is strongly regular up to level ${K}$ if ${P_c^M(0):=T(0)\in \bigcap\limits_{k=0}^{K-1} T^{-k}(W)}$ and, for each ${1\leq k\leq K}$, one has

$\displaystyle \sum\limits_{\substack{0<\ell\leq k \\ N(T^{\ell}(0))>M}} N(T^{\ell}(0)) \leq 2^{-\sqrt{M}} \sum\limits_{\ell=1}^k N(T^{\ell}(0)) \ \ \ \ \ (3)$

A parameter ${c}$ is called strongly regular if it is strongly regular of all levels ${K\in\mathbb{N}}$.

Remark 4 Let ${c\in (c^{(M)}, c^{(M-1)})}$ be a strongly regular parameter. It takes a while before ${\{P_c^j(0)\}_{j\in\mathbb{N}}}$ encounters a non-simple regular interval: if ${N(T^k(0))>M}$ (or, equivalently, ${T^k(0)\in (-\widetilde{\alpha}^{(M-2)}, \widetilde{\alpha}^{(M-2)})}$), then (3) implies that

$\displaystyle N(T^k(0))\leq \frac{2^{-\sqrt{M}}}{1-2^{-\sqrt{M}}}(N_k-M)$

where ${N_{k+1}:=\sum\limits_{\ell=0}^{k} N(T^{\ell}(0)) = M + \sum\limits_{\ell=1}^k N(T^{\ell}(0))}$. In particular, ${N_{k+1}\geq 2^{\sqrt{M}} M}$, so that the first ${2^{\sqrt{M}} M}$ iterates of ${0}$ encounter ${A}$ exclusively at simple regular intervals.

3. Regularity of strongly regular parameters

Let us now outline the proof of the fact that strongly regular parameters are regular.

Posted by: matheuscmss | June 21, 2016

## Symbolic dynamics for non-uniformly hyperbolic surface maps with discontinuities

Yuri Lima and I have just uploaded to ArXiv our paper “Symbolic dynamics for non-uniformly hyperbolic maps with discontinuities”. The main motivation for our paper is the question of extending the celebrated (Brin prize) work of Sarig on symbolic models/Markov partitions for smooth surface diffeomorphisms to the context of billiard maps: indeed, the main result of our paper is a partial solution to a problem appearing in page 346 of Sarig’s article.

An interesting corollary of our results is a refinement of a theorem of Chernov on the number of periodic points of certain billiard maps. More precisely, for a certain class of billiard maps ${f:M\rightarrow M}$, Chernov proved that

$\displaystyle \liminf\limits_{n\rightarrow\infty}\frac{1}{n}\log\#\textrm{Per}_n(f)\geq h$

where ${\textrm{Per}_n(f)}$ is the set of periodic points of ${f}$ with period ${n}$ and ${h}$ is the Kolmogorov-Sinai entropy of the Liouville measure. From our main results, Yuri and I can show that the billiard maps ${f:M\rightarrow M}$ studied by Chernov actually satisfy:

$\displaystyle \exists\, C>0, \, p\in\mathbb{N} \textrm{ such that } \#\textrm{Per}_{np}(f)\geq C e^{hnp} \, \forall \, n\in\mathbb{N}$

Remark 1 Our improvement of Chernov’s theorem is “similar in spirit” to Sarig’s improvement of Katok’s theorem on the number of periodic points for smooth surface diffeomorphisms: see Theorem 1.1 in Sarig’s paper for more details.

Below the fold, we give a slightly simplified version of the main result in our paper and we explain some steps of its proof.

Posted by: matheuscmss | June 6, 2016

## Zorich conjecture for hyperelliptic Rauzy-Veech groups

Artur Avila, Jean-Christophe Yoccoz and I have just uploaded to ArXiv our paper Zorich conjecture for hyperelliptic Rauzy-Veech groups.

This is the first installment of a series of two articles on the Kontsevich-Zorich cocycle over certain ${SL(2,\mathbb{R})}$-invariant loci in moduli spaces of translation surfaces obtained from cyclic cover constructions (inspired from the works of Veech and McMullen).

More precisely, the second paper of this series (still in preparation) studies the Kontsevich-Zorich cocycle over ${SL(2,\mathbb{R})}$-orbits of certain cyclic covers of translation surfaces in hyperelliptic components called ${\mathcal{H}(2g-2)^{hyp}}$ and ${\mathcal{H}(g-1,g-1)^{hyp}}$ in the literature. (The curious reader can find more explanations about this forthcoming paper in this old blog post here [cf. Remark 7].)

Of course, before studying the cyclic covers, we need to obtain some good description of the Kontsevich-Zorich cocycle on the hyperelliptic components and this is the purpose of the first article of the series.

Since the first paper of this series is not long, this post will just give a brief “reader’s guide” rather than entering into the technical details.

Remark 1 In the sequel, we will assume some familiarity with translation surfaces.

