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John E. McCarthy

Publications and source records attributed to John E. McCarthy.

At least 19 recordsLinked to original sources
arXiv2026

A Caratheodory theorem for the bidisk via Hilbert space methods

If $\ph$ is an analytic function bounded by 1 on the bidisk $\D^2$ and $τ\in\tb$ is a point at which $\ph$ has an angular gradient $\nabla\ph(τ)$ then $\nabla\ph(\la) \to \nabla\ph(τ)$ as $\la\toτ$ nontangentially in $\D^2$. This is an analog for the bidisk of a classical theorem of Carathéodory for the disk. For $\ph$ as above, if $τ\in\tb$ is such that the $\liminf$ of $(1-|\ph(\la)|)/(1-\|\la\|)$ as $\la\toτ$ is finite then the directional derivative $D_{-\de}\ph(τ)$ exists for all appropriate directions $\de\in\C^2$. Moreover, one can associate with $\ph$ and $τ$ an analytic function $h$ in the Pick class such that the value of the directional derivative can be expressed in terms of $h$.

math.CV↗
arXiv2026

From Clouatre-Ostermann-Ransford to Okubo-Ando

We prove that if $θ$ is a continuous unital homomorphism of an operator algebra $A$ into $B(\mathcal{H})$, and $β$ is in the dual space of $A$, then the completely bounded norm of $θ$ is less than or equal to the maximum of $1$ and the completely bounded norm of $θ+ βI $. As an application, we give another proof of the Okubo--Ando theorem.

math.FA↗
arXiv2025

Random commuting matrices

We define a random commuting $d$-tuple of $n$-by-$n$ matrices to be a random variable that takes values in the set of commuting $d$-tuples and has a distribution that is a rapidly decaying continuous weight on this algebraic set. In the Hermitian case, we characterize the eigenvalue distribution as $n$ tends to infinity. In the non-Hermitian case, we get a formula that holds if the set is irreducible. We show that there are qualitative differences between the single matrix case and the several commuting matrices case.

math.PR↗
arXiv2025

Caratheodory sets in the tridisk

We characterize all algebraic subsets of the tridisk that are Caratheodory sets, that is the intrinsic Caratheodory metric on the set equals the Caratheodory metric for the tridisk. We show that such sets are either retracts, or are isomorphic to one particular exceptional set.

math.CV↗
arXiv2024

Non-commutative manifolds, the free square root and symmetric functions in two non-commuting variables

The richly developed theory of complex manifolds plays important roles in our understanding of holomorphic functions in several complex variables. It is natural to consider manifolds that will play similar roles in the theory of holomorphic functions in several non-commuting variables. In this paper we introduce the class of \emph{nc-manifolds}, the mathematical objects that at each point possess a neighborhood that has the structure of an \emph{nc-domain} in the \emph{$d$-dimensional nc-universe $\m^d$}. We illustrate the use of such manifolds in free analysis through the construction of the non-commutative Riemann surface for the matricial square root function. A second illustration is the construction of a non-commutative analog of the elementary symmetric functions in two variables. For any symmetric domain in $\m^2$ we construct a 2-dimensional non-commutative manifold such that the symmetric holomorphic functions on the domain are in bijective correspondence with the holomorphic functions on the manifold. We also derive a version of the classical Newton-Girard formulae for power sums of two non-commuting variables.

math.CV↗
arXiv2023

Random anti-commuting Hermitian matrices

We consider pairs of anti-commuting $2p$-by-$2p$ Hermitian matrices that are chosen randomly with respect to a Gaussian measure. Generically such a pair decomposes into the direct sum of $2$-by-$2$ blocks on which the first matrix has eigenvalues $\pm x_j$ and the second has eigenvalues $\pm y_j$. We call $\{ (x_j, y_j) \}$ the skew spectrum of the pair. We derive a formula for the probability density of the skew spectrum, and show that the elements are repelling.

