Posted by: matheuscmss | June 30, 2015

## Hausdorff dimension of the graphs of the classical Weierstrass functions (after Weixiao Shen)

About two weeks ago, Weixiao Shen gave the talk “Hausdorff dimension of the graphs of the classical Weierstrass functions” during the Third Palis-Balzan International Symposium on Dynamical Systems.

In the sequel, I will transcript my notes from Shen’s talk.

1. Introduction

In Real Analysis, the classical Weierstrass function is

$\displaystyle W_{\lambda,b}(x) = \sum\limits_{n=0}^{\infty} \lambda^n \cos(2\pi b^n x)$

with ${1/b < \lambda < 1}$.

Note that the Weierstrass functions have the form

$\displaystyle f^{\phi}_{\lambda,b}(x) = \sum\limits_{n=0}^{\infty} \lambda^n \phi(b^n x)$

where ${\phi}$ is a ${\mathbb{Z}}$-periodic ${C^2}$-function.

Weierstrass (1872) and Hardy (1916) were interested in ${W_{\lambda,b}}$ because they are concrete examples of continuous but nowhere differentiable functions.

Remark 1 The graph of ${f^{\phi}_{\lambda,b}}$ tends to be a “fractal object” because ${f^{\phi}_{\lambda,b}}$ is self-similar in the sense that

$\displaystyle f^{\phi}_{\lambda, b}(x) = \phi(x) + \lambda f^{\phi}_{\lambda,b}(bx)$

We will come back to this point later.

Remark 2 ${f^{\phi}_{\lambda,b}}$ is a ${C^{\alpha}}$-function for all ${0\leq \alpha < \frac{-\log\lambda}{\log b}}$. In fact, for all ${x,y\in[0,1]}$, we have

$\displaystyle \frac{f^{\phi}_{\lambda, b}(x) - f^{\phi}_{\lambda,b}(y)}{|x-y|^{\alpha}} = \sum\limits_{n=0}^{\infty} \lambda^n b^{n\alpha} \left(\frac{\phi(b^n x) - \phi(b^n y)}{|b^n x - b^n y|^{\alpha}}\right),$

so that

$\displaystyle \frac{f^{\phi}_{\lambda, b}(x) - f^{\phi}_{\lambda,b}(y)}{|x-y|^{\alpha}} \leq \|\phi\|_{C^{\alpha}} \sum\limits_{n=0}^{\infty}(\lambda b^{\alpha})^n:=C(\phi,\alpha,\lambda,b) < \infty$

whenever ${\lambda b^{\alpha} < 1}$, i.e., ${\alpha < -\log\lambda/\log b}$.

The study of the graphs of ${W_{\lambda,b}}$ as fractal sets started with the work of Besicovitch-Ursell in 1937.

Remark 3 The Hausdorff dimension of the graph of a ${C^{\alpha}}$-function ${f:[0,1]\rightarrow\mathbb{R}}$ is

$\displaystyle \textrm{dim}(\textrm{graph}(f))\leq 2 - \alpha$

Indeed, for each ${n\in\mathbb{N}}$, the Hölder continuity condition

$\displaystyle |f(x)-f(y)|\leq C|x-y|^{\alpha}$

leads us to the “natural cover” of ${G=\textrm{graph}(f)}$ by the family ${(R_{j,n})_{j=1}^n}$ of rectangles given by

$\displaystyle R_{j,n}:=\left[\frac{j-1}{n}, \frac{j}{n}\right] \times \left[f(j/n)-\frac{C}{n^{\alpha}}, f(j/n)+\frac{C}{n^{\alpha}}\right]$

Nevertheless, a direct calculation with the family ${(R_{j,n})_{j=1}^n}$ does not give us an appropriate bound on ${\textrm{dim}(G)}$. In fact, since ${\textrm{diam}(R_{j,n})\leq 4C/n^{\alpha}}$ for each ${j=1,\dots, n}$, we have

$\displaystyle \sum\limits_{j=1}^n\textrm{diam}(R_{j,n})^d\leq n\left(\frac{4C}{n^{\alpha}}\right)^d = (4C)^{1/\alpha} < \infty$

for ${d=1/\alpha}$. Because ${n\in\mathbb{N}}$ is arbitrary, we deduce that ${\textrm{dim}(G)\leq 1/\alpha}$. Of course, this bound is certainly suboptimal for ${\alpha<1/2}$ (because we know that ${\textrm{dim}(G)\leq 2 < 1/\alpha}$ anyway).Fortunately, we can refine the covering ${(R_{j,n})}$ by taking into account that each rectangle ${R_{j,n}}$ tends to be more vertical than horizontal (i.e., its height ${2C/n^{\alpha}}$ is usually larger than its width ${1/n}$). More precisely, we can divide each rectangle ${R_{j,n}}$ into ${\lfloor n^{1-\alpha}\rfloor}$ squares, say

$\displaystyle R_{j,n} = \bigcup\limits_{k=1}^{\lfloor n^{1-\alpha}\rfloor}Q_{j,n,k},$

such that every square ${Q_{j,n,k}}$ has diameter ${\leq 2C/n}$. In this way, we obtain a covering ${(Q_{j,n,k})}$ of ${G}$ such that

$\displaystyle \sum\limits_{j=1}^n\sum\limits_{k=1}^{\lfloor n^{1-\alpha}\rfloor} \textrm{diam}(Q_{j,n,k})^d \leq n\cdot n^{1-\alpha}\cdot\left(\frac{2}{n}\right)^d\leq (2C)^{2-\alpha}<\infty$

for ${d=2-\alpha}$. Since ${n\in\mathbb{N}}$ is arbitrary, we conclude the desired bound

$\displaystyle \textrm{dim}(G)\leq 2-\alpha$

A long-standing conjecture about the fractal geometry of ${W_{\lambda,b}}$ is:

Conjecture (Mandelbrot 1977): The Hausdorff dimension of the graph of ${W_{\lambda,b}}$ is

$\displaystyle 1<\textrm{dim}(\textrm{graph}(W_{\lambda,b})) = 2 + \frac{\log\lambda}{\log b} < 2$

Remark 4 In view of remarks 2 and 3, the whole point of Mandelbrot’s conjecture is to establish the lower bound

$\displaystyle \textrm{dim}(\textrm{graph}(W_{\lambda,b})) \geq 2 + \frac{\log\lambda}{\log b}$

Remark 5 The analog of Mandelbrot conjecture for the box and packing dimensions is known to be true: see, e.g., these papers here and here).

In a recent paper (see here), Shen proved the following result:

Theorem 1 (Shen) For any ${b\geq 2}$ integer and for all ${1/b < \lambda < 1}$, the Mandelbrot conjecture is true, i.e.,

$\displaystyle \textrm{dim}(\textrm{graph}(W_{\lambda,b})) = 2 + \frac{\log\lambda}{\log b}$

Remark 6 The techniques employed by Shen also allow him to show that given ${\phi:\mathbb{R}\rightarrow\mathbb{R}}$ a ${\mathbb{Z}}$-periodic, non-constant, ${C^2}$ function, and given ${b\geq 2}$ integer, there exists ${K=K(\phi,b)>1}$ such that

$\displaystyle \textrm{dim}(\textrm{graph}(f^{\phi}_{\lambda,b})) = 2 + \frac{\log\lambda}{\log b}$

for all ${1/K < \lambda < 1}$.

Remark 7 A previous important result towards Mandelbrot’s conjecture was obtained by Barańsky-Barány-Romanowska (in 2014): they proved that for all ${b\geq 2}$ integer, there exists ${1/b < \lambda_b < 1}$ such that

$\displaystyle \textrm{dim}(\textrm{graph}(W_{\lambda,b})) = 2 + \frac{\log\lambda}{\log b}$

for all ${\lambda_b < \lambda < 1}$.

