arXiv subjects

Soumitra Ghara

Publications and source records attributed to Soumitra Ghara.

16 recordsLinked to original sources

arXiv2026

Circular operators and their strong circularity

Circular operators have been studied extensively since the work of R. Gellar, who conjectured that every circular operator on a complex separable Hilbert space is strongly circular. In this short note, we show that circularity and strong circularity coincide for bounded operators that are finite or countably infinite direct sums of irreducible operators. This considerably narrows the search for potential counterexamples to Gellar's conjecture. As an application, we prove that every circular operator in the Cowen-Douglas class is strongly circular. In addition, we obtain several general results on circular operators that reveal the significance of the hyper-range and the Cauchy dual.

math.FA↗

arXiv2026

Unbounded Toeplitz operators and finite rank de Branges-Rovnyak spaces

Motivated by the recent developments of de Branges-Rovnyak spaces, we investigate the function theoretic aspects of finite rank de Branges-Rovnyak spaces $H(B)$ generated by row-valued Schur functions $B$. We provide a generalization of Sarason's fundamental work by characterizing finite rank $H(B)$-spaces as the domain of the adjoint of the Toeplitz operators $T_\varphi^*$ with symbol $\varphi = BA^{-1}$, where $A$ is an matrix-valued outer function satisfying $A^*A+B^*B = I$ a.e. on the unit circle. We derive a norm formula for functions in $H(B)$-space and provide a concrete realization of this norm in terms of the Taylor coefficients of the function and the symbol $\varphi$. As an application, we characterize all symbols $B$ for which $H^\infty \subseteq H(B)$ in terms of the boundary behavior of $I-BB^*$, thereby extending Sarason's criterion for the classical de Branges-Rovnyak spaces.

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arXiv2025

Weakly $\mathcal U(d)$-homogeneous commuting tuple of bounded operators

We introduce and study the weakly $\mathcal U(d)$-homogeneous commuting tuple of operators. We provide a sufficient condition under which a weakly $\mathcal U(d)$-homogeneous tuple is similar to a $\mathcal U(d)$-homogeneous tuple. Further, we focus our attention to multishifts and completely characterize weakly $\mathcal U(d)$-homogeneous multishifts. In particular, we show that a multishift is weakly $\mathcal U(d)$-homogeneous if and only if it similar to a $\mathcal U(d)$-homogeneous multishift. The results for multishifts are further refined for the class of spherically balanced multishifts.

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arXiv2024

A short note on similarity of operator-valued multishifts

A complete characterization of the similarity between two operator-valued multishifts with invertible operator weights is obtained purely in terms of operator weights. This generalizes several existing results of the unitary equivalence of two (multi)shifts. Further, we utilize the aforementioned similarity criteria to determine the similarity between two tuples of operators of multiplication by the coordinate functions on certain reproducing kernel Hilbert spaces determined by diagonal kernels.

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arXiv2023

Cesàro summability of Taylor series in higher order weighted Dirichlet type spaces

For a positive integer $m$ and a finite non-negative Borel measure $μ$ on the unit circle, we study the Hadamard multipliers of higher order weighted Dirichlet-type spaces $\mathcal H_{μ, m}$. We show that if $α>\frac{1}{2},$ then for any $f$ in $\mathcal H_{μ, m},$ the sequence of generalized Ces{à}ro sums $\{σ_n^α[f]\}$ converges to $f$. We further show that if $α=\frac{1}{2}$ then for the Dirac delta measure supported at any point on the unit circle, the previous statement breaks down for every positive integer $m$.

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arXiv2023

Dirichlet-type spaces of the bidisc and Toral $2$-isometries

We introduce and study Dirichlet-type spaces $\mathcal D(μ_1, μ_2)$ of the unit bidisc $\mathbb D^2,$ where $μ_1, μ_2$ are finite positive Borel measures on the unit circle. We show that the coordinate functions $z_1$ and $z_2$ are multipliers for $\mathcal D(μ_1, μ_2)$ and the complex polynomials are dense in $\mathcal D(μ_1, μ_2).$ Further, we obtain the division property and solve Gleason's problem for $\mathcal D(μ_1, μ_2)$ over a bidisc centered at the origin. In particular, we show that the commuting pair $\mathscr M_z$ of the multiplication operators $\mathscr M_{z_1},$ $\mathscr M_{z_2}$ on $\mathcal D(μ_1, μ_2)$ defines a cyclic toral $2$-isometry and $\mathscr M^*_z$ belongs to the Cowen-Douglas class ${\bf B}_1(\mathbb D^2_r)$ for some $r >0.$ Moreover, we formulate a notion of wandering subspace for commuting tuples and use it to obtain a bidisc analog of Richter's representation theorem for cyclic analytic $2$-isometries. In particular, we show that a cyclic analytic toral $2$-isometric pair $T$ with cyclic vector $f_0$ is unitarily equivalent to $\mathscr M_z$ on $\mathcal D(μ_1, μ_2)$ if and only if $\ker T^*,$ spanned by $f_0,$ is a wandering subspace for $T.$