math.PR↗
arXiv2023

A note on composition operators on model spaces

Motivated by the study of composition operators on model spaces launched by Mashreghi and Shabankha we consider the following problem: for a given inner function $ϕ\not\in\mathsf{Aut}(\mathbb D)$, find a non-constant inner function $Ψ$ satisfying the functional equation $Ψ\circϕ=τΨ$, where $τ$ is a unimodular constant. We prove that this problem has a solution if and only if $ϕ$ is of positive hyperbolic step. More precisely, if this condition holds, we show that there is an infinite Blaschke product $B$ satisfying the equation for $τ=1$. If in addition, $ϕ$ is parabolic, we prove that the problem has a solution $Ψ$ for $any$ unimodular $τ$. Finally, we show that if $ϕ$ is of zero hyperbolic step, then no non-constant Bloch function $f$ and no unimodular constant $τ$ satisfy $f\circϕ=τf$.

math.CV↗
arXiv2022

Conferences -- An Owner's Manual

I share my opinions on how to get the most out of conferences. I have some advice on giving talks, but most of the essay is devoted to the much larger share of the conference not spent giving talks: attending talks and socializing.

math.HO↗
arXiv2022

Multiplier tests and subhomogeneity of multiplier algebras

Multipliers of reproducing kernel Hilbert spaces can be characterized in terms of positivity of $n \times n$ matrices analogous to the classical Pick matrix. We study for which reproducing kernel Hilbert spaces it suffices to consider matrices of bounded size $n$. We connect this problem to the notion of subhomogeneity of non-selfadjoint operator algebras. Our main results show that multiplier algebras of many Hilbert spaces of analytic functions, such as the Dirichlet space and the Drury-Arveson space, are not subhomogeneous, and hence one has to test Pick matrices of arbitrarily large matrix size $n$. To treat the Drury-Arveson space, we show that multiplier algebras of certain weighted Dirichlet spaces on the disc embed completely isometrically into the multiplier algebra of the Drury-Arveson space.

math.FA↗
arXiv2022

A generalization of Hardy's operator and an asymptotic Muntz-Szasz Theorem

The Hardy operator has all the monomial functions as eigenvectors. We study bounded operators on L^2 that take monomial functions to multiples of other monomials, with a shifted exponent. We prove that they all leave the space of functions vanishing on [0,s] invariant. We prove an asymptotic Muntz-Szasz theorem, characterizing the set of functions that are limits of linear combinations of monomials with exponents between n and 2n.

math.FA↗
arXiv2022

Monomial Operators

We study monomial operators on $ L^2[0,1]$, that is bounded linear operators that map each monomial $x^n$ to a multiple of $x^{p_n}$ for some $p_n$. We show that they are all unitarily equivalent to weighted composition operators on a Hardy space. We characterize what sequences $p_n$ can arise. In the case that $p_n$ is a fixed translation of $n$, we give a criterion for boundedness of the operator.

math.FA↗
arXiv2022

The Hardy-Weyl algebra

We study the algebra $\mathcal{A}$ generated by the Hardy operator $H$ and the operator $M_x$ of multiplication by $x$ on $L^2[0,1]$. We call $\mathcal{A}$ the Hardy-Weyl algebra. We show that its quotient by the compact operators is isomorphic to the algebra of functions that are continuous on $Λ$ and analytic on the interior of $Λ$ for a planar set $Λ$ = $[-1,0] \cup \bar{ \mathbb{D}(1,1)}$, which we call the lollipop. We find a Toeplitz-like short exact sequence for the $C^*$-algebra generated by $\mathcal{A}$. We study the operator $Z = H - M_x$, show that its point spectrum is $(-1,0] \cup \mathbb{D}(1,1)$, and that the eigenvalues grow in multiplicity as the points move to $0$ from the left.