The remainder of this post is dedicated to give some ideas of Shen’s proof of Theorem 1 by discussing the particular case when ${1/b<\lambda<2/b}$ and ${b\in\mathbb{N}}$ is large.

Posted by: matheuscmss | June 13, 2015

## Third Palis-Balzan International Symposium on Dynamical Systems & Workshop on Combinatorics, Number Theory and Dynamical Systems

It is a pleasure to announce the following two conferences (that I’m helping to organize):

The Third Palis-Balzan International Symposium on Dynamical Systems closes the five-year long Project Palis-Balzan – Dynamical Systems, Chaotic Behaviour-Uncertainty, sponsored by the Balzan Foundation, related to the prestigious award conferred to Jacob Palis (and IMPA) by the Balzan Foundation in 2010.

A detailed description of the program and the titles and abstracts of talks of this conference can be found here and here.

The Workshop on Combinatorics, Number Theory and Dynamical Systems is the second edition of an event organized by C. Mauduit, C. G. Moreira, Y. Lima, J.-C. Yoccoz and myself back in 2013.

The full list of speakers for the 2015 edition of this workshop can be found here.

I guess that this is all I have to say for now (but you can look at their respective webpages for updated information). See you in Paris or Rio!

Posted by: matheuscmss | May 20, 2015

## Decay of correlations for flows and Dolgopyat’s estimate II.b

Last time, we reduced the proof of the exponential mixing property for expanding semiflows to the following Dolgopyat-like estimate:

Proposition 1 Let ${T}$ be an uniformly expanding Markov map on ${\Delta:=(0,1)}$ and let ${r:\Delta\rightarrow\mathbb{R}^+}$ be a good roof function with exponential tails.Then, there exist ${\sigma_0'>0}$, ${T_0\geq 1}$, ${\beta<1}$ and ${C>0}$ such that the iterates ${L_s^k}$ of the weighted transfer operator ${L_su(x):=\sum\limits_{T(y)=x} e^{-sr(y)}\frac{1}{|T'(y)|}u(y)}$ satisfy

$\displaystyle \|L_s^k u\|_{L^2}\leq C\beta^k\left(\|u\|_{C^0}+\frac{1}{\max\{1,|t|\}}\|Du\|_{C^0}\right):= C\beta^k \|u\|_{1,t}$

for all ${k\in\mathbb{N}}$ and ${s=\sigma+it}$ with ${|\sigma|\leq\sigma_0'}$, ${|t|\geq T_0}$.

Remark 1 We use the same terminology from the previous post of this series.

Roughly speaking, the basic idea behind the exponential contraction property in Proposition 1 is that “oscillations produce cancellations”. In particular, the analysis of the “size” of ${L_s^k u}$ is divided into two regimes:

• (A) If ${u}$ exhibits a high oscillation at scale ${\frac{1}{|t|}}$ (in the sense that ${\|Du\|_{C^0}\gg |t|\|u\|_{C^0}}$), then we will have a “cancelation” (significant reduction of the size of ${L_s^k u}$) thanks to classical methods (Lasota-Yorke inequality);
• (B) If the oscillation of ${u}$ at scale ${\frac{1}{|t|}}$ is not high, then we will have a “cancelation” thanks to Dolgopyat’s mechanism, i.e., a combination of high oscillations of Birkhoff sums ${r^{(n)}(x)}$ of the roof function ${r}$ (coming from the fact that ${r}$ is not a ${C^1}$-coboundary) and the big phases ${e^{-itr^{(n)}(y)}}$, ${|t|\geq T_0}$, of the terms ${e^{-sr^{(n)}(y)}}$ in the formula defining ${L_s^n u(x)}$.

In the remainder of this post, we will formalize this outline of proof of Proposition 1. More precisely, the next section contains a discussion of Lasota-Yorke inequality and the regime (A), and the last section is devoted to Dolgopyat’s cancelation mechanism and the regime (B).

Posted by: matheuscmss | May 17, 2015

## Decay of correlations for flows and Dolgopyat’s estimate II.a

In a series of two posts, we will revisit our previous discussion on the exponential mixing property for hyperbolic flows via a technique called Dolgopyat’s estimate.

Here, our main goal is to provide a little bit more of details on how this technique works by offering a “guided tour” through Sections 2 and 7 of a paper of Avila-Goüezel-Yoccoz.

For this sake, we organize this short series of posts as follows. In next section, we introduce a prototypical class of semiflows exhibiting exponential mixing. After that, we state the main exponential mixing result of this post (an analog of Theorem 7.3 of Avila-Goüezel-Yoccoz paper) for such semiflows, and we reduce the proof of this mixing property to a Dolgopyat-like estimate on weighted transfer operators. Finally, the next post of the series will be entirely dedicated to sketch the proof of the Dolgopyat-like estimate.

1. Expanding semiflows

Recall that a suspension flow is a semiflow ${T_t:\Delta_r\rightarrow \Delta_r}$, ${t\in\mathbb{R}_+}$, associated to a base dynamics (discrete-time dynamical system) ${T:\Delta\rightarrow\Delta}$ and a roof function ${r:\Delta\rightarrow\mathbb{R}^+}$ in the following way. We consider ${\Delta_r:=(\Delta\times\mathbb{R}^+)/\sim}$ where ${\sim}$ is the equivalence relation induced by ${(T(x),0)\sim (x,r(x))}$, and we let ${T_t}$ be the semiflow on ${\Delta_r}$ induced by

$\displaystyle (x,s)\in \Delta\times\mathbb{R}^+\mapsto (x,s+t)\in\Delta\times\mathbb{R}^+$

Geometrically, ${T_t}$, ${0\leq t<\infty}$ flows up the point ${(x,s)}$, ${0\leq s, linearly (by translation) in the fiber ${\{x\}\times\mathbb{R}^+}$ until it hits the “roof” (the graph of ${r}$) at the point ${(x,r(x))}$. At this moment, one is sent back (by the equivalence relation ${\sim}$) to the basis ${\Delta\times\{0\}}$ at the point ${(T(x),0)\sim (x,r(x))}$, and the semiflow restarts again.

A more concise way of writing down ${T_t}$ is the following: denoting by ${\Delta_r:=\{(x,t):x\in \Delta, 0\leq t, one defines ${T_t(x,s) := (T^n x, s+t-r^{(n)}(x))}$ where ${r^{(n)}(x)}$ is the Birkhoff sum

$\displaystyle r^{(n)}(x):=\sum\limits_{k=0}^{n-1} r(T^k x) \ \ \ \ \ (1)$

and ${n}$ is the unique integer such that

$\displaystyle r^{(n)}(x)\leq s+t

In this post, we want to study the decay of correlations of expanding semiflows, that is, a suspension flow ${T_t}$ so that the base dynamics ${T}$ is an uniformly expanding Markov map and the roof function ${r}$ is a good roof function with exponential tails in the following sense.

Remark 1 Avila-Gouëzel-Yoccoz work in greater generality than the setting of this post: in fact, they allow ${\Delta}$ to be a John domain and they prove results for excellent hyperbolic semiflows (which are more common in “nature”), but we will always take ${\Delta=(0,1)}$ and we will study exclusively expanding semiflows (which are obtained from excellent hyperbolic semiflows by taking the “quotient along stable manifolds”) in order to simplify our exposition.