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arXiv2023

Commuting Tuple of Multiplication Operators Homogeneous under the Unitary Group

Let $\mathcal U(d)$ be the group of $d\times d$ unitary matrices. We find conditions to ensure that a $\mathcal U(d)$-homogeneous $d$-tuple $\boldsymbol T$ is unitarily equivalent to multiplication by the coordinate functions on some reproducing kernel Hilbert space $\mathcal H_K(\mathbb B_d, \mathbb C^n) \subseteq \mbox{\rm Hol}(\mathbb B_d, \mathbb C^n)$, $n= \dim \cap_{j=1}^d \ker T^*_{j}.$ We describe this class of $\mathcal U(d)$-homogeneous operators, equivalently, non-negative kernels $K$ quasi-invariant under the action of $\mathcal U(d)$. We classify quasi-invariant kernels $K$ transforming under $\mathcal U(d)$ with two specific choice of multipliers. A crucial ingredient of the proof is that the group $SU(d)$ has exactly two inequivalent irreducible unitary representations of dimension $d$ and none in dimensions $2, \ldots , d-1$, $d\geq 3$. We obtain explicit criterion for boundedness, reducibility and mutual unitary equivalence among these operators.

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arXiv2023

Summability and duality

We formalize the observation that the same summability methods converge in a Banach space $X$ and its dual $X^*$. At the same time we determine conditions under which these methods converge in the weak and weak*-topologies on $X$ and $X^*$ respectively. We also derive a general limitation theorem, which yields a necessary condition for the convergence of a summability method in $X$. These results are then illustrated by applications to a wide variety of function spaces, including spaces of continuous functions, Lebesgue spaces, the disk algebra, Hardy and Bergman spaces, the BMOA space, the Bloch space, and de Branges-Rovnyak spaces. Our approach shows that all these applications flow from just two abstract theorems.

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arXiv2022

A local Douglas formula for higher order weighted Dirichlet-type integrals

We prove a local Douglas formula for higher order weighted Dirichlet-type integrals. With the help of this formula, we study the multiplier algebra of the associated higher order weighted Dirichlet-type spaces $\mathcal H_{\pmbμ},$ induced by an $m$-tuple $\pmb μ=(μ_1,\ldots,μ_{m})$ of finite non-negative Borel measures on the unit circle. In particular, it is shown that any weighted Dirichlet-type space of order $m,$ for $m\geqslant 3,$ forms an algebra under pointwise product. We also prove that every non-zero closed $M_z$-invariant subspace of $\mathcal H_{\pmbμ},$ has codimension $1$ property if $m\geqslant 3$ or $μ_2$ is finitely supported. As another application of local Douglas formula obtained in this article, it is shown that for any $m\geqslant 2,$ weighted Dirichlet-type space of order $m$ does not coincide with any de Branges-Rovnyak space $\mathcal H(b)$ with equivalence of norms.

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arXiv2022

Analyticity, rank one perturbations and the invariance of the left spectrum

We address the question of the analyticity of a rank one perturbation of an analytic operator. If $\mathscr M_z$ is the bounded operator of multiplication by $z$ on a functional Hilbert space $\mathscr H_κ$ and $f \in \mathscr H$ with $f(0)=0,$ then $\mathscr M_z + f \otimes 1$ is always analytic. If $f(0) \neq 0,$ then the analyticity of $\mathscr M_z + f \otimes 1$ is characterized in terms of the membership to $\mathscr H_κ$ of the formal power series obtained by multiplying $f(z)$ by $\frac{1}{f(0)-z}.$ As an application, we discuss the problem of the invariance of the left spectrum under rank one perturbation. In particular, we show that the left spectrum $σ_l(T + f \otimes g)$ of the rank one perturbation $T + f \otimes g,$ $\,g \in \ker(T^*),$ of a cyclic analytic left invertible bounded linear operator $T$ coincides with the left spectrum of $T$ except the point $\inp{f}{g}.$ In general, the point $\inp{f}{g}$ may or may not belong to $σ_l(T + f \otimes g).$ However, if it belongs to $σ_l(T + f \otimes g) \backslash \{0\},$ then it is a simple eigenvalue of $T + f \otimes g.$

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arXiv2022

The relationship of the Gaussian curvature with the curvature of a Cowen-Douglas operator

It has been recently shown that if $K$ is a sesqui-analytic scalar valued non-negative definite kernel on a domain $Ω$ in $\mathbb C^m$, then the function $\big(K^2\partial_i\bar{\partial}_j\log K\big )_{i,j=1}^ m,$ is also a non-negative definite kernel on $Ω$. In this paper, we discuss two consequences of this result. The first one strengthens the curvature inequality for operators in the Cowen-Douglas class $B_1(Ω)$ while the second one gives a relationship of the reproducing kernel of a submodule of certain Hilbert modules with the curvature of the associated quotient module.