math.OA↗
arXiv2022

Complete Norm Preserving Extensions of Holomorphic Functions

We show that for every connected analytic subvariety $V$ there is a pseudoconvex set $Ω$ such that every bounded matrix-valued holomorphic function on $V$ extends isometrically to $Ω$. We prove that if $V$ is two analytic disks intersecting at one point, if every bounded scalar valued holomorphic function extends isometrically to $Ω$, then so does every matrix-valued function. In the special case that $Ω$ is the symmetrized bidisk, we show that this cannot be done by finding a linear isometric extension from the functions that vanish at one point.

math.CV↗
arXiv2022

Free outer functions in complete Pick spaces

Jury and Martin establish an analogue of the classical inner-outer factorization of Hardy space functions. They show that every function $f$ in a Hilbert function space with a normalized complete Pick reproducing kernel has a factorization of the type $f=φg$, where $g$ is cyclic, $φ$ is a contractive multiplier, and $\|f\|=\|g\|$. In this paper we show that if the cyclic factor is assumed to be what we call free outer, then the factors are essentially unique, and we give a characterization of the factors that is intrinsic to the space. That lets us compute examples. We also provide several applications of this factorization.

math.FA↗
arXiv2021

Operator NC functions

We establish a theory of NC functions on a class of von Neumann algebras with a particular direct sum property, e.g. $B(\mathcal{H})$. In contrast to the theory's origins, we do not rely on appealing to results from the matricial case. We prove that the $k^{\text{th}}$ directional derivative of any NC function at a scalar point is a $k$-linear homogeneous polynomial in its directions. Consequences include the fact that NC functions defined on domains containing scalar points can be uniformly approximated by free polynomials as well as realization formulas for NC functions bounded on particular sets, e.g. the non-commutative polydisk and non-commutative row ball.

math.OA↗
arXiv2021

The common range of co-analytic Toeplitz operators on the Drury-Arveson space

We characterize the common range of the adjoints of cyclic multiplication operators on the Drury--Arveson space. We show that a function belongs to this common range if and only if its Taylor coefficients satisfy a simple decay condition. To achieve this, we introduce the uniform Smirnov class on the ball and determine its dual space. We show that the dual space of the uniform Smirnov class equals the dual space of the strictly smaller Smirnov class of the Drury-Arveson space, and that this in turn equals the common range of the adjoints of cyclic multiplication operators.

math.FA↗
arXiv2020

Weak products of complete Pick spaces

Let $\mathcal H$ be the Drury-Arveson or Dirichlet space of the unit ball of $\mathbb C^d$. The weak product $\mathcal H\odot\mathcal H$ of $\mathcal H$ is the collection of all functions $h$ that can be written as $h=\sum_{n=1}^\infty f_n g_n$, where $\sum_{n=1}^\infty \|f_n\|\|g_n\|<\infty$. We show that $\mathcal H\odot\mathcal H$ is contained in the Smirnov class of $\mathcal H$, i.e. every function in $\mathcal H\odot\mathcal H$ is a quotient of two multipliers of $\mathcal H$, where the function in the denominator can be chosen to be cyclic in $\mathcal H$. As a consequence we show that the map $\mathcal N \to clos_{\mathcal H\odot\mathcal H} \mathcal N$ establishes a 1-1 and onto correspondence between the multiplier invariant subspaces of $\mathcal H$ and of $\mathcal H\odot\mathcal H$. The results hold for many weighted Besov spaces $\mathcal H$ in the unit ball of $\mathbb C^d$ provided the reproducing kernel has the complete Pick property. One of our main technical lemmas states that for weighted Besov spaces $\mathcal H$ that satisfy what we call the multiplier inclusion condition any bounded column multiplication operator $\mathcal H \to \oplus_{n=1}^\infty \mathcal H$ induces a bounded row multiplication operator $\oplus_{n=1}^\infty \mathcal H \to \mathcal H$. For the Drury-Arveson space $H^2_d$ this leads to an alternate proof of the characterization of interpolating sequences in terms of weak separation and Carleson measure conditions.

math.FA↗