Remarks on the non-commutative Khintchine inequalities for 0<p<20<p<2

We show that the validity of the non-commutative Khintchine inequality for some $q$ with $1<q<2$ implies its validity (with another constant) for all $1\le p<q$. We prove this for the inequality involving the Rademacher functions, but also for more general "lacunary" sequences, or even non-commutative analogues of the Rademacher functions. For instance, we may apply it to the "Z(2)-sequences" previously considered by Harcharras. The result appears to be new in that case. It implies that the space $\ell^n_1$ contains (as an operator space) a large subspace uniformly isomorphic (as an operator space) to $R_k+C_k$ with $k\sim n^{\frac12}$. This naturally raises several interesting questions concerning the best possible such $k$. Unfortunately we cannot settle the validity of the non-commutative Khintchine inequality for $0<p<1$ but we can prove several would be corollaries. For instance, given an infinite scalar matrix $[x_{ij}]$, we give a necessary and sufficient condition for $[\pm x_{ij}]$ to be in the Schatten class $S_p$ for almost all (independent) choices of signs $\pm 1$. We also characterize the bounded Schur multipliers from $S_2$ to $S_p$. The latter two characterizations extend to $0<p<1$ results already known for $1\le p\le2$. In addition, we observe that the hypercontractive inequalities, proved by Carlen and Lieb for the Fermionic case, remain valid for operator space valued functions, and hence the Kahane inequalities are valid in this setting.Comment: Some more minor correction

Subgaussian sequences in probability and Fourier analysis

This is a review on subgaussian sequences of random variables, prepared for the Mediterranean Institute for the Mathematical Sciences (MIMS). We first describe the main examples of such sequences. Then we focus on examples coming from the harmonic analysis of Fourier series and we describe the connection of subgaussian sequences of characters on the unidimensional torus (or any compact Abelian group) with Sidon sets. We explain the main combinatorial open problem concerning such subgaussian sequences. We present the answer to the analogous question for subgaussian bounded mean oscillation (BMO) sequences on the unit circle. Lastly, we describe several very recent results that provide a generalization of the preceding ones when the trigonometric system (or its analogue on a compact Abelian group) is replaced by an arbitrary orthonormal system bounded in $L_\infty$.Comment: We added a short section on subgaussian BMO sequence

Spectral gap properties of the unitary groups: around Rider's results on non-commutative Sidon sets

We present a proof of Rider's unpublished result that the union of two Sidon sets in the dual of a non-commutative compact group is Sidon, and that randomly Sidon sets are Sidon. Most likely this proof is essentially the one announced by Rider and communicated in a letter to the author around 1979 (lost by him since then). The key fact is a spectral gap property with respect to certain representations of the unitary groups $U(n)$ that holds uniformly over $n$. The proof crucially uses Weyl's character formulae. We survey the results that we obtained 30 years ago using Rider's unpublished results. Using a recent different approach valid for certain orthonormal systems of matrix valued functions, we give a new proof of the spectral gap property that is required to show that the union of two Sidon sets is Sidon. The latter proof yields a rather good quantitative estimate. Several related results are discussed with possible applications to random matrix theory.Comment: v2: minor corrections, v3 more minor corrections v4) minor corrections, last section removed to be included in another paper in preparation with E. Breuillard v5) more minor corrections + two references added. The paper will appear in a volume dedicated to the memory of V. P. Havi

Analyzing Individual Proofs as the Basis of Interoperability between Proof Systems

We describe the first results of a project of analyzing in which theories formal proofs can be ex- pressed. We use this analysis as the basis of interoperability between proof systems.Comment: In Proceedings PxTP 2017, arXiv:1712.0089

Rigidity and L2L^2 cohomology of hyperbolic manifolds

When X=\Gamma\backslash \H^n is a real hyperbolic manifold, it is already known that if the critical exponent is small enough then some cohomology spaces and some spaces of $L^2$ harmonic forms vanish. In this paper, we show rigidity results in the borderline case of these vanishing results

Deduction modulo theory

This paper is a survey on Deduction modulo theor