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arXiv2021

The Cauchy dual subnormality problem via de Branges-Rovnyak spaces

The Cauchy dual subnormality problem (for short, CDSP) asks whether the Cauchy dual of a $2$-isometry is subnormal. In this paper, we address this problem for cyclic $2$-isometries. In view of some recent developments in operator theory on function spaces (see \cite{AM, LGR}), one may recast CDSP as the problem of subnormality of the Cauchy dual $\mathscr M'_z$ of the multiplication operator $\mathscr M_z$ acting on a de Branges-Rovnyak space $\mathcal H(B),$ where $B$ is a vector-valued rational function. The main result of this paper characterizes the subnormality of $\mathscr M'_z$ on $\mathcal H(B)$ provided $B$ is a vector-valued rational function with simple poles. As an application, we provide affirmative solution to CDSP for the Dirichlet-type spaces $\mathscr D(μ)$ associated with measures $μ$ supported on two antipodal points of the unit circle.

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arXiv2020

$\mathbb K$-homogeneous tuple of operators on bounded symmetric domains

Let $Ω$ be an irreducible bounded symmetric domain of rank $r$ in $\mathbb C^d.$ Let $\mathbb K$ be the maximal compact subgroup of the identity component $G$ of the biholomorphic automorphism group of the domain $Ω$. The group $\mathbb K$ consisting of linear transformations acts naturally on any $d$-tuple $\boldsymbol T=(T_1,\ldots, T_d)$ of commuting bounded linear operators. If the orbit of this action modulo unitary equivalence is a singleton, then we say that $\boldsymbol T$ is $\mathbb{K}$-homogeneous. In this paper, we obtain a model for all $\mathbb{K}$-homogeneous $d$-tuple $\boldsymbol{T}$ as the operators of multiplication by the coordinate functions $z_1,\ldots ,z_d$ on a reproducing kernel Hilbert space of holomorphic functions defined on $Ω$. Using this model we obtain a criterion for (i) boundedness, (ii) membership in the Cowen-Douglas class (iii) unitary equivalence and similarity of these $d$-tuples. In particular, we show that the adjoint of the $d$-tuple of multiplication by the coordinate functions on the weighted Bergman spaces are in the Cowen-Douglas class $B_1(Ω)$. For a bounded symmetric domain $Ω$ of rank $2$, an explicit description of the operator $\sum_{i=1}^d T_i^*T_i$ is given. In general, based on this formula, we make a conjecture giving the form of this operator.

math.FA↗

arXiv2019

Decomposition of the tensor product of two Hilbert modules

Given a pair of positive real numbers $α, β$ and a sesqui-analytic function $K$ on a bounded domain $Ω\subset \mathbb C^m$, in this paper, we investigate the properties of the sesqui-analytic function $\mathbb K^{(α, β)}:= K^{α+β}\big(\partial_i\bar{\partial}_j\log K\big )_{i,j=1}^ m,$ taking values in $m\times m$ matrices. One of the key findings is that $\mathbb K^{(α, β)}$ is non-negative definite whenever $K^α$ and $K^β$ are non-negative definite. In this case, a realization of the Hilbert module determined by the kernel $\mathbb K^{(α,β)}$ is obtained. Let $\mathcal M_i$, $i=1,2,$ be two Hilbert modules over the polynomial ring $\mathbb C[z_1, \ldots, z_m]$. Then $\mathbb C[z_1, \ldots, z_{2m}]$ acts naturally on the tensor product $\mathcal M_1\otimes \mathcal M_2$. The restriction of this action to the polynomial ring $\mathbb C[z_1, \ldots, z_m]$ obtained using the restriction map $p \mapsto p_{|Δ}$ leads to a natural decomposition of the tensor product $\mathcal M_1\otimes \mathcal M_2$, which is investigated. Two of the initial pieces in this decomposition are identified.

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arXiv2018

The orbit of a bounded operator under the Möbius group modulo similarity equivalence

Let Möb denote the group of biholomorphic automorphisms of the unit disc and $(\mbox{Möb} \cdot T)$ be the orbit of a Hilbert space operator $T$ under the action of Möb. If the quotient $(\mbox{Möb} \cdot T)/\sim$, where $\sim$ is the similarity between two operators is a singleton, then the operator $T$ is said to be weakly homogeneous. In this paper, we obtain a criterion to determine if the operator $M_z$ of multiplication by the coordinate function $z$ on a reproducing kernel Hilbert space is weakly homogeneous. We use this to show that there exists a Möbius bounded weakly homogeneous operator which is not similar to any homogeneous operator, answering a question of Bagchi and Misra in the negative. Some necessary conditions for the Möbius boundedness of a weighted shift are also obtained. As a consequence, it is shown that the Dirichlet shift is not Möbius bounded.

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arXiv2017

On sum of two subnormal kernels

We show, by means of a class of examples, that if $K_1$ and $K_2$ are two positive definite kernels on the unit disc such that the multiplication by the coordinate function on the corresponding reproducing kernel Hilbert space is subnormal, then the multiplication operator on the Hilbert space determined by their sum $K_1+K_2$ need not be subnormal. This settles a recent conjecture of Gregory T. Adams, Nathan S. Feldman and Paul J. McGuire in the negative. We also discuss some cases for which the answer is affirmative.

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