1. Rauzy-Veech groups and Zorich conjecture

The starting point of our article is a description of certain combinatorial objects — called hyperelliptic Rauzy diagrams — coding the dynamics of the Kontsevich-Zorich cocycle on ${\mathcal{H}(2g-2)^{hyp}}$ and ${\mathcal{H}(g-1,g-1)^{hyp}}$.

Remark 2 By the time that the first version of our article was written, we thought that we found a new description of these diagrams, but Pascal Hubert kindly pointed out to us that G. Rauzy was aware of it.

One important feature of this description of hyperelliptic Rauzy diagrams is the fact that we can order these diagrams by “complexity” in such a way that two consecutive diagrams can be related to each other by an inductive procedure.

The behavior of the Kontsevich-Zorich cocycle (with respect to Masur-Veech measures) is described in general by the Rauzy-Veech algorithm: roughly speaking, this algorithm is a natural way to attach matrices (acting on homology groups) to the arrows of Rauzy diagrams, and, in this language, the action of the Kontsevich-Zorich cocycle is just the multiplication of the matrices attached to concatenations of arrows of Rauzy diagrams.

In particular, the features of the Kontsevich-Zorich cocycle (with respect to Masur-Veech measures) can be derived from the study of the so-called Rauzy-Veech groups, i.e., the groups generated by the matrices attached to the arrows of a given Rauzy diagram. For example, the celebrated paper of Avila and Viana solving affirmatively a conjecture of Kontsevich and Zorich proves the simplicity of the Lyapunov exponents of the Kontsevich-Zorich cocycle by establishing (inductively) the pinching and twisting properties for Rauzy-Veech groups.

In our article, we exploit the “inductive” description of hyperelliptic Rauzy diagrams to compute the hyperelliptic Rauzy-Veech groups.

Remark 3 Our arguments for the computation of hyperelliptic Rauzy-Veech groups were inspired from the calculations of ${U(p,q)}$-blocks of the Kontsevich-Zorich cocycle over cyclic covers in the second paper of this series. In other words, we first developed some parts of the second paper before writing the first paper.

An interesting corollary of this computation is the fact that hyperelliptic Rauzy-Veech groups are explicit finite-index subgroups of the symplectic groups ${\textrm{Sp}(2g,\mathbb{Z})}$, so that they are Zariski dense in ${\textrm{Sp}(2g,\mathbb{R})}$.

The Zariski density of (general) Rauzy-Veech groups in symplectic groups was conjectured by Zorich (see, e.g., Remark 6.12 in Avila-Viana paper) as a step towards the Kontsevich-Zorich conjecture established by Avila-Viana. Therefore, the previous paragraph means that Zorich conjecture is true for hyperelliptic Rauzy-Veech groups (and this justifies our choice for the title of our paper).

Here, it is worth to point out that Zorich conjecture asks more than what is needed to prove the Kontsevich-Zorich conjecture. Indeed, the Zariski denseness in symplectic groups imply the pinching and twisting properties of Avila-Viana, so that Zorich conjecture implies Kontsevich-Zorich conjecture. On the other hand, we saw in this previous blog post that a pinching and twisting group of symplectic matrices might not be Zariski dense: in other words, the techniques of Avila-Viana solve the Kontsevich-Zorich without addressing Zorich conjecture.

Thus, our proof of Zorich conjecture for hyperelliptic Rauzy-Veech groups gives an alternative proof of this particular case of Avila-Viana theorem.

2. Braid groups and A’Campo theorem

After a preliminary version of our article was complete, Martin Möller noticed some similarities between our characterization of hyperelliptic Rauzy-Veech groups and a result of A’Campo on the images of certain monodromy representations associated to hyperelliptic Riemann surfaces.

As it turns out, this is not a coincidence: we showed that the elements of the hyperelliptic Rauzy-Veech group associated to certain elementary loops on hyperelliptic Rauzy diagrams are induced by Dehn twists lifting the generators of a braid group; hence, this permits to recover our description of hyperelliptic Rauzy-Veech groups from A’Campo theorem.

Remark 4 In a certain sense, the previous paragraph is a sort of sanity test: the same groups found by A’Campo were rediscovered by us using different methods.

Closing this short post, let me point out that this relationship between loops in hyperelliptic Rauzy diagrams and Dehn twists in hyperelliptic surfaces reveals an interesting fact: the fundamental groups of the combinatorial model (hyperelliptic Rauzy diagram) coincides with the orbifold fundamental groups of ${\mathcal{H}(2g-2)^{hyp}}$ and ${\mathcal{H}(g-1,g-1)^{hyp}}$. In other words, the hyperelliptic Rauzy diagrams “see” the topology of objects (${\mathcal{H}(2g-2)^{hyp}}$ and ${\mathcal{H}(g-1,g-1)^{hyp}}$) coded by them.