Definition 1 Let ${\Delta=(0,1)}$, ${Leb}$ be the Lebesgue measure on ${\Delta}$, and ${\{\Delta^{(l)}\}_{l\in L}}$ be a finite or countable partition of ${\Delta}$ modulo zero into open subintervals. We say that ${T:\bigcup\limits_{l\in L} \Delta^{(l)}\rightarrow \Delta}$ is an uniformly expanding Markov map if

• ${\{\Delta^{(l)}\}}$ is a Markov partition: for each ${l\in L}$, the restriction of ${T}$ to ${\Delta^{(l)}}$ is a ${C^1}$-diffeomorphism between ${\Delta^{(l)}}$ and ${\Delta}$;
• ${T}$ is expanding: there exist a constant ${\kappa>1}$ and, for each ${l\in L}$, a constant ${C(l)>1}$ such that ${\kappa\leq|T'(x)|\leq C(l)}$ for each ${x\in\Delta^{(l)}}$;
• ${T}$ has bounded distortion: denoting by ${J(x)=1/|T'(x)|}$ the inverse of the Jacobian of ${T}$ and by ${\mathcal{H}=\{(T|_{\Delta^{(l)}})^{-1}\}_{l\in L}}$ the set of inverse branches of ${T}$, we require that ${\log J}$ is a ${C^1}$ function on each ${\Delta^{(l)}}$ and there exists a constant ${C>0}$ such that

$\displaystyle \left|\frac{h''(x)}{h'(x)}\right| = |D((\log J)\circ h)(x)|\leq C$

for all ${h\in \mathcal{H}}$ and ${x\in \Delta}$. (This condition is also called Renyi condition in the literature.)

Remark 2 Araújo and Melbourne showed recently that, for the purposes of discussing exponential mixing properties (for excellent hyperbolic semiflows with one-dimensional unstable subbundles), the bounded distortion (Renyi condition) can be relaxed: indeed, it suffices to require that ${\log J}$ is a Hölder function such that the Hölder constant of ${\log J\circ h}$ is uniformly bounded for all ${h\in \mathcal{H}}$.

Example 1 Let ${\Delta=\Delta^{(0)}\cup\Delta^{(1)}=(0,1/2)\cup(1/2,1)}$ be the finite partition (mod. ${0}$) of ${\Delta}$ provided by the two subintervals ${\Delta^{(l)}=(\frac{l}{2}, \frac{l+1}{2})}$, ${l=0, 1}$. The map ${T:\Delta^{(0)}\cup\Delta^{(1)}\rightarrow\Delta}$ given by ${T(x)=2x-l}$ for ${x\in\Delta^{(l)}}$ is an uniformly expanding Markov map (preserving the Lebesgue measure ${Leb}$).

An uniformly expanding map ${T}$ preserves an unique probability measure ${\mu}$ which is absolutely continuous with respect to the Lebesgue measure ${Leb}$. Moreover, the density ${d\mu/dLeb}$ is a ${C^1}$ function whose values are bounded away from ${0}$ and ${\infty}$, and ${\mu}$ is ergodic and mixing.

Indeed, the proof of these facts can be found in Aaronson’s book and it involves the study of the spectral properties of the so-called transfer (Ruelle-Perron-Frobenius) operator

$\displaystyle Lu(x) = \sum\limits_{T(y)=x} J(y) u(y) = \sum\limits_{h\in\mathcal{H}} J(hx) u(hx)$

(as it was discussed in Theorem 1 of Section 1 of this post in the case of a finite Markov partition ${\{\Delta^{(l)}\}_{l\in L}}$, ${\# L<\infty}$)

Definition 2 Let ${T:\bigcup\limits_{l\in L} \Delta^{(l)}\rightarrow \Delta}$ be an uniformly expanding Markov map. A function ${r:\bigcup\limits_{l\in L}\Delta^{(l)}\rightarrow\mathbb{R}^+}$ is a good roof function if

• there exists a constant ${\varepsilon>0}$ such that ${r(x)\geq\varepsilon}$ for all ${x}$;
• there exists a constant ${C>0}$ such that ${|D(r\circ h)(x)|\leq C}$ for all ${x}$ and all inverse branch ${h\in\mathcal{H}}$ of ${T}$;
• ${r}$ is not a ${C^1}$coboundary: it is not possible to write ${r = \psi + \phi\circ T - \phi}$ where ${\psi:\Delta\rightarrow\mathbb{R}}$ is constant on each ${\Delta^{(l)}}$ and ${\phi:\Delta\rightarrow\mathbb{R}}$ is ${C^1}$.

Remark 3 Intuitively, the condition that ${r}$ is not a coboundary says that it is not possible to change variables to make the roof function into a piecewise constant function. Here, the main point is that we have to avoid suspension flows with piecewise constant roof functions (possibly after conjugation) in order to have a chance to obtain nice mixing properties (see this post for more comments).

Definition 3 A good roof function ${r:\bigcup\limits_{l\in L}\Delta^{(l)}\rightarrow\mathbb{R}^+}$ has exponential tails if there exists ${\sigma_0 > 0}$ such that ${\int_{\Delta} e^{\sigma_0 r} d Leb < \infty}$.

The suspension flow ${T_t}$ associated to an uniformly expanding Markov map ${T:\bigcup\limits_{l\in L}\Delta^{(l)}\rightarrow\Delta}$ and a good roof function ${r:\bigcup\limits_{l\in L}\Delta^{(l)}\rightarrow\mathbb{R}^+}$ with exponential tails preserves the probability measure

$\displaystyle \mu_r:=\mu\otimes Leb/\mu\otimes Leb(\Delta_r)$

on ${\Delta_r}$. Note that ${\mu_r}$ is absolutely continuous with respect to ${Leb_r:= Leb\otimes Leb}$ (because ${\mu}$ is absolutely continuous with respect to ${Leb}$).

Remark 4 All integrals in this post are always taken with respect to ${Leb}$ or ${Leb_r}$ unless otherwise specified.

Remark 5 In the sequel, AGY stands for Avila-Gouëzel-Yoccoz.

2. Statement of the exponential mixing result

Let ${(T_t)_{t\in\mathbb{R}}}$ be an expanding semiflow.

Theorem 4 There exist constants ${C>0}$, ${\delta>0}$ such that

$\displaystyle \left|\int U\cdot V\circ T_t \, d Leb_r - \left(\int U \, d Leb_r\right) \left(\int V\,d\mu_r\right)\right|\leq C e^{-\delta t}\|U\|_{C^1}\|V\|_{C^1}$

for all ${t\geq 0}$ and for all ${U, V\in C^1(\Delta_r)}$.

Remark 6 By applying this theorem with ${U(x,t)\cdot \frac{d\mu}{d Leb}(x)}$ in the place of ${U}$, we obtain the classical exponential mixing statement:

$\displaystyle \left|\int U\cdot V\circ T_t \, d\mu_r - \left(\int U \, d \mu_r\right) \left(\int V\,d\mu_r\right)\right|\leq C e^{-\delta t}\|U\|_{C^1}\|V\|_{C^1}$

Remark 7 This theorem is exactly Theorem 7.3 in AGY paper except that they work with observables ${U}$ and ${V}$ belonging to Banach spaces ${\mathcal{B}_0}$ and ${\mathcal{B}_1}$ which are slightly more general than ${C^1}$ (in the sense that ${C^1\subset \mathcal{B}_0\subset \mathcal{B}_1}$). In fact, AGY need to deal with these Banach spaces because they use their Theorem 7.3 to deduce a more general result of exponential mixing for excellent hyperbolic semiflows (see their paper for more explanations), but we will not discuss this point here.

The remainder of this post is dedicated to the proof of Theorem 4.

