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Kamran Sharifi

Publications and source records attributed to Kamran Sharifi.

16 recordsLinked to original sources
arXiv2025

A Riesz-Fredholm type theorem on certain Hilbert C*-modules

Let $C$ be compact modular operator on a Hilbert C*-module $E$ satisfying property $\mathbb{[H]}$ [{\it J. Math. Phys.} {\bf 49} (2008), 033519], and let $ L :=I-C$. We prove the existence of a unique natural number $r$ for which $L^r$ is an EP operator on $E$. Moreover, we show that the kernel of $L^r$ is a finitely generated submodule of $E$ and that $E$ admits the decomposition $E=Ker(L^r) \oplus Ran(L^r)$. These results provide a framework for analyzing the solvability of the equation $x-Cx=f$ on $E$.

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arXiv2025

Invariant submodules of modular operators and Lomonosov type theorem for Hilbert C*-modules

In this paper, we introduce the notion of invariant submodule in the theory of Hilbert C*-modules and study some basic properties of bounded adjointable operators and their generalized inverses which have nontrivial invariant submodules. We demonstrate the representation of the solution set of an operator equation on Hilbert C*-modules by taking advantage of invariant submodules. In particular, we consider the special cases of finite dimensional C*-algebras and C*-algebras of compact operators as the underling C*-algebra to simplify our results, and obtain a Lomonosov type theorem for compact operators on some Hilbert C*-modules.

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arXiv2024

EP modular operators and their products

We study first EP modular operators on Hilbert C*-modules and then we provide necessary and sufficient conditions for the product of two EP modular operators to be EP. These enable us to extend some results of Koliha [{\it Studia Math.} {\bf 139} (2000), 81--90.] for an arbitrary C*-algebra and the C*-algebras of compact operators.

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arXiv2021

Some examples of EP operators

We give some examples of EP and non-EP operators to show that the main results of Mohammadzadeh Karizakia {\it et al} [Some results about EP modular operators, {\it Linear and Multilinear Algebra}, DOI: 10.1080/03081087.2020.1844613] are not correct even in the case of Hilbert spaces.

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arXiv2016

Some remarks on derivations on the algebra of operators in Hilbert pro-C*-bimodules

Suppose $A$ is a pro-C*-algebra. Let $L_{A}(E)$ be the pro-C*-algebra of adjointable operators on a Hilbert $A$-module $E$ and let $K_{A}(E)$ be the closed two sided $*$-ideal of all compact operators on $E$. We prove that if $E$ be a full Hilbert $A$-module, the innerness of derivations on $K_{A}(E)$ implies the innerness of derivations on $L_{A}(E)$. We show that if $A$ is a commutative pro-C*-algebra and $E$ is a Hilbert $A$-bimodule then every derivation on $K_{A}(E)$ is zero. Moreover, if $A$ is a commutative $σ$-C*-algebra and $E$ is a Hilbert $A$-bimodule then every derivation on $L_{A}(E)$ is zero, too.

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arXiv2016

Atiyah-Jänich theorem for $σ$-C*-algebras

K-theory for $ σ$-C*-algebras (countable inverse limits of C*-algebras) has been investigated by N. C. Phillips [{\it K-Theory} {\bf 3} (1989), 441--478]. We use his representable K-theory to show that the space of Fredholm modular operators with coefficients in an arbitrary unital $ σ$-C*-algebra $A$, represents the functor $X \mapsto {\rm RK}_{0}(C(X, A))$ from the category of countably compactly generated spaces to the category of abelian groups.

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arXiv2016

Completely positive maps on Hilbert modules over pro-C*-algebras

We derive Paschke's GNS construction for completely positive maps on unital pro-C*-algebras from the KSGNS construction, presented by M. Joita [J. London Math. Soc. {\bf 66} (2002), 421--432], and then we deduce an analogue of Stinespring theorem for Hilbert modules over pro-C*-algebras. Also, we obtain a Radon-Nikodym type theorem for operator valued completely positive maps on Hilbert modules over pro-C*-algebras.

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arXiv2014

The reverse order law for Moore-Penrose inverses of operators on Hilbert C*-modules

Suppose $T$ and $S$ are bounded adjointable operators between Hilbert C*-modules admitting bounded Moore-Penrose inverse operators. Some necessary and sufficient conditions are given for the reverse order law $(TS)^{ †} =S^{ †} T^{ †}$ to hold. In particular, we show that the equality holds if and only if $Ran(T^{\ast}TS) \subseteq Ran(S)$ and $Ran(SS^{\ast}T^{\ast}) \subseteq Ran(T^{\ast}),$ which was studied first by Greville [{\it SIAM Rev. 8 (1966) 518--521}] for matrices.

