Positive answer to the invariant and hyperinvariant subspaces problems for hyponormal operators

The question whether every operator on infinite-dimensional Hilbert space 𝐻 has a nontrivial invariant subspace or a nontrivial hyperinvariant subspace is one of the most difficult problems in operator theory. This problem is open for more than half a century. A subnormal operator has a nontrivial invariant subspace, but the existence of nontrivial invariant subspace for a hyponormal operator 𝑇 still open. In this paper we give an affirmative answer of the existence of a nontrivial hyperinvariant subspace for a hyponormal operator. More generally, we show that a large classes of operators containing the class of hyponormal operators have nontrivial hyperinvariant subspaces. Finally, every generalized scalar operator on a Banach space 𝑋 has a nontrivial invariant subspace.

On Reflexivity of Certain Hyponormal Operators with Double Commutant Property

Sarason did pioneer work on the reflexivity and purpose of this paper is to discuss the reflexivity of different class of contractions. Among contractions it is now known that C11 contractions with finite defect indices, C.o contractions with unequal defect indices and C1. contractions with at least one finite defect indices are reflexive. More over the characterization of reflexive operators among co contractions and completely non unitary weak contractions with finite defect indices has been reduced to that of S (F), the compression of the shift on H2 ⊖ F H2, F is inner. The present work is mainly focused on the reflexivity of contractions whose characteristic function is constant. This class of operator include many other isometries, co-isometries and their direct sum. We shall also discuss the reflexivity of hyponormal contractions, reflexivity of C1. contractions and weak contractions. It is already known that normal operators isometries, quasinormal and sub-normal operators are reflexive. We partially generalize these results by showing that certain hyponormal operators with double commutant property are reflexive. In addition, reflexivity of operators which are direct sum of a unitary operator and C.o contractions with unequal defect indices,is proved Each of this kind of operator is reflexive and satisfies the double commutant property with some restrictions.

The class of (n,m)power-A-quasi-hyponormal operators in semi-Hilbertian space

The concept of K-quasi-hyponormal operators on semi-Hilbertian space is defined by Ould Ahmed Mahmoud Sid Ahmed and Abdelkader Benali in [7]. This paper is devoted to the study of new class of operators on semi-Hilbertian space H, ∥. ∥Acalled (n,m)power-A-quasi-hyponormal denoted [(n,m)QH]A.We give some basic properties of these operators and some examples are also given .An operator T ∈ BA(H) is said to be (n,m) power-A-quasi-hyponormal for some positive operator A and for some positive integers n and m if T⋕((T⋕)mTn— Tn(T⋕)m)T≥A or equivalently AT⋕((T⋕)mTn— Tn(T⋕)m)T≥0

CONDITIONS IMPLYING NORMALITY AND HYPO-NORMALITY OF OPERATORS IN HILBERT SPACE

The aim of this paper is to give sufficient conditions on two normal and hyponormal operators (bounded or not), defined on a Hilbert space, which make their algebraic sum hyponormal (only in bounded case). The results are accompanied by some interesting examples and counter examples.