Posted by: matheuscmss | May 29, 2016

## Yoccoz proof of Jakobson theorem I

Almost twenty years ago, Jean-Christophe Yoccoz gave several lectures (at ETH and Collège de France, for instance) on a proof of Jakobson theorem (based on the so-called Yoccoz puzzles), and he distributed hard copies of his lecture notes whenever they were requested (for example, he gave me such a copy when I became his post-doctoral student in 2007).

More recently, he gave a couple of lectures at Collège de France aiming to generalize his proof of Jakobson theorem to obtain some results improving upon the Wang-Young theory of rank one attractors.

Of course, his lectures (whose videos are available at his website) started by recalling several elements of his proof of Jakobson theorem and I decided to take the opportunity to write a series of posts about Jean-Christophe’s proof of Jakobson theorem (based on his hand-written lecture notes from 1997).

For the first installment of this series, we’ll review some aspects of the dynamics of the one-dimensional quadratic maps and, after that, we’ll state Jakobson’s theorem, describe the three main steps of Yoccoz proof of Jakobson theorem and implement the first step of the strategy.

1. Introduction

The dynamics of one-dimensional affine maps ${F_{a,b}(z)=az+b}$ is fairly easy understand. The change of variables provided by the translation ${T_c(z)=z+c}$ transforms ${F_{a,b}}$ into ${(T_c^{-1}\circ F_{a,b}\circ T_c)(z) = az + ((a-1)c+b)}$. If ${a\neq 1}$, the choice ${c=-b/(a-1)}$ shows that the affine map ${F_{a,b}}$ is conjugated to its linear part ${F_{a,0}(z)=az}$. If ${a=1}$, ${F_{a,b}(z)=z+b}$ is a translation.

In other words, the dynamics of polynomial maps ${P:\mathbb{C}\rightarrow\mathbb{C}}$ is not very interesting when ${\textrm{deg}(P)=1}$. On the other hand, we shall see below that quadratic maps are already sufficiently complicated to produce all kinds of rich dynamical behaviors.

The quadratic family is ${P_c(z)=z^2+c}$ where ${z\in\mathbb{C}}$ and ${c\in\mathbb{C}}$.

Remark 1 This family is sometimes presented in the literature as ${c-z^2}$ or ${cz(1-z)}$. As it turns out, these presentations are equivalent to each other: indeed, the affine change of variables ${h(z)=Az+B}$ converts ${P_c}$ into

$\displaystyle (h^{-1}\circ P_c\circ h)(z) = Az^2+2Bz+\frac{B^2+c-B}{A}$

The dynamics of ${P_c}$ near infinity is easy to understand: in fact, since ${P_c(z)\approx z^2}$ for ${|z|\gg 1}$, one has that ${\infty}$ attracts the orbit ${\{P_c^n(z)\}_{n\in\mathbb{N}}}$ of any ${z\in\mathbb{C}}$ with ${|z|\gg 1}$.

This means that the interesting dynamics of ${P_c}$ occurs in the filled-in Julia set:

$\displaystyle K(c):=\{z\in\mathbb{C}: P_c^n(z)\not\rightarrow\infty \textrm{ as } n\rightarrow\infty\}$

Note that ${K(c)}$ is totally invariant, that is, ${P_c(K(c)) = K(c) = P_c^{-1}(K(c))}$. Also, ${K(c)}$ is a compact set because ${|z|>R}$ implies that ${|P_c(z)|>R|z|-|c|>(R-\frac{|c|}{R})|z|}$ and ${R-\frac{|c|}{R}>1}$ whenever ${R>\frac{1+\sqrt{1+4|c|}}{2}:=R_c}$, so that

$\displaystyle K(c)=\bigcap\limits_{n\in\mathbb{N}} P_c^{-n}(\mathbb{D}_{R_c}(0))$

(where ${\mathbb{D}_R(0):=\{z\in\mathbb{C}: |z|\leq R\}}$).

Moreover, ${K(c)\neq\emptyset}$ because it contains all periodic points of ${P_c}$ (i.e., all solutions of the algebraic equations ${P_c^n(z)=z}$, ${n\in\mathbb{N}}$).

Remark 2 ${K(c)}$ is a full compact set, i.e., ${\mathbb{C}-K(c)}$ is connected: indeed, this happens because the maximum principle implies that a bounded open set ${U\subset \mathbb{C}}$ with boundary ${\partial U\subset K(c)}$ must be completely contained in ${K(c)}$ (i.e., ${U\subset K(c)}$).

The dynamics of ${P_c}$ on ${K(c)}$ is influenced by the behaviour of the orbit ${\{P_c^n(0)\}_{n\in\mathbb{N}}}$ of the critical point ${0\in\mathbb{C}}$. More precisely, let us consider the Mandelbrot set

$\displaystyle M:=\{c\in\mathbb{C}:P_c^n(0)\not\rightarrow\infty\}:=\{c\in\mathbb{C}: 0\in K(c)\}$

Posted by: matheuscmss | March 16, 2016

## Counting torus fibrations on a K3 surface (after S. Filip)

Last week, Simion Filip gave the talk “Counting torus fibrations on a K3 surface” at Université Paris 11 (Orsay). This post is a transcription of my notes from his lecture and, of course, all typos/mistakes are my sole responsibility.

