In this talk we will don't speak about Joseph-Louis Lagrange (1736-1813) but about Lagrange's reception at the nineteenth Century. "Who read Lagrange at this Times?", "Why and How?", "What does it mean being a mathematician or doing mathematics at this Century" are some of the questions of our conference. We will give some elements of answers and the case Lagrange will be a pretext in order to explain what are doing historians of mathematics: searching archives and – thanks to a methodology – trying to understand, read and write the Past.
Lagrange - mathematical press - complete works - bibliographic index of mathematical sciences (1894-1912) - Liouville - Boussinesq - Terquem[-]

Motivated by the spectrogram (or short-time Fourier transform) basic principles of linear algebra are explained, preparing for the more general case of Gabor frames in time-frequency analysis. The importance of the singular value decomposition and the four spaces associated with a matrix is pointed out, and based on this the pseudo-inverse (leading later to the dual Gabor frame) and the Loewdin (symmetric) orthogonalization are explained.
CIRM - Chaire Jean-Morlet 2014 - Aix-Marseille Université[-]

Generalizing results of Rossi and Vergne for the holomorphic discrete series on symmetric domains, on the one hand, and of Chailuek and Hall for Toeplitz operators on the ball, on the other hand, we establish existence of analytic continuation of weighted Bergman spaces, in the weight (Wallach) parameter, as well as of the associated Toeplitz operators (with sufficiently nice symbols), on any smoothly bounded strictly pseudoconvex domain. Still further extension to Sobolev spaces of holomorphic functions is likewise treated.[-]

One of the important "products" of wavelet theory consists in the insight that it is often beneficial to consider sparsity in signal processing applications. In fact, wavelet compression relies on the fact that wavelet expansions of real-world signals and images are usually sparse. Compressive sensing builds on sparsity and tells us that sparse signals (expansions) can be recovered from incomplete linear measurements (samples) efficiently. This finding triggered an enormous research activity in recent years both in signal processing applications as well as their mathematical foundations. The present talk discusses connections of compressive sensing and time-frequency analysis (the sister of wavelet theory). In particular, we give on overview on recent results on compressive sensing with time-frequency structured random matrices.

Keywords: compressive sensing - time-frequency analysis - wavelets - sparsity - random matrices - $\ell_1$-minimization - radar - wireless communications[-]

I present a joint work with S. Popa and D. Shlyakhtenko introducing a cohomology theory for quasi-regular inclusions of von Neumann algebras. In particular, we define $L^2$-cohomology and $L^2$-Betti numbers for such inclusions. Applying this to the symmetric enveloping inclusion of a finite index subfactor, we get a cohomology theory and a definition of $L^2$-Betti numbers for finite index subfactors, as well as for arbitrary rigid $C^*$-tensor categories. For the inclusion of a Cartan subalgebra in a $II_1$ factor, we recover Gaboriau's $L^2$-Betti numbers for equivalence relations.[-]

This is joint work with Éric Ricard. We give a proof of the Khintchine inequalities in non- commutative $L_p$-spaces for all $0 < p < 1$. This case remained open since the first proof given by Francoise Lust-Piquard in 1986 for $1 < p < \infty$. These inequalities are valid for the Rademacher functions or Gaussian random variables, but also for more general sequences, e.g. for lacunary Fourier series or the analogues of Gaussian variables in free probability.

The Khintchine inequalities for non-commutative $L_p$-spaces play an important roˆle in the recent developments in non-commutative Functional Analysis, and in particular in Operator Space Theory. Just like their commutative counterpart for ordinary $L_p$-spaces, they are a crucial tool to understand the behavior of unconditionally convergent series of random variables, or random vectors, in non-commutative $L_p$. The commutative version for $p = 1$ is closely related to Grothendieck's Theorem. In the most classical setting, the non-commutative Khintchine inequalities deal with Rademacher series of the form

$S=\sum_kr_k(t)x_k$

where $(r_k)$ are the Rademacher functions on the Lebesgue interval where the coefficients $x_k$ are in the Schatten $q$-class or in a non-commutative $L_q$-space associated to a semifinite trace $\tau$. Let us denote simply by $||.||_q$ the norm (or quasi-norm) in the latter Banach (or quasi-Banach) space, that we will denote by $L_q(\tau)$. When $\tau$ is the usual trace on $B(\ell_2)$, we recover the Schatten $q$-class. By Kahane's well known results, $S$ converges almost surely in norm if it converges in $L_q(dt;L_q(\tau))$. Thus to characterize the almost sure norm-convergence for series such as $S$, it suffices to produce a two sided equivalent of $||S||_{L_q(dt;L_q(\tau))}$ when $S$ is a finite sum, and this is precisely what the non-commutative Khintchine inequalities provide :
For any $0 < q < \infty$ there are positive constants $\alpha_q,\beta_q$ such that for any finite set $(x_1, . . . , x_n)$ in $L_q(\tau)$ we have

$(\beta_q)^{-1}|||(x_k)|||_q\leq\left(\int||S(t)||^q_qdt\right)^{1/q}\leq\alpha_q|||(x_k)|||_q$

where $|||(x_k)|||_q$ is defined as follows :
If $2\le q<\infty$

$|||x_k|||_q \overset{def}{=} \max\lbrace ||(\sum x^*_k x_k)^{1/2} ||_q, ||(\sum x_kx^*_k)^{1/2}||_q\rbrace$ (1)

and if $0\le q<2$:

$|||x|||_q \overset{def}{=} \underset{x_k=a_k+b_k}{inf} \lbrace ||(\sum a^*_ka_k)^{1/2} ||_q + ||(\sum b_kb^*_k)^{1/2}||_q\rbrace$. (2)

Note that $\beta=1$ if $q\ge2$, while $\alpha_q=1$ if $q\le2$ and the corresponding one sided bounds are easy. The difficulty is to verify the other side.[-]

A determinantal point process governed by a Hermitian contraction kernel $K$ on a measure space $E$ remains determinantal when conditioned on its configuration on a subset $B \subset E$. Moreover, the conditional kernel can be chosen canonically in a way that is "local" in a non-commutative sense, i.e. invariant under "restriction" to closed subspaces $L^2(B) \subset P \subset L^2(E)$. Using the properties of the canonical conditional kernel we establish a conjecture of Lyons and Peres: if $K$ is a projection then almost surely all functions in its image can be recovered by sampling at the points of the process.
Joint work with Alexander Bufetov and Yanqi Qiu.[-]

Two important examples of the determinantal point processes associated with the Hilbert spaces of holomorphic functions are the Ginibre point process and the set of zeros of the Gaussian Analytic Functions on the unit disk. In this talk, I will talk such class of determinantal point processes in greater generality. The main topics concerned are the equivalence of the reduced Palm measures and the quasi-invariance of these point processes under certain natural group action of the group of compactly supported diffeomorphisms of the phase space. This talk is based partly on the joint works with Alexander I. Bufetov and partly on a more recent joint work with Alexander I. Bufetov and Shilei Fan.[-]