arXiv subjects
Publications and source records attributed to Javier Parcet.
We provide an operator space version of Maurey's factorization theorem. The main tool is an embedding result of independent interest. Applications for operator spaces and noncommutative Lp spaces are included.
We prove weak type inequalities for a large class of noncommutative square functions. In conjunction with BMO type estimates, interpolation and duality, we will obtain the corresponding equivalences in the whole Lp scale. The main novelty of our approach relies on a row/column valued theory for noncommutative martingale transforms and operator-valued Calderon-Zygmund operators. This seems to be new in the noncommutative setting and might be regarded as a first step towards a vector-valued noncommutative theory. Some examples and applications are also explored.
Let $f_1, f_2, ..., f_n$ be a family of independent copies of a given random variable f in a probability space $(Ω, \mathcal{F}, μ)$. Then, the following equivalence of norms holds whenever $1 \le q \le p < \infty$ $$\Big(\int_Ω \Big[ \sum_{k=1}^n |f_k|^q \Big]^{\frac{p}{q}} d μ\Big)^{\frac1p} \sim \max_{r \in \{p,q\}} {n^{\frac1r} \Big(\int_Ω|f|^r dμ\Big)^{\frac1r}}.$$ We prove a noncommutative analogue of this inequality for sums of free random variables over a given von Neumann subalgebra. This formulation leads to new classes of noncommutative function spaces which appear in quantum probability as square functions, conditioned square functions and maximal functions. Our main tools are Rosenthal type inequalities for free random variables, noncommutative martingale theory and factorization of operator-valued analytic functions. This allows us to generalize this inequality as a result for noncommutative L_p in the category of operator spaces. Moreover, the use of free random variables produces the right formulation for $p=\infty$, which has not a commutative counterpart.
Given any $1 < q \le 2$, we use new free probability techniques to construct a completely isomorphic embedding of $\ell_q$ (equipped with its natural operator space structure) into the predual of a sufficiently large QWEP von Neumann algebra.
In this paper we obtain the weak type (1,1) boundedness of Calderon-Zygmund operators acting over operator-valued functions. Our main tools for its solution are a noncommutative form of Calderon-Zygmund decomposition in conjunction with a pseudo-localization principle for singular integrals, which is new even in the classical setting and of independent interest. Perhaps because of the hidden role of pseudo-localization and almost orthogonality, this problem has remained open for quite some time. We also consider Calderon-Zygmund operators associated to certain operator-valued kernels.
We show that a reflexive subspace of the predual of a von Neumann algebra embeds into a noncommutative Lp space for some p>1. This is a noncommutative version of Rosenthal's result for commutative Lp spaces. Similarly for 1 < q < 2, an infinite dimensional subspace X of a noncommutative Lq space either contains lq or embeds in Lp for some q < p < 2. The novelty in the noncommutative setting is a double sided change of density.
Let 1 \le p < q \le 2 and let M be any von Neumann algebra. We use recent techniques from free harmonic analysis to construct a completely isomorphic embedding of Lq(M) (equipped with its natural operator space structure) into Lp(A) for some sufficiently large von Neumann algebra A. We show that hyperfiniteness and the QWEP are preserved in our construction.
Let $\mathcal{A}$ denote the reduced amalgamated free product of a family $\mathsf{A}_1, \mathsf{A}_2, ..., \mathsf{A}_n$ of von Neumann algebras over a von Neumann subalgebra $\Be$ with respect to normal faithful conditional expectations $\Es_k: \mathsf{A}_k \to \Be$. We investigate the norm in $L_p(\Al)$ of homogeneous polynomials of a given degree $d$. We first generalize Voiculescu's inequality to arbitrary degree $d \ge 1$ and indices $1 \le p \le \infty$. This can be regarded as a free analogue of the classical Rosenthal inequality. Our second result is a length-reduction formula from which we generalize recent results of Pisier, Ricard and the authors. All constants in our estimates are independent of $n$ so that we may consider infinitely many free factors. As applications, we study square functions of free martingales. More precisely we show that, in contrast with the Khintchine and Rosenthal inequalities, the free analogue of the Burkholder-Gundy inequalities does not hold on $L_\infty(\Al)$. At the end of the paper we also consider Khintchine type inequalities for Shlyakhtenko's generalized circular systems.
An analogue of Taylor's formula, which arises by substituting the classical derivative by a divided difference operator of Askey-Wilson type, is developed here. We study the convergence of the associated Taylor series. Our results complement a recent work by Ismail and Stanton. Quite surprisingly, in some cases the Taylor polynomials converge to a function which differs from the original one. We provide explicit expressions for the integral remainder. As application, we obtain some summation formulas for basic hypergeometric series. As far as we know, one of them is new. We conclude by studying the different forms of the binomial theorem in this context.
Let $\Free_n$ denote the free group with $n$ generators $g_1, g_2, ..., g_n$. Let $λ$ stand for the left regular representation of $\Free_n$ and let $τ$ be the standard trace associated to $λ$. Given any positive integer $d$, we study the operator space structure of the subspace $\Word_p(n,d)$ of $L_p(τ)$ generated by the family of operators $λ(g_{i_1}g_{i_2} ... g_{i_d})$ with $1 \le i_k \le n$. Moreover, our description of this operator space holds up to a constant which does not depend on $n$ or $p$, so that our result remains valid for infinitely many generators. We also consider the subspace of $L_p(τ)$ generated by the image under $λ$ of the set of reduced words of length $d$. Our result extends to any exponent $1 \le p \le \infty$ a previous result of Buchholz for the space $\Word_{\infty}(n,d)$. The main application is a certain interpolation theorem, valid for any degree $d$ (extending a result of the second author restricted to $d=1$). In the simplest case $d=2$, our theorem can be stated as follows: consider the space $\mathcal{K}_p$ formed of all block matrices $a=(a_{ij})$ with entries in the Schatten class $S_p$, such that $a$ is in $S_p$ relative to $\ell_2 \otimes \ell_2$ and moreover such that $(\sum_{ij} a_{ij}^* a_{ij} )^{1/2}$ and $(\sum_{ij} a_{ij} a_{ij}^*)^{1/2}$ both belong to $S_p$. We equip $\mathcal{K}_p$ with the maximum of the three corresponding norms. Then, for $2 \le p \le \infty$ we have $\mathcal{K}_p \simeq (\mathcal{K}_2, \mathcal{K}_\infty)_θ$ with $1/p = (1-θ)/2$.