1. Introduction

A classical problem in Dynamical Systems is the investigation of closed trajectories in billiards in polygons.

In the case of rational polygons (i.e., polygons whose angles belong to ${\mathbb{Q}\pi}$), it is known that closed trajectories are abundant. A popular way to establish this fact passes through the procedure of unfolding a rational polygon into a flat surface: roughly speaking, instead of letting the trajectories reflect on the boundary of the polygon, we reflect (finitely many times) the boundaries of the polygon in order to obtain straight line trajectories on a flat surface. See this excellent survey of Masur-Tabachnikov for more details.

For our purposes, we recall that a flat surface is the data ${(X,\omega,\Omega)}$ of a Riemann surface ${X}$, a non-trivial holomorphic ${1}$-form ${\Omega}$ and the flat metric ${\omega}$ thought as the Kähler form ${\omega=\frac{i}{2}\Omega\wedge\overline{\Omega}}$. Note that ${\Omega}$ has zeros, i.e., ${\omega}$ has (conical) singularities, whenever ${X}$ has genus ${>1}$ (by Riemann-Roch theorem).

Some of the key features concerning closed trajectories in flat surfaces are:

• closed geodesics of the flat metric come in families of parallel trajectories called cylinders in the literature;
• such closed geodesics occur in a dense set of directions in the unit circle ${S^1}$;
• Eskin and Masur showed the following asymptotics for the problem of counting cylinders: there exists a constant ${c>0}$ such that the number of (cylinders of) closed trajectories of length ${\leq L}$ is ${\sim c\cdot L^2}$.

The goal of this post is to generalize this picture to higher dimensions or, more precisely, to K3 surfaces.

2. K3 surfaces

Definition 1 A compact complex ${2}$-dimensional manifold ${X}$ is a K3 surface if

• (i) ${X}$ admits a (global) nowhere vanishing holomorphic ${2}$-form ${\Omega}$;
• (ii) the first Betti number ${b_1(X)}$ is zero (this is equivalent to ${\pi_1(X)=\{0\}}$ in this context).

Two basic examples of K3 surfaces are:

Example 1 Quartic surfaces in ${\mathbb{P}^3(\mathbb{C})}$, i.e., ${\{F=0\}\subset \mathbb{P}^3(\mathbb{C})}$, ${F}$ is a polynomial of degree ${\textrm{deg}(F)=4}$.

Example 2 (Kummer examples) Let ${A=\mathbb{C}^2/\Lambda}$ be a complex torus (i.e., ${\Lambda}$ is a lattice of ${\mathbb{C}^2}$). Then, ${A_0= A/\{\pm\textrm{Id}_A\}}$ has a subset $\textrm{sing}$ of ${16}$ singular points, and the blow-up ${X=Bl_{\textrm{sing}}(A_0)}$ is a K3 surface.

Some basic properties of K3 surfaces include:

• all K3’s are diffeomorphic;
• all K3’s are Kähler (Siu);
• ${H^2(X,\mathbb{Z})}$ has rank ${22}$, the Hodge intersection form has signature ${(3,19)}$ on ${H^2(X,\mathbb{Z})}$, and ${H^2(X,\mathbb{Z})}$ is an even unimodular lattice;
• the Hodge decomposition of ${H^2(X,\mathbb{C})}$ is ${H^2(X,\mathbb{C}) = H^{2,0}\oplus H^{1,1}\oplus H^{0,2}}$, where ${H^{2,0}=\mathbb{C}\cdot\Omega}$ has rank ${1}$ and signature ${(1,0)}$, ${H^{1,1}}$ has rank ${20}$ and signature ${(1,19)}$, and ${H^{0,2}=\mathbb{C}\cdot\overline{\Omega}}$ has rank ${1}$ and signature ${(1,0)}$;
• the data in the previous two items determine the K3 surface (by Torelli theorem).

See, e.g., the lecture notes of D. Huybrechts for more details.

3. Special Lagrangian submanifolds

The natural generalization of closed trajectories on flat surfaces are special Lagrangian submanifolds (SLags).

Definition 2 Let ${(X, \omega, \Omega)}$, ${X}$ is a compact complex ${n}$-dimensional manifold, ${\omega}$ is a Kähler form, ${\Omega}$ is a holomorphic ${n}$-form. A real ${n}$-dimensional submanifold ${L\subset X}$ is a special Lagrangian submanifold (SLag) if

• ${L}$ is Lagrangian, i.e., ${\omega|_{L}=0}$;
• ${L}$ is special, i.e., ${\Omega|_{L} = d \textrm{Vol}_{\textrm{Riemannian}}}$.

Remark 1 Special Lagrangians are minimal submanifolds (in the sense that they minimize the volume in their cohomology class).

The next example justifies the claim that special Lagrangian submanifolds are the analog of closed trajectories on flat surfaces.