Posted by: matheuscmss | May 10, 2015

## The Hausdorff measure (at adequate scale) of simply connected planar domains

Some of the partial advances obtained by Jacob Palis, Jean-Christophe Yoccoz and myself on the computation of Hausdorff dimensions of stable and unstable sets of non-uniformly hyperbolic horseshoes (announced in this blog post here and this survey article here) are based on the following lemma:

Lemma 1 Let ${f:B\rightarrow\mathbb{R}^2}$ be a ${C^1}$ diffeomorphism from the closed unit ball ${B:= \{(x,y)\in\mathbb{R}^2: x^2 + y^2\leq 1\}}$ of ${\mathbb{R}^2}$ into its image.Let ${K\geq 1}$ and ${L\geq 1}$ be two constants such that ${\|Df(p)\|\leq K}$ and ${\textrm{Jac}(f)(p):=|\det Df(p)|\leq L}$ for all ${p\in B}$.

Then, for each ${1\leq d\leq 2}$, the ${d}$-dimensional Hausdorff measure ${H^d_{\sqrt{2}}(f(B))}$ at scale ${\sqrt{2}}$ of ${f(B)}$ satisfies

$\displaystyle H^d_{\sqrt{2}}(f(B)) := \inf\limits_{\substack{\bigcup\limits_{i\in \mathbb{N}} U_i \supset f(B), \\ \textrm{diam}(U_i)\leq \sqrt{2}}}\sum\limits_{i\in\mathbb{N}}\textrm{diam}(U_i)^d \leq 170\pi \cdot \max\{K,L\}^{2-d} \cdot L^{d-1} \ \ \ \ \ (1)$

Remark 1 In fact, this is not the version of the lemma used in practice by Palis, Yoccoz and myself. Indeed, for our purposes, we need the estimate

$\displaystyle H^d_{r\sqrt{2}}(g(B_r))\leq 170\pi\cdot r^d\cdot K^{2-d}\cdot L^{d-1}$

where ${B_r}$ is the ball of radius ${r}$ centered at the origin and ${g}$ is a ${C^1}$ diffeomorphism such that ${\|Dg\|\leq K}$ and ${\textrm{Jac}(g)\leq L}$ for ${1\leq L\leq K}$. Of course, this estimate is deduced from the lemma above by scaling, i.e., by applying the lemma to ${f = h_r^{-1}\circ g\circ h_r }$ where ${h_r:\mathbb{R}^2\rightarrow\mathbb{R}^2}$ is the scaling ${h_r(p)=rp}$.

Nevertheless, we are not completely sure if we should write down an article just with our current partial results on non-uniformly hyperbolic horseshoes because our feeling is that these results can be significantly improved by the following heuristic reason.

In a certain sense, Lemma 1 says that one of the “worst” cases (where the estimate (1) becomes “sharp” [modulo the multiplicative factor ${170\pi}$]) happens when ${f}$ is an affine hyperbolic conservative map ${f_K(x,y)=(Kx,\frac{1}{K}y)}$ (say ${K\geq 1}$): indeed, since ${[-\sqrt{\frac{1}{2}}, \sqrt{\frac{1}{2}}]\times [-\sqrt{\frac{1}{2}}, \sqrt{\frac{1}{2}}]\subset B\subset [-1,1]\times [-1,1]}$, the most “economical” way to cover ${f_K(B)}$ using a countable collection of sets of diameters ${\leq \sqrt{2}}$ is basically to use ${K^2}$ squares of sizes ${1/K}$ (which gives an estimate ${H^d_{\sqrt{2}}(f(B))\leq K^2(1/K)^d = K^{2-d}}$).

However, in the context of (expectional subsets of stable sets of) non-uniformly hyperbolic horseshoes, we deal with maps ${f}$ obtained by successive compositions of affine-like hyperbolic maps and a certain folding map (corresponding to “almost tangency” situations). In particular, we work with maps ${f}$ which are very different from affine hyperbolic maps and, thus, one can expect to get slightly better estimates than Lemma 1 in this setting.

In summary, Jacob, Jean-Christophe and I hope to improve the results announced in this survey here, so that Lemma 1 above will become a “deleted scene” of our forthcoming paper.

On the other hand, this lemma might be useful for other purposes and, for this reason, I will record its (short) proof in this post.

1. Proof of Lemma 1

The proof of (1) is based on the following idea. By studying the intersection of ${f(B)}$ with dyadic squares on ${\mathbb{R}^2}$, we can interpret the measure ${H^d_{\sqrt{2}}(f(B))}$ as a sort of ${L^d}$-norm of a certain function. Since ${1\leq d\leq 2}$, we can control this ${L^d}$-norm in terms of the ${L^1}$ and ${L^2}$ norms (by interpolation). As it turns out, the ${L^1}$-norm, resp. ${L^2}$-norm, is controlled by the features of the derivative ${Df}$, resp. Jacobian determinant ${Jac(f)}$, and this morally explains the estimate (1).

Let us now turn to the details of this argument. Denote by ${U:=f(B)}$ and ${\partial U}$ its boundary. For each integer ${k\geq 0}$, let ${\Delta_k}$ be the collection of dyadic squares of level ${k}$, i.e., ${\Delta_k}$ is the collection of squares of sizes ${1/2^k}$ with corners on the lattice ${(1/2^k)\cdot\mathbb{Z}^2}$.

Consider the following recursively defined cover of ${U}$. First, let ${\mathcal{C}_0}$ be the subset of squares ${Q\in \Delta_0}$ such that

$\displaystyle \textrm{area}(Q\cap U)\geq \frac{1}{5}$

Next, for each ${k>0}$, we define inductively ${\mathcal{C}_k}$ as the subset of squares ${Q\in\Delta_k}$ such that ${Q}$ is not contained in some ${Q'\in\mathcal{C}_l}$ for ${0\leq l < k}$, and ${Q}$ intersects a significant portion of ${U}$ in the sense that

$\displaystyle \textrm{area}(Q\cap U)\geq \frac{1}{5}\textrm{area}(Q) \ \ \ \ \ (2)$

In other words, we start with ${U}$ and we look at the collection ${\mathcal{C}_0}$ of dyadic squares of level ${0}$ intersecting it in a significant portion. If the squares in ${\mathcal{C}_0}$ suffice to cover ${U}$, we stop the process. Otherwise, we consider the dyadic squares of level ${0}$ not belonging to ${\mathcal{C}_0}$, we divide each of them into four dyadic squares of level ${1}$, and we build a collection ${\mathcal{C}_1}$ of such dyadic squares of level ${1}$ intersecting in a significant way the remaining part of ${U}$ not covered by ${\mathcal{C}_0}$, etc.

Remark 2 In this construction, we are implicitly assuming that ${U=f(B)}$ is not entirely contained in a dyadic square ${Q\in\bigcup\limits_{k=0}^{\infty}\Delta_k}$. In fact, if ${U\subset Q}$, then the trivial bound ${H^d_{\sqrt{2}}(U)\leq \textrm{diam}(Q)^d\leq (\sqrt{2})^d\leq 2}$ (for ${1\leq d\leq 2}$) is enough to complete the proof of the lemma.

In this way, we obtain a countable collection ${\bigcup\limits_{k=0}^{\infty} \mathcal{C}_k:=(U_i)_{i\in\mathbb{N}}}$ covering ${U=f(B)}$ such that ${\textrm{diam}(U_i)\leq \sqrt{2}}$ and

$\displaystyle H^d_{\sqrt{2}}(f(B))\leq \sum\limits_{i}\textrm{diam}(U_i)^d = \sum\limits_{k=0}^{\infty} N_k\left(\frac{1}{2^k}\right)^d \ \ \ \ \ (3)$

where ${N_k:=(\sqrt{2})^d\#\mathcal{C}_k}$.