We provide an analogue of Gundy's decomposition for L1-bounded non-commutative martingales. An important difference from the classical case is that for any L1-bounded non-commutative martingale, the decomposition consists of four martingales. This is strongly related with the row/column nature of non-commutative Hardy spaces of martingales. As applications, we obtain simpler proofs of the weak type (1,1) boundedness for non-commutative martingale transforms and the non-commutative analogue of Burkholder's weak type inequality for square functions. A sequence (x_n) in a normed space X is called 2-co-lacunary if there exists a bounded linear map from the closed linear span of (x_n) to l2 taking each x_n to the n-th vector basis of l2. We prove (using our decomposition) that any relatively weakly compact martingale difference sequence in L1(M,τ) whose sequence of norms is bounded away from zero is 2-co-lacunary, generalizing a result of Aldous and Fremlin to non-commutative L1-spaces.
Given a probability space $(Ω, \mathsf{A}, μ)$, let $\mathsf{A}_1, \mathsf{A}_2, ...$ be a filtration of $σ$-subalgebras of $\mathsf{A}$ and let $\mathsf{E}_1, \mathsf{E}_2, ...$ denote the corresponding family of conditional expectations. Given a martingale $f = (f_1, f_2, ...)$ adapted to this filtration and bounded in $L_p(Ω)$ for some $2 \le p < \infty$, Burkholder's inequality claims that $$\|f\|_{L_p(Ω)} \sim_{\mathrm{c}_p} \Big\| \Big(\sum_{k=1}^\infty \mathsf{E}_{k-1}(|df_k|^2) \Big)^{1/2} \Big\|_{L_{p}(Ω)} + \Big(\sum_{k=1}^\infty \|df_k\|_p^p \Big)^{1/p}.$$ Motivated by quantum probability, Junge and Xu recently extended this result to the range $1 < p < 2$. In this paper we study Burkholder's inequality for $p=1$, for which the techniques (as we shall explain) must be different. Quite surprisingly, we obtain two non-equivalent estimates which play the role of the weak type $(1,1)$ analog of Burkholder's inequality. As application, we obtain new properties of Davis decomposition for martingales.
We emulate the Rademacher functions on any non-commutative compact group requiring the resulting system to have pairwise disjoint spectra.
We investigate the norm of sums of independent vector-valued random variables in noncommutative Lp spaces. This allows us to obtain a uniform family of complete embeddings of the Schatten class Sq^n in Sp(lq^m) with optimal order m = n^2. Using these embeddings we show the surprising fact that the sharp type (cotype) index in the sense of operator spaces for Lp[0,1] is min(p,p') (max(p,p')). Similar techniques are used to show that the operator space notions of B-convexity and K-convexity are equivalent.
In this paper we extend a recent Pisier's inequality for p-orthogonal sums in non-commutative Lebesgue spaces. To that purpose, we generalize the notion of p-orthogonality to the class of multi-indexed families of operators. This kind of families appear naturally in certain non-commutative Khintchine type inequalities associated with free groups. Other p-orthogonal families are given by the homogeneous operator-valued polynomials in the Rademacher variables or the multi-indexed martingale difference sequences. As in Pisier's result, our tools are mainly combinatorial.
In this paper we present a unified approach to the spectral analysis of an hypergeometric type operator whose eigenfunctions include the classical orthogonal polynomials. We write the eigenfunctions of this operator by means of a new Taylor formula for operators of Askey-Wilson type. This gives rise to some expressions for the eigenfunctions, which are unknown in such a general setting. Our methods also give a general Rodrigues formula from which several well known formulas of Rodrigues type can be obtained directly. Moreover, other new Rodrigues type formulas come out when seeking for regular solutions of the associated functional equations. The main difference here is that, in contrast with the formulas appearing in the literature, we get non-ramified solutions which are useful for applications in combinatorics. Another fact, that becomes clear in this paper, is the role played by the theory of elliptic functions in the connection between ramified and non-ramified solutions.
Let $Ω$ denote a non-empty finite set. Let $S(Ω)$ stand for the symmetric group on $Ω$ and let us write $P(Ω)$ for the power set of $Ω$. Let $ρ: S(Ω) \to U(L^2(P(Ω)))$ be the left unitary representation of $S(Ω)$ associated with its natural action on $P(Ω)$. We consider the algebra consisting of those endomorphisms of $L^2(P(Ω))$ which commute with the action of $ρ$. We find an attractive basis $B$ for this algebra. We obtain an expression, as a linear combination of $B$, for the product of any two elements of $B$. We obtain an expression, as a linear combination of $B$, for the adjoint of each element of $B$. It turns out the Fourier transform on $P(Ω)$ is an element of our algebra; we give the matrix which represents this transform with respect to $B$.
The main purpose of this paper is to study the validity of the Hausdorff-Young inequality for vector-valued functions defined on a non-commutative compact group. The natural context for this research is that of operator spaces. This leads us to formulate a whole new theory of Fourier type and cotype for the category of operator spaces. The present paper is the first step in this program, where the basic theory is presented, the main examples are analyzed and some questions are posed.