Example 3 Consider the case ${n=1}$, i.e., ${X}$ is a flat surface. In this situation, all real ${1}$-dimensional submanifolds are Lagrangian. On the other hand, since ${\Omega}$ is locally ${dz}$ (away from its divisors) in this setting, we see that a special Lagrangian is a horizontal geodesic of the flat metric. In particular, if we replace ${\Omega}$ by ${e^{i\theta}\Omega}$, then the SLags become the straight line trajectories at angle ${\theta}$ on ${X}$.

In a similar vein, special Lagrangian fibrations are the analog of cylinders of closed horizontal trajectories on flat surfaces.

4. Special Lagrangian fibrations

Definition 3 A fibration ${X\rightarrow B}$ of ${X}$ over a real ${n}$-dimensional base ${B}$ is SLag if its fibers are compact ${n}$-dimensional SLags submanifolds of ${X}$. The volume ${V\in\mathbb{R}_+}$ of such a fibration is ${V=\int_L\Omega}$ where ${L}$ is any fiber of ${X\rightarrow B}$.

Remark 2 The fibers of SLag fibrations are torii. One can compare this with the Arnold-Liouville theorem saying that the fibers of a fibration ${M\rightarrow B}$ of a symplectic manifold ${M}$ by compact Lagrangian submanifolds are necessarily torii. In particular, the base ${B}$ has an integral affine structure whose structural group is the semi-direct product of ${GL(\mathbb{Z}^n)}$ by ${\mathbb{Z}^n}$.

Remark 3 Similarly to the case ${n=1}$, a typical K3 surface doesn’t admit a SLag fibration.

K3 surfaces possess a significant amount of relevant structures. For example, a particular case of Yau’s solution to Calabi’s conjecture says that:

Theorem 4 (Yau) Let ${X}$ be a K3 surface equipped with a Kähler form ${\omega}$. Then, there exists an unique ${\omega'}$ in the same cohomology class of ${\omega}$ in ${H^2(X,\mathbb{R})}$ such that ${\omega'}$ induces a Ricci-flat metric.

Moreover, K3 surfaces ${X}$ with Ricci-flat metrics ${g}$ are hyperKähler manifolds, i.e., they admit three complex structures ${I, J, K}$ such that ${(X,g,I,J,K)}$ has the following properties:

• ${g}$ is a Ricci-flat Riemannian metric;
• ${I, J, K}$ are complex structures satisfying the usual quaternionic relations: ${IJ=-JI=K}$;
• ${I, J, K}$ are compatible with ${g}$: the forms ${\omega_{\ast}=g(\dot, \ast\dot)}$ are closed (i.e. ${d\omega_{\ast}=0}$) for ${\ast=I, J, K}$.

Equivalently, we can write the data ${(X,g,I,J,K)}$ of a hyperKähler manifold as ${(X,\omega,\Omega)}$ where ${\omega=\omega_I}$ and ${\Omega = \omega_J+i\omega_K}$. In this way, we obtain a presentation of K3 surfaces bearing some similarities with our definition of flat surfaces.

5. Statement of the main result

In this setting, we change direction of SLag fibrations (in analogy with the case of closed trajectories in flat surfaces) through the notion of twistor families. More concretely, we consider the sphere ${\mathbb{S}^2:=\{(x,y,z)\in\mathbb{R}^3: x^2+y^2+z^2=1\}}$ and we denote by

$\displaystyle \mathfrak{X} = X\times \mathbb{S}^2$

where the fiber ${X\times\{t\}}$ is equipped with the complex structure ${I_t=xI+yJ+zK}$ for ${t=(x,y,z)}$.

At this point, we are almost ready to state the main result of this post: for technical reasons, we will give an impressionistic statement before explaining the true theorem in Remark 4 below.

Theorem 5 (Filip) Fix a (generic) twistor family ${\mathfrak{X}=X\times\mathbb{S}^2}$. Then, there exists ${c>0}$ and ${\delta>0}$ such that

$\displaystyle \#\{t\in\mathbb{S}^2: X\times\{t\} \textrm{ admits a SLag fibration of volume }\leq V\} =$

$\displaystyle c\cdot V^{20} + O(V^{20-\delta})$

as ${V\rightarrow\infty}$.

Remark 4 This statement is not quite true in the sense that one should count “equators in the twistor sphere” rather than counting points ${t\in\mathbb{S}^2}$. Indeed, this is so because if a complex structure ${I_t}$ admits a SLag fibration at some point ${x}$ in the equator ${(\mathbb{R}t)^{\perp}\cap \mathbb{S}^2}$, then one also has SLag fibrations with the same “angle” ${x}$ as ${I_t}$ varies along the equator ${(\mathbb{R}x)^{\perp}\cap \mathbb{S}^2}$.

At this point, Filip started running out of time, and, for this reason, he offered the following sketch of proof of his theorem.