By thinking of this expression as a ${L^d}$-norm and by applying interpolation between the ${L^1}$ and ${L^2}$ norms, we obtain that

$\displaystyle \sum\limits_{k=0}^{\infty} N_k\left(\frac{1}{2^k}\right)^d\leq \left(\sum\limits_{k=0}^{\infty} \frac{N_k}{2^k}\right)^{2-d} \left(\sum\limits_{k=0}^{\infty} \frac{N_k}{(2^k)^2}\right)^{d-1} \ \ \ \ \ (4)$

This reduces our task to estimate these ${L^1}$ and ${L^2}$ norms. We begin by observing that the ${L^2}$-norm is easily controlled in terms of the Jacobian of ${f}$ (thanks to the condition (2)):

$\displaystyle \sum\limits_{k=0}^{\infty} \frac{N_k}{(2^k)^2} = (\sqrt{2})^d\sum\limits_{k}\sum\limits_{Q\in\mathcal{C}_k} \textrm{area}(Q) \ \ \ \ \ (5)$

$\displaystyle \begin{array}{rcl} &\leq & (\sqrt{2})^d\sum\limits_{k}\sum\limits_{Q\in\mathcal{C}_k} 5\cdot \textrm{area}(Q\cap U) \\ &\leq& 10 \cdot \textrm{area}(U) = 10 \int_B \textrm{Jac}(f) \\ &\leq& 10\pi\cdot L \end{array}$

for any ${1\leq d\leq 2}$. In particular, we have that

$\displaystyle N_0\leq 10\pi L$

From this estimate, we see that the ${L^1}$-norm satisfies

$\displaystyle \sum\limits_{k=0}^{\infty} \frac{N_k}{2^k} = N_0+\sum\limits_{k=1}^{\infty} \frac{N_k}{2^k}\leq 10\pi L+\sum\limits_{k>0} \frac{N_k}{2^k} \ \ \ \ \ (6)$

Thus, we have just to estimate the series ${\sum\limits_{k>0} \frac{N_k}{2^k}}$. We affirm that this series is controlled by the derivative of ${f}$. In order to prove this, we need the following claim:

Claim. For each ${k>0}$ and ${Q\in\mathcal{C}_k}$, one has

$\displaystyle \textrm{length}(Q\cap \partial U)\geq \frac{1}{20}\cdot\frac{1}{2^k} \ \ \ \ \ (7)$

Proof of Claim. Note that ${U}$ can not contain ${Q}$: indeed, since ${Q\subset Q'}$ for some dyadic square ${Q'\in\Delta_{k-1}}$ of level ${k-1\geq 0}$ (and, thus, ${4\cdot \textrm{area}(Q) = \textrm{area}(Q')}$), if ${Q\subset U}$, then ${\textrm{area}(Q'\cap U)\geq \textrm{area}(Q\cap U) = \textrm{area}(Q)=\frac{1}{4}\textrm{area}(Q')}$, a contradiction with the definition of ${Q\in\mathcal{C}_k}$. Because we are assuming that ${U}$ is not contained in ${Q}$ (cf. Remark 2) and we also have that ${Q}$ intersects (a significant portion of) ${U}$, we get that

$\displaystyle \partial U\cap \partial Q\neq \emptyset$

For the sake of contradiction, suppose that ${\textrm{length}(\partial U\cap Q)<\frac{1}{20\cdot 2^k}}$. Since ${\partial U}$ intersects ${\partial Q}$, the ${\frac{1}{20\cdot 2^k}}$-neighborhood ${V_k}$ of ${\partial Q}$ contains ${\partial U\cap Q}$. This means that

• (a) either ${Q-V_k}$ is contained in ${U}$
• (b) or ${Q-V_k}$ is disjoint from ${U}$

However, we obtain a contradiction in both cases. Indeed, in case (a), we get that a dyadic square ${Q'}$ of level ${k-1}$ containing ${Q}$ satsifies

$\displaystyle \textrm{area}(Q'\cap U)\geq \textrm{area}(Q-V_k) = \left(1-2\cdot\frac{1}{20}\right)^2\textrm{area}(Q) = \frac{81}{400}\textrm{area}(Q'),$

a contradiction with the definition of ${Q\in\mathcal{C}_k}$. Similarly, in case (b), we obtain that

$\displaystyle \textrm{area}(Q\cap U)\leq \textrm{area}(Q\cap V_k) = \left(1-\frac{81}{100}\right)\textrm{area}(Q) < \frac{1}{5}\textrm{area}(Q),$

This completes the proof of the claim. ${\square}$

Coming back to the calculation of the series ${\sum\limits_{k>0} N_k/2^k}$, we observe that the estimate (7) from the claim and the fact that ${\|Df\|\leq K}$ imply:

$\displaystyle \begin{array}{rcl} \sum\limits_{k>0} \frac{N_k}{2^k} &=& (\sqrt{2})^d \sum\limits_{k>0}\sum\limits_{Q\in\mathcal{C}_k} \frac{1}{2^k} \\ &\leq& 20(\sqrt{2})^d\sum\limits_{k>0}\sum\limits_{Q\in\mathcal{C}_k}\textrm{length}(\partial U\cap Q) \\ &\leq& 20(\sqrt{2})^d 2 \cdot \textrm{length}(\partial U) \\ &\leq& 80 K\cdot \textrm{length}(\partial B) = 160\pi K \end{array}$

By plugging this estimate into (6), we deduce that the ${L^1}$-norm verifies

$\displaystyle \sum\limits_{k=0}^{\infty} \frac{N_k}{2^k}\leq 170\pi \max\{K,L\} \ \ \ \ \ (8)$

Finally, from (3), (4), (5) and (8), we conclude that

$\displaystyle H^d_{\sqrt{2}}(f(B))\leq (170\pi)^{2-d}(10\pi)^{d-1}\max\{K,L\}^{2-d} L^{d-1}\leq 170\pi \max\{K,L\}^{2-d} L^{d-1}$

This ends the proof of the lemma.

Posted by: matheuscmss | April 24, 2015

## Some comments on the conjectures of Ivanov and Putman-Wieland

During the graduate workshop on moduli of curves (organized by Samuel Grushevsky, Robert Lazarsfeld, and Eduard Looijenga last July 2014), Alex Wright gave a minicourse on the ${SL(2,\mathbb{R})}$-orbits on moduli spaces of translation surfaces (the videos of the lectures and the corresponding lecture notes are available here and here).

These lectures by Alex Wright made Eduard Looijenga ask if some “remarkable” translation surfaces could help in solving the following question.

Let ${S}$ be a ramified finite cover of the two-torus ${T}$ (say branched at only one point ${0\in T}$). Denote by ${H}$ the subspace of ${H_1(S,\mathbb{Q})}$ generated by the homology classes of all simple closed loops on ${S}$ covering such a curve on ${T}$.

Question 1. Is it true that one always has ${H=H_1(S,\mathbb{Q})}$ in this setting?

By following Alex Wright’s advice, Eduard Looijenga wrote me asking if I knew the answer to this question. I replied to him that my old friend Eierlegende Wollmilchsau provided a negative answer to his question, and I directed him to the papers of Forni (from 2006), Herrlich-Schmithüsen (from 2008) and our joint paper with Yoccoz (from 2010) for detailed explanations.

In a subsequent email, Eduard told me that my answer was a good indication that notable translation surfaces could be interesting for his purposes: indeed, the Eierlegende Wollmilchsau is precisely the example described in the appendix of a paper by Andrew Putman and Ben Wieland from 2013 where Question 1 was originally solved.

After more exchanges of emails, I learned from Eduard that his question was motivated by the attempts of Putman-Wieland (in the paper quoted above) to attack the following conjecture of Nikolai Ivanov (circa 1991):

Conjecture (Ivanov). Let ${g\geq 3}$ and ${n\geq0}$. Consider ${\Gamma}$ a finite-index subgroup of the mapping-class group ${\textrm{Mod}_{g,n}}$ of isotopy classes of homeomorphisms of a genus ${g}$ surface ${S}$ fixing pointwise a set ${\Sigma=\{x_1,\dots, x_n\}}$ of marked points. Then, there is no surjective homomorphism from ${\Gamma}$ to ${\mathbb{Z}}$.