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arXiv2011

Generic properties of module maps and characterizing inverse limits of C*-algebras of compact operators

We study closedness of the range, adjointability and generalized invertibility of modular operators between Hilbert modules over locally C*-algebras of coefficients. Our investigations and the recent results of M. Frank [Characterizing C*-algebras of compact operators by generic categorical properties of Hilbert C*-modules, {\it J. K-Theory} {\bf 2} (2008), 453-462] reveal a number of equivalence properties of the category of Hilbert modules over locally C*-algebras which characterize precisely the inverse limit of C*-algebras of the C*-algebra of compact operators.

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arXiv2011

The product of operators with closed range in Hilbert C*-modules

Suppose $T$ and $S$ are bounded adjointable operators with close range between Hilbert C*-modules, then $TS$ has closed range if and only if $Ker(T)+Ran(S)$ is an orthogonal summand, if and only if $Ker(S^*)+Ran(T^*)$ is an orthogonal summand. Moreover, if the Dixmier (or minimal) angle between $Ran(S)$ and $Ker(T) \cap [Ker(T) \cap Ran(S)]^{\perp}$ is positive and $ \bar{Ker(S^*)+Ran(T^*)} $ is an orthogonal summand then $TS$ has closed range.

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arXiv2010

Normality of adjointable module maps

Normality of bounded and unbounded adjointable operators are discussed. Suppose $T$ is an adjointable operator between Hilbert C*-modules which has polar decomposition, then $T$ is normal if and only if there exists a unitary operator $ \mathcal{U}$ which commutes with $T$ and $T^*$ such that $T=\mathcal{U} \, T^*.$ Kaplansky's theorem for normality of the product of bounded operators is also reformulated in the framework of Hilbert C*-modules.

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arXiv2010

Generalized inverses and polar decomposition of unbounded regular operators on Hilbert $C^*$-modules

In this note we show that an unbounded regular operator $t$ on Hilbert $C^*$-modules over an arbitrary $C^*$ algebra $ \mathcal{A}$ has polar decomposition if and only if the closures of the ranges of $t$ and $|t|$ are orthogonally complemented, if and only if the operators $t$ and $t^*$ have unbounded regular generalized inverses. For a given $C^*$-algebra $ \mathcal{A}$ any densely defined $\mathcal A$-linear closed operator $t$ between Hilbert $C^*$-modules has polar decomposition, if and only if any densely defined $\mathcal A$-linear closed operator $t$ between Hilbert $C^*$-modules has generalized inverse, if and only if $\mathcal A$ is a $C^*$-algebra of compact operators.

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arXiv2010

Adjointability of densely defined closed operators and the Magajna-Schweizer Theorem

In this notes unbounded regular operators on Hilbert $C^*$-modules over arbitrary $C^*$-algebras are discussed. A densely defined operator $t$ possesses an adjoint operator if the graph of $t$ is an orthogonal summand. Moreover, for a densely defined operator $t$ the graph of $t$ is orthogonally complemented and the range of $P_FP_{G(t)^\bot}$ is dense in its biorthogonal complement if and only if $t$ is regular. For a given $C^*$-algebra $\mathcal A$ any densely defined $\mathcal A$-linear closed operator $t$ between Hilbert $C^*$-modules is regular, if and only if any densely defined $\mathcal A$-linear closed operator $t$ between Hilbert $C^*$-modules admits a densely defined adjoint operator, if and only if $\mathcal A$ is a $C^*$-algebra of compact operators. Some further characterizations of closed and regular modular operators are obtained. Changes 1: Improved results, corrected misprints, added references. Accepted by J. Operator Theory, August 2007 / Changes 2: Filled gap in the proof of Thm. 3.1, changes in the formulations of Cor. 3.2 and Thm. 3.4, updated references and address of the second author.

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arXiv2009

The gap between unbounded regular operators

We study and compare the gap and the Riesz topologies of the space of all unbounded regular operators on Hilbert C*-modules. We show that the space of all bounded adjointable operators on Hilbert C*-modules is an open dense subset of the space of all unbounded regular operators with respect to the gap topology. The restriction of the gap topology on the space of all bounded adjointable operators is equivalent with the topology which is generated by the usual operator norm. The space of regular selfadjoint Fredholm operators on Hilbert C*-modules over the C*-algebra of compact operators is path-connected with respect to the gap topology, however, the result may not be true for some Hilbert C*-modules.

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arXiv2008

The Atkinson Theorem in Hilbert C*-Modules Over C*-Algebras of Compact Operators

In this paper the concept of unbounded Fredholm operators on Hilbert C*- modules over an arbitrary C*-algebra is discussed and the Atkinson theorem is generalized for bounded and unbounded Feredholm operators on Hilbert C*-modules over C*-algebras of compact operators. In the framework of Hilbert C*-modules over C*-algebras of compact operators, the index of an unbounded Fredholm operator and the index of its bounded transform are the same.

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