The first step is to reduce to counting elliptic fibrations (i.e., holomorphic fibrations ${X\rightarrow\mathbb{P}^1}$ whose fibers are elliptic curves).

The second step is to show that if the twistor family is not too special, then the counting problem reduces to the “linear algebra level” of the rank ${22}$ vector space ${H^2(X)}$ equipped with a form of signature ${(3,19)}$.

Finally, this last counting problem can be solved through quantitative equidistribution results on ${\Gamma\backslash SO(3,19)}$ for the action of a certain ${1}$-parameter subgroup ${a_s}$ on the quotient of the stabilizer in ${SO(3,19)}$ of a null vector in ${\mathbb{R}^{3,19}}$ by a lattice ${\Gamma_0}$ (that is, the quotient of the semidirect product of ${SO(3,18)}$ and ${\mathbb{R}^{18}}$ by${\Gamma_0}$).

Posted by: matheuscmss | January 23, 2016

## Some remarks on Sarnak’s question about thin Kontsevich-Zorich monodromies

Sometime ago, Alex Eskin and Alex Wright told me about the following question posed to them by Peter Sarnak:

How frequent are thin groups among Kontsevich-Zorich monodromies?

Instead of explaning the meaning of Sarnak’s question in general, we shall restrict ourselves to the case of Kontsevich-Zorich (KZ) monodromies associated to square-tiled surfaces.

More concretely, let ${X=(M,\omega)}$ be a square-tiled surface (also called origamis) of genus ${g\geq 1}$, i.e., ${p:M\rightarrow\mathbb{R}^2/\mathbb{Z}^2}$ is a finite branched covering which is unramified off ${0\in\mathbb{R}^2/\mathbb{Z}^2}$ and ${\omega = p^*(dz)}$ is the pullback of ${dz=dx+i dy}$ on ${\mathbb{R}^2/\mathbb{Z}^2 \simeq \mathbb{C}/\mathbb{Z}\oplus i\mathbb{Z}}$. We have a natural representation

$\displaystyle \rho_X:\textrm{Aff}(X)\rightarrow Sp(H_1^{(0)}(X,\mathbb{Z}))\simeq Sp(2g-2,\mathbb{Z})$

from the group ${\textrm{Aff}(X)}$ of affine homeomorphisms of ${X}$ to the group ${Sp(H_1^{(0)}(X,\mathbb{Z}))}$ of symplectic matrices of the subspace ${H_1^{(0)}(X,\mathbb{Z})}$ of integral homology classes of ${X}$ projecting to zero under ${p}$. In this setting, the Kontsevich-Zorich monodromy ${\Gamma_X}$ (associated to the ${SL(2,\mathbb{R})}$-orbit of ${X}$ in the moduli space of translation surfaces) is the image of ${\rho_X}$, i.e.,

$\displaystyle \Gamma_X := \rho_X(\textrm{Aff}(X))$

(See e.g. these posts here for more background material on square-tiled surfaces.)

By following Sarnak’s terminology, we will say that ${\Gamma_X}$ is a thin group if ${\Gamma_X}$ is an infinite index subgroup of ${Sp(2g-2,\mathbb{Z})}$ whose Zariski closure is

$\displaystyle \overline{\Gamma_X}^{\textrm{Zariski}} = Sp(2g-2,\mathbb{R})$

In the particular case of square-tiled surfaces, Sarnak’s question above is related to the following two problems:

• (a) find examples of square-tiled surfaces ${X}$ whose KZ monodromies ${\Gamma_X}$ are thin;
• (b) decide whether the “majority” of square-tiled surfaces in a given connected component ${\mathcal{C}}$ of a stratum of the moduli spaces of unit area translation surfaces has thin KZ monodromy (here, “majority” could mean “all but finitely many” or “almost full probability as the number of squares/tiles grows”.)

The goal of this post is to record (below the fold) some discussions with Vincent Delecroix and certain participants of MathOverFlow around item (a).

Remark 1 While we will not give answers to items (a) and/or (b) in this post, we decided to write it down anyway with the hope that it might be of interest to some readers of this blog: in fact, by the end of this post, we will show the following conditional statement: if the group generated by the matrices

$\displaystyle A=\left(\begin{array}{cccc} 1 & 0 & 0 & 0 \\ 0 & 1 & 0 & 0 \\ 0 & 0 & -1 & 1 \\ 0 & 0 & -1 & 0 \end{array}\right), \quad B=\left(\begin{array}{cccc} -1 & 0 & 0 & -1 \\ 0 & 0 & -1 & 0 \\ 0 & 1 & -1 & 0 \\ 1 & 0 & 0 & 0 \end{array}\right)$

has infinite-index in ${Sp(4,\mathbb{Z})}$, then a certain square-tiled surface of genus ${3}$ answers item (a) affirmatively.

Remark 2 Some “evidence” supporting a positive answer to item (b) is provided by this recent paper of Fuchs-Rivin where it is shown that two “randomly chosen” elements (in ${SL(n,\mathbb{Z})}$) “tend” to generate thin groups.

