arXiv subjects

Todd Kemp

Publications and source records attributed to Todd Kemp.

30 records · Page 2Linked to original sources

arXiv2013

Liberation of Projections

We study the liberation process for projections: $(p,q)\mapsto (p_t,q)= (u_tpu_t^\ast,q)$ where $u_t$ is a free unitary Brownian motion freely independent from $\{p,q\}$. Its action on the operator-valued angle $qp_tq$ between the projections induces a flow on the corresponding spectral measures $μ_t$; we prove that the Cauchy transform of the measure satisfies a holomorphic PDE. We develop a theory of subordination for the boundary values of this PDE, and use it to show that the spectral measure $μ_t$ possesses a piecewise analytic density for any $t>0$ and any initial projections of trace $\frac12$. We us this to prove the Unification Conjecture for free entropy and information in this trace $\frac12$ setting.

math.FA↗

arXiv2013

The Large-$N$ Limits of Brownian Motions on $\mathbb{GL}_N$

We introduce a two-parameter family of diffusion processes $(B_{r,s}^N(t))_{t\ge 0}$, $r,s>0$, on the general linear group $\mathbb{GL}_N$ that are Brownian motions with respect to certain natural metrics on the group. At the same time, we introduce a two-parameter family of free Itô processes $(b_{r,s}(t))_{t\ge 0}$ in a faithful, tracial $W^\ast$-probability space, and we prove that the full process $(B^N_{r,s}(t))_{t\ge 0}$ converges to $(b_{r,s}(t))_{t\ge 0}$ in noncommutative distribution as $N\to\infty$ for each $r,s>0$. The processes $(b_{r,s}(t))_{t\ge 0}$ interpolate between the free unitary Brownian motion when $(r,s)=(1,0)$, and the free multiplicative Brownian motion when $r=s=\frac12$; we thus resolve the open problem of convergence of the Brownian motion on $\mathbb{GL}_N$ posed by Biane in 1997.

math.PR↗

arXiv2013

Heat Kernel Empirical Laws on $\mathbb{U}_N$ and $\mathbb{GL}_N$

This paper studies the empirical measures of eigenvalues and singular values for random matrices drawn from the heat kernel measures on the unitary groups $\mathbb{U}_N$ and the general linear groups $\mathbb{GL}_N$, for $N\in\mathbb{N}$. It establishes the strongest known convergence results for the empirical eigenvalues in the $\mathbb{U}_N$ case, and the first known almost sure convergence results for the eigenvalues and singular values in the $\mathbb{GL}_N$ case. The limit noncommutative distribution associated to the heat kernel measure on $\mathbb{GL}_N$ is identified as the projection of a flow on an infinite-dimensional polynomial space. These results are then strengthened from variance estimates to $L^p$ estimates for even integers $p$.

math.PR↗

arXiv2013

The Large-$N$ Limit of the Segal--Bargmann Transform on $\mathbb{U}_N$

We study the (two-parameter) Segal--Bargmann transform $\mathbf{B}_{s,t}^N$ on the unitary group $\mathbb{U}_N$, for large $N$. Acting on matrix valued functions that are equivariant under the adjoint action of the group, the transform has a meaningful limit $\mathscr{G}_{s,t}$ as $N\to\infty$, which can be identified as an operator on the space of complex Laurent polynomials. We introduce the space of {\em trace polynomials}, and use it to give effective computational methods to determine the action of the heat operator, and thus the Segal--Bargmann transform. We prove several concentration of measure and limit theorems, giving a direct connection from the finite-dimensional transform $\mathbf{B}_{s,t}^N$ to its limit $\mathscr{G}_{s,t}$. We characterize the operator $\mathscr{G}_{s,t}$ through its inverse action on the standard polynomial basis. Finally, we show that, in the case $s=t$, the limit transform $\mathscr{G}_{t,t}$ is the ``free Hall transform'' $\mathscr{G}^t$ introduced by Biane.

math.FA↗

arXiv2012

Duality in Segal-Bargmann Spaces

For $α>0$, the Bargmann projection $P_α$ is the orthogonal projection from $L^2(γ_α)$ onto the holomorphic subspace $L^2_{hol}(γ_α)$, where $γ_α$ is the standard Gaussian probability measure on $\C^n$ with variance $(2α)^{-n}$. The space $L^2_{hol}(γ_α)$ is classically known as the Segal-Bargmann space. We show that $P_α$ extends to a bounded operator on $L^p(γ_{αp/2})$, and calculate the exact norm of this scaled $L^p$ Bargmann projection. We use this to show that the dual space of the $L^p$-Segal-Bargmann space $L^p_{hol}(γ_{αp/2})$ is an $L^{p'}$ Segal-Bargmann space, but with the Gaussian measure scaled differently: $(L^p_{hol}(γ_{αp/2}))^* \cong L^{p'}_{hol}(γ_{αp'/2})$ (this was shown originally by Janson, Peetre, and Rochberg). We show that the Bargmann projection controls this dual isomorphism, and gives a dimension-independent estimate on one of the two constants of equivalence of the norms.