Remark 1 This conjecture came from the belief that mapping-class groups should behave in many aspects like lattices in higher-rank Lie groups and it is known that such lattices do not surject on ${\mathbb{Z}}$ because they satisfy Kazhdan property (T). Nevertheless, Jorgen Andersen recently proved that the mapping-class groups ${\textrm{Mod}_{g,n}}$ do not have Kazhdan property (T) when ${g\geq 2}$.

Remark 2 It was proved by John McCarthy and Feraydoun Taherkhani that the analog for ${g=2}$ of Ivanov’s conjecture fails.

In fact, Putman-Wieland proposed the following strategy to study Ivanov’s conjecture. First, they introduced the following conjecture:

Conjecture (Putman-Wieland). Fix ${g\geq 2}$ and ${n\geq 0}$. Given a finite-index characteristic subgroup ${K}$ of the fundamental group ${\pi_1(S_{g,n}, x_{n+1})}$ of a surface ${S=S_{g,n}}$ of genus ${g}$ with ${n}$ punctures ${x_1, \dots, x_n}$, denote by ${S_K\rightarrow S}$ the associated finite cover, and let ${\overline{S_K}}$ be the compact surface obtained from ${S_K}$ by filling its punctures.

Then, the natural action on ${H_1(\overline{S_K},\mathbb{Q})-\{0\}}$ of the group of lifts to ${\overline{S_K}}$ of isotopy classes of diffeomorphisms of ${S_{g,n}}$ fixing ${x_1,\dots, x_{n+1}}$ pointwise has no finite orbits.

Remark 3 This conjecture is closely related (for reasons that we will not explain in this post) to a natural generalization of Question 1 to general ramified finite covers ${S\rightarrow T}$.

Remark 4 The analog of Putman-Wieland conjecture in genus ${g=1}$ is false: the same counterexample to Question 1 (namely, the Eierlegende Wollmilchsau) serves to answer negatively this genus 1 version of Putman-Wieland conjecture.

Remark 5 In the context of Putman-Wieland conjecture, one has a representation ${\textrm{Mod}_{g,n+1}\rightarrow \textrm{Aut}(H_1(\overline{S_K}, \mathbb{Q}))}$ (induced by the lifts of elements of ${\textrm{Mod}_{g,n+1}}$ to ${\overline{S_K}}$). This representation is called a higher Prym representation by Putman-Wieland. In this language, Putman-Wieland conjecture asserts that higher Prym representations have no non-trivial finite orbits when ${g\geq 2}$ and ${n\geq 0}$.

Secondly, they proved that:

Theorem 1 (Putman-Wieland) Fix ${g\geq 3}$ and ${n\geq 0}$.

• (a) If Putman-Wieland conjecture holds for every finite-index characteristic subgroup ${K}$ of ${\pi_1(S_{g-1, n+1}, x_{n+2})}$, then Ivanov conjecture is true for any finite-index sugroup ${\Gamma}$ of ${\textrm{Mod}_{g,n}}$.
• (b) If Ivanov conjecture holds for every finite-index subgroup of ${\textrm{Mod}_{g,n+2}}$, then Putman-Wieland conjecture is true for any finite-index characteristic subgroup ${K}$ of ${\pi_1(S_{g,n+1}, x_{n+2})}$.

Moreover, if Ivanov conjecture is true for all finite-index subgroups of ${\textrm{Mod}_{g,n}}$ for all ${n\geq0}$, then it is also true for all finite-index subgroups of ${\textrm{Mod}_{G,m}}$ with ${G\geq g}$, ${m\geq 0}$.

In other words, Putman-Wieland proposed to approach an algebraic problem (Ivanov conjecture) via the study of a geometric problem (Putman-Wieland conjecture) because these two problems are “essentially” equivalent.

In particular, this gives the following concrete route to establish Ivanov conjecture:

• (I) if we want to show that Ivanov conjecture is true for all ${g\geq 3}$ and ${n\geq 0}$, then it suffices to prove Putman-Wieland conjecture for ${g=2}$ (and all ${n\geq 0}$); indeed, this is so because item (a) of Putman-Wieland theorem would imply that Ivanov conjecture is true for ${g=3}$ (and all ${n\geq0}$) in this setting, and, hence, the last paragraph of Putman-Wieland theorem would allow to conclude the validity of Ivanov conjecture in general.
• (II) if we want to show that Ivanov conjecture is false for some ${g\geq 3}$ and ${m\geq 2}$, then it suffices to construct a counterexample to Putman-Wieland conjecture for ${g=3}$ and ${n=m-1}$.

Once we got at this point in our email conversations, Eduard told me that Question 1 was just a warmup towards his main question:

Question 2. Are there remarkable translation surfaces giving counterexamples to Putman-Wieland conjecture?

By inspecting my list of “preferred” translation surfaces, I noticed that I knew such an example: in fact, there is exactly one member in a family of translation surfaces that I’m studying with Artur Avila and Jean-Christophe Yoccoz (for other purposes) which is a counterexample to Putman-Wieland conjecture in genus ${g=2}$ (and ${n=6}$).

In other words, one of the translation surfaces in a forthcoming paper joint with Artur and Jean-Christophe answers Question 2.

Remark 6 This shows that Putman-Wieland’s strategy (I) above does not work (because their conjecture is false in genus ${2}$). Of course, this does not mean that Ivanov conjecture is false: in fact, by Putman-Wieland strategy (II), one needs a counterexample to Putman-Wieland conjecture in genus ${g\geq 3}$ (rather than in genus ${g=2}$). Here, it is worth to point out that Artur, Jean-Christophe and I have no good candidates of counterexamples to Putman-Wieland conjecture in genus ${g\geq 3}$ and/or Ivanov conjecture.

Below the fold, we focus on the case ${g=2}$ and ${n=6}$ of Putman-Wieland conjecture.

Update (September 7, 2015): Last June 2015, Eduard gave a talk on algebro-geometrical aspects of mapping-class groups, and he wrote the following summary here where the connections between (a natural generalization of) Question 1 and the conjectures of Ivanov and Putman-Wieland are discussed.

Posted by: matheuscmss | March 31, 2015

## Subadditive cocycles and horofunctions (after Gouëzel and Karlsson)

Last March 25th, Sébastien Gouëzel gave the talk “Subadditive cocycles and horofunctions” at the Ergodic Theory and Dynamical Systems seminar of LAGA , Université Paris 13.

As it is always the case with Sébastien’s expositions, he managed to communicate very clearly the ideas of a mathematically profound subject (and, by the way, this topic is not directly related to his excellent Bourbaki seminar talk from March 21st).

In the sequel, I’ll transcript my lecture notes for Sébastien’s talk. Of course, all errors and mistakes are my entire responsibility.

1. Introduction

Let us warmup by giving a proof of the following theorem:

Theorem 1 (Kohlberg-Neyman (1981)) Let ${\phi:\mathbb{R}^d\rightarrow\mathbb{R}^d}$ be a weak contraction of the Euclidean space ${(\mathbb{R}^d, \|.\|)}$ in the sense that

$\displaystyle \|\phi(x)-\phi(y)\|\leq \|x-y\|$

for all ${x,y\in\mathbb{R}^d}$.Then, the sequence

$\displaystyle \frac{\phi^n(0)}{n}$

converges as ${n\rightarrow\infty}$.

Remark 1 The origin ${0\in\mathbb{R}^d}$ can be replaced by any point ${x_0\in\mathbb{R}^n}$ because

$\displaystyle \|\phi^n(x_0)-\phi^n(0)\|\leq \|x_0\|$

so that

$\displaystyle \lim\limits_{n\rightarrow\infty}\frac{\phi^n(x_0) - \phi^n(0)}{n} = 0$

As the reader might suspect, the fact that such an “innocent-looking” result was proved only in 1981 (in this paper here) indicates that its proof is not easy to find if we don’t use the “correct” setup.