Posted by: matheuscmss | November 18, 2015

## Harmonic quasi-isometries (after Benoist and Hulin)

Last September 28, Yves Benoist gave a beautiful talk on the occasion of the workshop Geometry and Dynamics on Moduli Spaces (that is, one of the four 2015 Clay Research Workshops) about his joint work with Dominique Hulin on a generalized version of the so-called Schoen-Li-Wang conjecture (on harmonic maps within bounded distance to given quasi-isometries of symmetric spaces of rank one).

Remark 1 Still concerning beautiful talks delivered in this conference, I strongly recommend taking a look at this video of Peter Scholze’s talk on cohomology of algebraic varieties: indeed, I think that he was extremely sucessful in communicating his results to a broad audience of non-experts in Algebraic Geometry (such as myself).

This post is a transcription of my notes for Yves’ lecture and, as usual, all errors/mistakes below are my entire responsibility.

Posted by: matheuscmss | October 11, 2015

## Soficity, short cycles and the Higman group (after Helfgott-Juschenko)

Last August 25th, Harald Helfgott gave the talk “Soficity, short cycles and the Higman group” (based on joint work with Kate Juschenko) during the Second Workshop on Combinatorics, Number Theory and Dynamical Systems coorganized by Christian Mauduit, Carlos Gustavo Moreira, Yuri Lima and myself at IMPA (Brazil).

This post is a transcription of my notes for Harald’s talk, and, evidently, all mistakes/errors are my responsibility.

1. Some notations

We denote by ${Sym(n)}$ the group of all permutations of ${\{1,\dots, n\}}$.

For ${g_1, g_2\in Sym(n)}$, we define their distance ${d_n(g_1, g_2)}$ by

$\displaystyle d_n(g_1, g_2):=\frac{1}{n}\#\{1\leq i\leq n: g_1(i)\neq g_2(i)\}$

Definition 1 (Gromov; Weiss) Let ${G}$ be a group. Given ${\delta>0}$ and ${S\subset G}$ finite, we say that ${\phi:S\rightarrow Sym(n)}$ is a ${(S,n,\delta)}$-sofic representation whenever

• (a) ${\phi}$ is an “approximate homomorphism”: ${d_n(\phi(g)\phi(h),\phi(gh))<\delta}$ for all ${g,h\in S}$ with ${gh\in S}$;
• (b) ${\phi(g)\in Sym(n)}$ has “few” fixed points for ${g\neq e}$: ${d_n(\phi(g),id)>1-\delta}$ for all ${g\in S-\{e\}}$.

We say that a group ${G}$ is sofic if it has ${(S,n,\delta)}$-sofic representation for all ${S\subset G}$ finite, all ${\delta>0}$ (and some ${n\in\mathbb{N}}$).

Basic examples of sofic groups are: finite groups, amenable groups, etc. In general, it is known that several families of groups are sofic, but it is an important open problem to construct (or show the existence of) non-sofic groups.

The goal of this post is to discuss a candidate for non-sofic group and its connections to Number Theory.

1.1. Higman groups

For ${m\geq 2}$, let

$\displaystyle H_m := \langle a_1,\dots, a_m : a_i^{-1} a_{i+1} a_i = a_{i+1}^2 \textrm{ for all } i\in\mathbb{Z}/m\mathbb{Z} \rangle$

The groups ${H_2}$ and ${H_3}$ are trivial, and the group ${H_4}$ is the so-called Higman group.

Remark 1 Several statements in this post can be generalized for ${H_m}$ for all ${m\geq 4}$, but for the sake of exposition we will stick to ${H_4}$.

Theorem 2 (Helfgott-Juschenko) Assume that ${H_4}$ is sofic.Then, for every ${\varepsilon>0}$, there exists ${n\in\mathbb{N}}$ and a bijection ${f:\mathbb{Z}/n\mathbb{Z}\rightarrow\mathbb{Z}/n\mathbb{Z}}$ such that

• (a) ${f}$ is an “almost exponential function”: ${f(x+1)=2f(x)}$ for all ${x\in S}$ where ${S\subset\mathbb{Z}/n\mathbb{Z}}$ is a subset of cardinality ${|S|\geq (1-\varepsilon)n}$.
• (b) ${f^4(x):=f\circ f\circ f\circ f(x)=x}$ for all ${x\in\mathbb{Z}/n\mathbb{Z}}$.

Remark 2 The existence of functions ${f}$ as above is “unlikely” when ${\varepsilon>0}$ is small. More precisely, it is possible to show that there are no bijections ${f:\mathbb{Z}/n\mathbb{Z}\rightarrow\mathbb{Z}/n\mathbb{Z}}$ satisfying item (a) with ${\varepsilon=0}$ and item (b) when ${n=p^5}$ is the fifth power of a prime ${p}$ (cf. Remark 4 below for a more precise statement).

In other words, if we could take ${\varepsilon=0}$ and ${n=p^5}$ in the statement of Theorem 2, the non-soficity of the Higman group ${H_4}$ would follow.