math.FA↗

arXiv2012

Wigner chaos and the fourth moment

We prove that a normalized sequence of multiple Wigner integrals (in a fixed order of free Wigner chaos) converges in law to the standard semicircular distribution if and only if the corresponding sequence of fourth moments converges to 2, the fourth moment of the semicircular law. This extends to the free probabilistic, setting some recent results by Nualart and Peccati on characterizations of central limit theorems in a fixed order of Gaussian Wiener chaos. Our proof is combinatorial, analyzing the relevant noncrossing partitions that control the moments of the integrals. We can also use these techniques to distinguish the first order of chaos from all others in terms of distributions; we then use tools from the free Malliavin calculus to give quantitative bounds on a distance between different orders of chaos. When applied to highly symmetric kernels, our results yield a new transfer principle, connecting central limit theorems in free Wigner chaos to those in Gaussian Wiener chaos. We use this to prove a new free version of an important classical theorem, the Breuer-Major theorem.

math.PR↗

arXiv2009

Enumeration of non-crossing pairings on bit strings

A non-crossing pairing on a bitstring matches 1s and 0s in a manner such that the pairing diagram is nonintersecting. By considering such pairings on arbitrary bitstrings $1^{n_1} 0^{m_1} ... 1^{n_r} 0^{m_r}$, we generalize classical problems from the theory of Catalan structures. In particular, it is very difficult to find useful explicit formulas for the enumeration function $ϕ(n_1, m_1, ..., n_r, m_r)$, which counts the number of pairings as a function of the underlying bitstring. We determine explicit formulas for $ϕ$, and also prove general upper bounds in terms of Fuss-Catalan numbers by relating non-crossing pairings to other generalized Catalan structures (that are in some sense more natural). This enumeration problem arises in the theory of random matrices and free probability.

math.CO↗

arXiv2008

Resolvents of R-Diagonal Operators

We consider the resolvent $(λ-a)^{-1}$ of any $R$-diagonal operator $a$ in a $\mathrm{II}_1$-factor. Our main theorem gives a universal asymptotic formula for the norm of such a resolvent. En route to its proof, we calculate the $R$-transform of the operator $|λ-c|^2$ where $c$ is Voiculescu's circular operator, and give an asymptotic formula for the negative moments of $|λ-a|^2$ for any $R$-diagonal $a$. We use a mixture of complex analytic and combinatorial techniques, each giving finer information where the other can give only coarse detail. In particular, we introduce {\em partition structure diagrams}, a new combinatorial structure arising in free probability.

math.OA↗

arXiv2008

Hypercontractivity for log-subharmonic functions

We prove strong hypercontractivity (SHC) inequalities for logarithmically subharmonic functions on $\RR^n$ and different classes of measures: Gaussian measures on $\RR^n$, symmetric Bernoulli and symmetric uniform probability measures on $\RR$, as well as their convolutions. Surprisingly, a slightly weaker strong hypercontractivity property holds for {\em any} symmetric measure on $\RR$. For all measures on $\R$ for which we know the (SHC) holds, we prove that a log--Sobolev inequality holds in the log-subharmonic category with a constant {\em smaller} than the one for Gaussian measure in the classical context. This result is extended to all dimensions for compactly-supported measures.

math.FA↗

arXiv2008

R-diagonal dilation semigroups

This paper addresses extensions of the complex Ornstein-Uhlenbeck semigroup to operator algebras in free probability theory. If $a_1,...,a_k$ are $\ast$-free $\mathscr{R}$-diagonal operators in a $\mathrm{II}_1$ factor, then $D_t(a_{i_1}... a_{i_n}) = e^{-nt} a_{i_1}... a_{i_n}$ defines a dilation semigroup on the non-self-adjoint operator algebra generated by $a_1,...,a_k$. We show that $D_t$ extends (in two different ways) to a semigroup of completely positive maps on the von Neumann algebra generated by $a_1,...,a_k$. Moreover, we show that $D_t$ satisfies an optimal ultracontractive property: $\|D_t\colon L^2\to L^\infty\| \sim t^{-1}$ for small $t>0$.

math.FA↗

arXiv2005

Strong Haagerup inequalities for free R-diagonal elements

In this paper, we generalize Haagerup's inequality (on convolution norm in the free group) to a very general context of R-diagonal elements in a tracial von Neumann algebra; moreover, we show that in this "holomorphic" setting, the inequality is greatly improved from its originial form. We give an elementary combinatorial proof of a very special case of our main result, and then generalize these techniques. En route, we prove a number of moment and cumulant estimates for R-diagonal elements that are of independent interest. Finally, we use our strong Haagerup inequality to prove a strong ultracontractivity theorem.

math.OA↗

arXiv2005

Strong hypercontractivity in non-commutative holomorphic spaces

We introduce holomorphic algebras $H_q$ in the context of the q-Gaussian algebra $Γ_q$ of Bozejko, Kümmerer, and Speicher, and give a q-Segal-Bargmann transform for them. We then prove a strong hypercontractivity theorem, generalizing Janson's strong (holomorphic) hypercontractivity, from $L^2(H_q) \to L^r(H_q)$ for r an even integer.

math.OA↗