For the purposes of this post, we will show Theorem 1 using a argument of Karlsson (from 2001).

Posted by: matheuscmss | March 15, 2015

## First Bourbaki seminar of 2015 (III): Ambrosio’s talk

For the third (and last) installment of this series of posts (which started here) on the first Bourbaki seminar of 2015, we will discuss a talk that brought me back some good memories from the time I did my PhD at IMPA when I took a course on Geometric Measure Theory (taught by Hermano Frid) based on the books of Evans-Gariepy (for the introductory part of the course) and Guisti (for the core part of the course).

In fact, our goal today is to revisit Luigi Ambrosio’s Bourbaki seminar talk entitled “The regularity theory of area-minimizing integral currents (after Almgren-DeLellis-Spadaro)”. Here, besides the original works of Almgren and DeLellis-Spadaro (in these five papers here), the main references are the video of Ambrosio’s talk and his lecture notes (both in English).

Disclaimer. As usual, all errors and mistakes are my entire responsibility.

1. Introduction

This post is centered around solutions to the so-called Plateau’s problem.

A formulation of Plateau’s problem in dimension ${m}$ and codimension ${n}$ is the following. Given a ${(m+n)}$-dimensional Riemannian manifold and a ${(m-1)}$-dimensional compact embedded oriented submanifold ${\Gamma\subset M}$ (without boundary), find a ${m}$-dimensional embedded oriented submanifold ${\Sigma\subset M}$ with boundary ${\partial\Sigma=\Gamma}$ such that

$\displaystyle \textrm{vol}_m(\Sigma)\leq \textrm{vol}_m(\widetilde{\Sigma})$

for all oriented ${m}$-dimensional submanifold ${\widetilde{\Sigma}}$ with ${\partial\widetilde{\Sigma} = \Gamma}$. (Here, ${\textrm{vol}_m(A)}$ denotes the ${m}$-dimensional volume of ${A}$).

This formulation of Plateau’s problem allows for several variants. Moreover, the solution to Plateau’s problem is very sensitive on the precise mathematical formulation of the problem (and, in particular, on the dimension ${m}$ and codimension ${n}$).

Example 1 The works of Douglas and Radó (based on the conformal parametrization method and some compactness arguments) provided a solution to (a formulation of) Plateau’s problem when ${m=2}$ and the boundary ${\Gamma}$ is circular (i.e., ${\Gamma}$ is parametrized by the round circle ${S^1=\{ (x,y)\in\mathbb{R}^2: x^2+y^2=1 \}}$). Unfortunately, the techniques employed by Douglas and Radó do not work for arbitrary dimension ${m}$ and codimension ${n}$.

Example 2 The following example gives an idea on the difficulties that one might found while trying to solve Plateau’s problem (in the formulation given above).Let us consider the case ${m=n=2}$, ${\Gamma\subset\mathbb{R}^4\simeq \mathbb{C}^2}$ defined as

$\displaystyle \Gamma:=\{(\zeta^2,\zeta^3)\in\mathbb{C}^2: |\zeta|=1\}$

The singular immersed disk

$\displaystyle D:=\{(z, w)\in\mathbb{C}^2: z^3=w^2, |z|\leq 1\}$

satisfies ${\partial D=\Gamma}$ and the so-called calibration method can be applied to prove that

$\displaystyle \mathcal{H}^2(D)<\mathcal{H}^2(\Sigma)$

for all smooth oriented ${2}$-dimensional submanifold ${\Sigma\subset\mathbb{R}^4}$ with ${\partial\Sigma = \Gamma}$. (Here, ${\mathcal{H}^2}$ stands for the ${2}$-dimensional Hausdorff measure on ${\mathbb{R}^4}$.)

The example above motivates the introduction of weak solutions (including immersed submanifolds) to Plateau’s problem, so that the main question becomes the existence and regularity of weak solutions.

This point of view was adopted by several authors: for example, De Giorgi studied the notion of sets of finite perimeter when ${n=1}$, and Federer and Fleming introduced the notion of currents.

Remark 1 The “PDE counterpart” of this point of view is the study of existence and regularity of weak solutions of PDEs in Sobolev spaces.

In arbitrary dimensions and codimensions, De Giorgi’s regularity theory provides weak solutions to Plateau’s problem which are smooth on open and dense subsets.

As it was pointed out by Federer, this is not a satisfactory regularity statement. Indeed, De Giorgi’s regularity theory allows (in principle) weak solutions to Plateau’s problem whose singular set (i.e., the subset of points where the weak solution is not smooth) could be “large” in the sense that its (${m}$-dimensional) Hausdorff measure could be positive.

In this context, Almgren wrote two preprints which together had more than 1000 pages (published in posthumous way here) where it was shown that the singular set of weak solutions to Plateau’s problem has codimension ${2}$ at least:

Theorem 1 (Almgren) Let ${T}$ be an integral rectifiable ${m}$-dimensional current in a ${(m+n)}$-dimensional ${C^5}$ Riemannian manifold ${M^{m+n}}$.If ${T}$ is area-minimizing (i.e., a weak solution to Plateau’s problem), then there exists a closed subset ${\textrm{sing}(T)}$ of ${T}$ such that:

• ${\textrm{sing}(T)}$ has codimension ${\geq 2}$: the Hausdorff dimension of ${\textrm{sing}(T)}$ is ${\leq (m-2)}$, and
• ${\textrm{sing}(T)}$ is the singular set of ${T}$: the subset ${T\cap (M-(\textrm{supp}(\partial T)\cup\textrm{sing}(T))}$ is induced by a smooth oriented ${m}$-dimensional submanifold of ${M}$.

We will explain the notion of area-minimizing integral rectifiable currents (appearing in the statement above) in a moment. For now, let us just make some historical remarks. Ambrosio has the impression that some parts of Almgren’s work were not completely reviewed, even though several experts have used some of the ideas and techniques introduced by Almgren. For this reason, the simplifications (of about ${1/3}$) and, more importantly, improvements of Almgren’s work obtained by DeLellis-Spadaro in a series of five articles (quoted in the very beginning of the pots) were very much appreciated by the experts of this field.

Closing this introduction, let us present the plan for the remaining sections of this post:

• the next section reviews the construction of solutions to the generalized Plateau problem via Federer-Fleming compactness theorem for integral rectifiable currents;
• then, in the subsequent section, we revisit some aspects of the regularity theory of weak solutions to Plateau’s problem in codimension ${n=1}$; in particular, we will see in this setting a stronger version of Theorem 1;
• after that, in the last section of this post, we make some comments on the works of Almgren and DeLellis-Spadaro on the regularity theory for Plateau’s problem in codimension ${n\geq 2}$; in particular, we will sketch the proof of Theorem 1 above.

Remark 2 For the sake of exposition, from now on we will restrict ourselves to the study of Plateau’s problem in the case of Euclidean ambient spaces, i.e., ${M^{m+n} = \mathbb{R}^{m+n}}$. In fact, this is not a great loss in generality because the statements and proofs in the Euclidean setting ${M^{m+n} = \mathbb{R}^{m+n}}$ can be adapted to arbitrary Riemannian manifolds ${M^{m+n}}$ with almost no extra effort.

Posted by: matheuscmss | March 7, 2015

## Journée Surfaces Plates

Luca Marchese and I are organizing a one-day event called “Journée Surfaces Plates” at LAGA, Université Paris 13 next May 20th, 2015.