Unfortunately, the techniques of Helfgott and Juschenko do not allow us to take ${\varepsilon=0}$ in Theorem 2, but they permit to control the integer ${n}$. More concretely, as we are going to see in Theorem 4 below, the integer ${n}$ can be chosen from any fixed sequence ${0 which is thick in the following sense:

Definition 3 A sequence ${0 of positive integers is thick if for every ${\varepsilon>0}$ there exists ${K>0}$ such that

$\displaystyle \frac{n_{k+1}}{n_k}\leq 1+\varepsilon$

for all ${k>K}$.

Remark 3 It does not take much to be a thick sequence: for example, the sequences ${\{n^3\}_{n\in\mathbb{N}}}$ and ${\{p^5\}_{p \textrm{ prime}}}$ are thick.

As we already announced, the main result of Helfgott-Juschenko is the following improvement of Theorem 2:

Theorem 4 (Helfgott-Juschenko) Assume that ${H_4}$ is sofic and let ${0 be a thick sequence.Then, for every ${\varepsilon>0}$, there exists ${k}$ and a bijection ${f:\mathbb{Z}/n_k\mathbb{Z}\rightarrow\mathbb{Z}/n_k\mathbb{Z}}$ such that

• (a) ${f(x+1)=2f(x)}$ for all ${x\in S}$ where ${S\subset\mathbb{Z}/n_k\mathbb{Z}}$ is a subset of cardinality ${|S|\geq (1-\varepsilon)n_k}$.
• (b) ${f^4(x):=f\circ f\circ f\circ f(x)=x}$ for all ${x\in\mathbb{Z}/n_k\mathbb{Z}}$.

Remark 4 The non-soficity of Higman group ${H_4}$ would follow from this theorem if the bijection ${f:\mathbb{Z}/n_k\mathbb{Z}\rightarrow \mathbb{Z}/n_k\mathbb{Z}}$, ${n_k\in\{p^5\}_{p\textrm{ prime}}}$, provided by this statement could be taken so that

• (a*) ${f(x+1)=2f(x)}$ for all ${x\in S}$ where ${S\subset\mathbb{Z}/n_k\mathbb{Z}}$ is a subset of cardinality ${|S|\geq n_k-n_k^{1/6}}$.
• (b) ${f^4(x):=f\circ f\circ f\circ f(x)=x}$ for all ${x\in\mathbb{Z}/n_k\mathbb{Z}}$.

Indeed, this is so because Glebsky and Holden-Robinson proved that there is no ${f}$ verifying (a*) and (b).

Remark 5 A natural question related to Theorem 4 is: what happens with fewer iterations in item (b)? In this situation, it is possible to use the fact that the group ${H_3}$ is trivial to show that, for each ${\varepsilon>0}$, there exists ${n_{\varepsilon}\in\mathbb{N}}$ such that for any ${n\geq n_{\varepsilon}}$ there is no bijection ${f:\mathbb{Z}/n\mathbb{Z}\rightarrow \mathbb{Z}/n\mathbb{Z}}$ such that

• (a) ${f(x+1)=2f(x)}$ for all ${x\in S}$ for ${|S|\geq (1-\varepsilon)n}$.
• (b) ${f^3(x):=f\circ f\circ f(x)=x}$ for all ${x\in S}$ for ${|S|\geq (1-\varepsilon)n}$.

This last remark can be generalized as follows.

Theorem 5 (Helfgott-Juschenko) Let ${n, g\geq 2}$ be coprime. Consider the function ${f:\{0,1,\dots, n-1\}\rightarrow\{0,1,\dots, n-1\}}$ given by

$\displaystyle f(x)=g^x (\textrm{mod } n)$

Then, the equation

$\displaystyle f^3(x):=f\circ f\circ f(x)=x$

has at most ${o(n)}$ solutions ${x\in\{0,1,\dots, n-1\}}$.

Remark 6 This theorem improves on a result of Glebsky-Shparlinski.

Posted by: matheuscmss | September 27, 2015

## Third Bourbaki seminar of 2015: Sophie Morel

Three months ago, Sophie Morel gave her Bourbaki seminar talk “Construction de représentations galoisiennes [d’après Scholze]”.

As it turns out, Sophie Morel was kind towards the non-experts in this subject (like myself): indeed, a large part of the talk (see the video here) was introductory, while the more advanced material was delegated to the lecture notes (available here).

In the remainder of this post, I’ll try to summarize some of the topics discussed by Sophie Morel’s talk (using the video in the link above as the main source).

Disclaimer: Since I’m not an expert on this subject, all mistakes in this post are my responsibility.

Remark 1 If you are in the Oxford area this week, then you will have the opportunity to learn about the main results in Morel’s talk directly from the authors: indeed, Boxer and Scholze (resp.) will give talks tomorrow, resp. Wednesday on their works (see this schedule here of the corresponding 2015 Clay research conference.)