Generalized Cesàro Operators: Geometry of Spectra and Quasi-Nilpotency

For the class of Hardy spaces and standard weighted Bergman spaces of the unit disk, we prove that the spectrum of a generalized Cesàro operator $T_g$ is unchanged if the symbol $g$ is perturbed to $g+h$ by an analytic function $h$ inducing a quasi-nilpotent operator $T_h$, that is, spectrum of $T_h$ equals $\{0\}$. We also show that any $T_g$ operator that can be approximated in the operator norm by an operator $T_h$ with bounded symbol $h$ is quasi-nilpotent. In the converse direction, we establish an equivalent condition for the function $g \in \textbf{BMOA}$ to be in the $\textbf{BMOA}$ norm closure of $H^{\infty }$. This condition turns out to be equivalent to quasi-nilpotency of the operator $T_g$ on the Hardy spaces. This raises the question whether similar statement is true in the context of Bergman spaces and the Bloch space. Furthermore, we provide some general geometric properties of the spectrum of $T_{g}$ operators.

The Equations Defining Affine Grassmannians in Type A and a Conjecture of Kreiman, Lakshmibai, Magyar, and Weyman

The affine Grassmannian of $SL_n$ admits an embedding into the Sato Grassmannian, which further admits a Plücker embedding into the projectivization of Fermion Fock space. Kreiman, Lakshmibai, Magyar, and Weyman describe the linear part of the ideal defining this embedding in terms of certain elements of the dual of Fock space called shuffles, and they conjecture that these elements together with the Plücker relations suffice to cut out the affine Grassmannian. We give a proof of this conjecture in two steps; first, we reinterpret the shuffle equations in terms of Frobenius twists of symmetric functions. Using this, we reduce to a finite-dimensional problem, which we solve. For the 2nd step, we introduce a finite-dimensional analogue of the affine Grassmannian of $SL_n$, which we conjecture to be precisely the reduced subscheme of a finite-dimensional Grassmannian consisting of subspaces invariant under a nilpotent operator.

The properties of [∞,C]-isometric operators

In this paper we introduce the class of [?,C]-isometric operators and study various properties of this class. In particular, we show that if T is an [?,C]-isometric operator and Q is a quasi-nilpotent operator, then T + Q is an [?,C]-isometric operator under suitable conditions. Also, we show that the class of [?,C]-isometric operators is norm closed. Finally, we examine properties of products of [?,C]-isometric operators.

On [m,C]-isometric operators

In this paper we introduce an [m;C]-isometric operator T on a complex Hilbert space H and study its spectral properties. We show that if T is an [m,C]-isometric operator and N is an n-nilpotent operator, respectively, then T + N is an [m + 2n ? 2,C]-isometric operator. Finally we give a short proof of Duggal?s result for tensor product of m-isometries and give a similar result for [m,C]-isometric operators.

Polaroid Operators with Svep and Perturbations of Property (Gaw)

In this paper we establish for a bounded linear operator defined on a Banach space several sufficient and necessary conditions for which property (gaw) holds. In this work, we consider commutative perturbations by algebraic operator and quasinilpotent operator for T ∈ B(X ) such that T * satisfies property (gaw). We prove that if A is an algebraic and T ∈ PS(X ) is such that AT = TA, then ƒ(T * + A*) satisfies property (gaw) for every ƒ ∈ Hc(σ(T + A)). Moreover, we show that if Q is a quasi-nilpotent operator and T ∈ PS(X ) is such that TQ = QT, then ƒ(T * + Q*) satisfies the property (gaw) for every ƒ ∈ Hc(σ(T +Q)). At the end of this paper, we apply the obtained results to a number of subclasses of PS(X ).

Perturbation ofm-Isometries by Nilpotent Operators

We prove that ifTis anm-isometry on a Hilbert space andQann-nilpotent operator commuting withT, thenT+Qis a2n+m-2-isometry. Moreover, we show that a similar result form, q-isometries on Banach spaces is not true.