This event will consist into three talks by Giovanni Forni, Jean-Christophe Yoccoz and Anton Zorich, and a tentative schedule is available here. Also, it is likely that these talks will be recorded, and, in this case, I plan to update this (very short) post by providing a link for the eventual videos of these lectures.

Please note that any interested person can attend this event (as there are no inscription fees).  On the other hand, since our budget is very limited, unfortunately Luca and I  can not offer any sort of financial help (with local and/or travel expenses) to potential participants. In particular, we would ask you to use your own grants to support your eventual participation in “Journée Surfaces Plates”.

Posted by: matheuscmss | February 28, 2015

## First Bourbaki seminar of 2015 (II): Carron’s talk

For the second installment of this series of posts (which started here) on the first Bourbaki seminar of 2015, we will discuss Gilles Carron talk entitled “New utilisation of the maximum principles in Geometry (after B. Andrews, S. Brendle, J. Clutterbuck)”. Here, besides the original works of Andrews-Clutterbuck and Brendle (quoted below), the main references are the video of Carron’s talk and his lecture notes (both in French).

Disclaimer. All errors, mistakes or misattributions are my entire responsibility.

1. Introduction

Given a Riemannian ${n}$-dimensional manifold ${M}$, one can often study its Geometry by analyzing adequate smooth real functions ${f}$ on ${M}$ (such as scalar curvature). One of the techniques used to get some information about ${f}$ is the following observation (“baby maximum principle”): if ${f}$ has a local maximum at a point ${p}$, then we dispose of

• a first order information: the gradient of ${f}$ at ${p}$ vanishes; and
• a second order information: the Hessian of ${f}$ at ${p}$ has a sign (namely, it is negative definite).

In order to extract more information from this technique, one can appeal to the so-called doubling of variables method: instead of studying ${f}$, one investigates the local maxima of a “well-chosen” function ${g}$ on the double of variables (e.g., ${g:M\times M\rightarrow\mathbb{R}}$). In this way, we have new constraints because the gradient and Hessian of ${g}$ depend on more variables than those of ${f}$.

This idea of doubling the variables goes back to Kruzkov who used it to estimate the modulus of continuity of the derivative of solutions of a non-linear parabolic PDE (in one space dimension). In this post we shall see how this idea was ingeniously employed by Andrews and Clutterbuck (2011) and Brendle (2013) in two recent important works.

Theorem 1 (Andrews-Clutterbuck) Let ${\Omega\subset\mathbb{R}^n}$ be a convex domain of diameter ${D}$. Consider the Schrödinger operator ${-\Delta+V}$ where ${\Delta=\sum\limits_{i=1}^n\frac{\partial^2}{\partial^2 x_i}}$ is the Laplacian operator and ${V}$ is the operator induced by the multiplication by a convex function ${V:\Omega\rightarrow\mathbb{R}}$.Recall that the spectrum of ${-\Delta+V}$ with respect to Dirichlet condition on the boundary ${\partial\Omega}$ consists of a discrete set of eigenvalues of the form: ${\lambda_1<\lambda_2\leq \dots}$

In this setting, the fundamental gap ${\lambda_2-\lambda_1}$ of ${-\Delta+V}$ is bounded from below by

$\displaystyle \lambda_2 - \lambda_1 \geq 3\frac{\pi^2}{D^2}$

Remark 1 This theorem is sharp: ${\lambda_2-\lambda_1=3\frac{\pi^2}{D^2}}$ when ${\Omega=(-D/2, D/2)\subset\mathbb{R}}$ and ${V\equiv 0}$ (by Fourier analysis). In other terms, Andrews-Clutterbuck theorem is an optimal comparison theorem between the fundamental gap of general Schrödinger operators with the one-dimensional case.

Next, we state Brendle’s theorem:

Theorem 2 (Brendle) A minimal torus inside the round sphere ${S^3=\{(x_1,\dots, x_4\in\mathbb{R}^4: x_1^2+\dots+x_4^2=1\}}$ is isometric to Clifford torus ${\mathbb{T}=\{(x_1,\dots,x_4)\in\mathbb{R}^4: x_1^2+x_2^2 = x_3^2+x_4^2 = 1/2\}}$.

The sketches of proof of these results are presented in the next two Sections. For now, let us close this introductory section by explaining some of the motivations of these theorems.

1.1. The context of Andrews-Clutterbuck theorem

The interest of the fundamental gap ${\gamma=\lambda_2-\lambda_1}$ comes from the fact that it helps in the description of the long-term behavior of non-negative non-trivial solutions of the heat equation

$\displaystyle \frac{\partial}{\partial t} u(t,x) = \Delta u(t,x) - V(x)u(t,x), \quad (t,x)\in [0,\infty)\times \Omega$

with ${u\equiv 0}$ on ${\partial \Omega}$. More precisely, one has that

$\displaystyle u(t,x) = c \exp(\lambda_1t) f_1(x) (1+O(\exp(-\gamma t))$

where

• ${c}$ is an adequate constant,
• ${f_1}$ is the ground state of ${-\Delta+V}$, i.e., ${-\Delta f_1+V f_1=\lambda_1 f_1}$, ${f_1>0}$ on ${\textrm{int}(\Omega)}$, ${f_1=0}$ on ${\partial\Omega}$ and ${f_1}$ is normalized so that ${\int_{\Omega} f_1^2=1}$, and
• ${O(\exp(-\gamma t))}$ denotes (as usual) a quantity bounded from above by ${C\exp(-\gamma t)}$ for some constant ${C>0}$ and all ${t\geq 0}$.

The theorem of Andrews-Clutterbuck answers positively a conjecture of Yau and Ashbaugh-Benguria. This conjecture was based on a series of works in Mathematics and Physics: from the mathematical side, van den Berg observed during his study of the behavior of spectral functions in big convex domains (modeling Bose-Einstein condensation) that ${\lambda_2-\lambda_1\geq 3\pi^2/D^2}$ for the free Laplacian (${V\equiv 0}$) on several convex domains. After that, Singer-Wong-Yau-Yau proved that

$\displaystyle \lambda_2-\lambda_1\geq \frac{1}{4}\left(\frac{\pi^2}{D^2}\right)$

and Yu-Zhong improved this result by showing that

$\displaystyle \lambda_2-\lambda_1\geq \frac{\pi^2}{D^2}$

Furthermore, some particular cases of Andrews-Clutterbuck were previously known: for instance, Lavine proved the one-dimensional case ${\Omega\subset\mathbb{R}}$, and other authors studied the cases of convex domains with some (axial and/or rotational) symmetries in higher dimensions.

1.2. The context of Brendle theorem

The theorem of Brendle answers affirmatively a Lawson’s conjecture.

Lawson arrived at this conjecture after proving (in this paper here) that every compact oriented surface ${\Sigma}$ without boundary can be minimally embedded in ${S^3}$.

Remark 2 The analog of Lawson’s theorem is completely false in ${\mathbb{R}^3}$: using the maximum principle, one can show that there are no immersed compact minimal surfaces in ${\mathbb{R}^3}$.

Moreover, Lawson (in the same paper loc. cit.) showed that, if the genus of ${\Sigma}$ is not prime, then ${\Sigma}$ admits two non-isometric minimal embeddings in ${S^3}$.

On the other hand, Lawson’s construction in the case of genus ${1}$ produces only the Clifford torus (up to isometries). Nevertheless, Lawson proved (in this paper here) that if ${\Sigma\subset S^3}$ is a minimal torus, then there exists a diffeomorphism ${F:S^3\rightarrow S^3}$ taking ${\Sigma}$ to the Clifford torus ${\mathbb{T}}$: in other terms, there is no knotted minimal torus in ${S^3}$!

In this context, Lawson was led to conjecture that this diffeomorphism ${F:S^3\rightarrow S^3}$ could be taken to be an isometry, an assertion that was confirmed by Brendle.