Research Article | Open Access

Rasoul Eskandari, Farzollah Mirzapour, Ali Morassaei, "More on -Normal Operators in Hilbert Spaces", *Abstract and Applied Analysis*, vol. 2012, Article ID 204031, 11 pages, 2012. https://doi.org/10.1155/2012/204031

# More on -Normal Operators in Hilbert Spaces

**Academic Editor:**Gerd Teschke

#### Abstract

We study some properties of -normal operators and we present various inequalities between the operator norm and the numerical radius of -normal operators on Banach algebra *ℬ*() of all bounded linear operators , where is Hilbert space.

#### 1. Introduction

Throughout the paper, let denote the algebra of all bounded linear operators acting on a complex Hilbert space , denote the algebra of all self-adjoint operators in , and is the identity operator. In case of , we identify with the full matrix algebra of all matrices with entries in the complex field. An operator is called positive if is valid for any , and then we write . Moreover, by we mean for any . For , we say if . An operator is majorized by , if there exists a constant such that for all or equivalently [1].

For real numbers and with , an operator acting on a Hilbert space is called -*normal* [2, 3] if
An immediate consequence of above definition is
from which we obtain
for all .

Notice that, according to (1.1), if is -normal operator, then and majorize each other.

In [3], Moslehian posed two problems about -normal operators as follows.

For fixed and ,(i)give an example of an -normal operator which is neither normal nor hyponormal;(ii)is there any nice relation between norm, numerical radius, and spectral radius of an -normal operator?

Dragomir and Moslehian answered these problems in [2], as more as, they propounded a nice example of -normal operator that is neither normal nor hyponormal, as follows.

The matrix in is an -normal with and .

The *numerical radius * of an operator on is defined by
Obviously, by (1.4), for any we have
It is well known that is a norm on the Banach algebra of all bounded linear operators. Moreover, we have
For other results and historical comments on the numerical radius see [4].

The *antieigenvalue* of an operator defined by
The vector which takes is called an antieigenvector of . We refer more study on this matter to [4].

In this paper, we prove some properties of -normal operators and state various inequalities between the operator norm and the numerical radius of -normal operators in Hilbert spaces.

#### 2. Some Properties of -Normal Operators

In this section, we establish some properties of -normal operators. It is easy to see that if is an -normal then is -normal. We find numbers such that is -normal where is -normal.

We know by the Cauchy-Schwartz inequality that . Also we can write We define

We know that if is normal operator then is also normal.

Theorem 2.1. *Let be an -normal operator on a Hilbert space such that and . Then is -normal, if provided one of the following conditions holds:*(i)*,*(ii)*. *

*Proof. *In both of above cases, we show that
By the assumption (i), , we have for every with and , consequently we get , and therefore (2.3) is valid. On the other hand, if (ii) holds and we set then we get for every with and , consequently:
Since , we obtain
and so
Now, by using the last inequality, we have
This shows that (2.3) holds for (ii), too. Thus, for any with we have
and this completes the proof.

Corollary 2.2. *Let be an -normal operator. We have the following.*(i)*If then is -normal operator for any .*(ii)*If then is -normal operator for any .*

*Proof. *(i) By the definition of the first antieigenvalue of , for all we have
By using Theorem 2.1(i) we imply that is an -normal.

(ii) If , then
By using Theorem 2.1(i) we imply that is an -normal.

Corollary 2.3. *Let be an injective and -normal operator with . Then*(i)* is dense,*(ii)* is injective,*(iii)*if is surjective then is also -normal.*

*Proof. *Since the inequality (1.3) is valid, we obtain , and therefore , thus is a dense subspace of and is injective. This proves (i) and (ii).

To prove (iii), we note that since is surjective, we imply that is invertible. On the other hand we have . Also we know that if and are two positive and invertible operators with then . Since is -normal, by taking inverse from all sides of (1.1), we get
This means that is -normal, thus is -normal.

*Example 2.4. *Consider the following matrix in :
is an -normal operator, with parameters and . Then is -normal.

For we call
the *spectral radius* of , where is the spectrum of and it is known that [5, page 102].

Theorem 2.5. *Let be an -normal operator such that is -normal operator for every , too. Then, we have
*

*Proof. *For any we have
In particular, if is a self-adjoint operator then . Thus, by the definition of -normal operator, we have
By induction on , we imply that
from which we obtain
Therefore, we get . This completes the proof.

Below, we give an example of -normal operator such that it satisfies in Theorem 2.5.

*Example 2.6. *Assume that is a separable Hilbert space and is an orthonormal basis for . We define the operator as follows:
so
and by simple computation we get
Consequently, is -normal operator and also is -normal operator, for any integer . Thus we have and , hence (2.14) is valid.

#### 3. Inequalities Involving Norms and Numerical Radius

In this section we state some inequalities involving norms and numerical radius.

Theorem 3.1. *Let be an -normal operator.*(i)*For positive real numbers and with and we have
*(ii)*If or , then we have
where . *(iii)*If and for any with we have
then, we obtain
*

*Proof. *(i) We use the following known inequality:
which is valid for any where is a Hilbert space.

Now, if we take and in (3.5), then for any we get
Taking the supremum in (3.6) over with , we get the desired result (3.1).

(ii) We use the following inequality [6, Theorem 8, page 551]:
where and are two vectors in a Hilbert space and or .

Now, if we put and in (3.7), then we obtain
Now, taking the supremum over in (3.8), we get the desired result (3.2).

(iii) We use the following reverse of Schwarz's inequality:
which is valid for and , with (see [7]). We take and in (3.9) to get
Thus, we obtain
Now, taking the supremum over in recent inequality, we get the desired result (3.4).

Theorem 3.2. *Assume that is an -normal operator. Then, we have
*

*Proof. *By [2, Theorem 3.1], we have
and also
On the other hand, it is known [8] that for we have
By using this inequality we get
If we put in (3.14), we obtain
Thus we get
Now, we take in (3.13) to obtain
This completes the proof.

Theorem 3.3. *Assume that is an -normal operator. Then for any real with , we have
*

*Proof. *By [9, Theorem 2.6] (see also [10, Theorem 2.4]), we have
where , and . By taking , and in (3.21), we get
thus, we have
Finally, we take supremum over from both sides of
and we use triangle inequality for supremums to complete the proof.

Corollary 3.4. *Let be an -normal operator. Then, we have
*

*Proof. *By using the inequality (3.21) we get
We take in inequalities (3.20) and (3.26) to imply
Thus, . Now, taking supremum overall with , the desired inequality is obtained.

#### Acknowledgments

The authors would like to sincerely thank the anonymous referee for several useful comments improving the paper and also Professor Mehdi Hassani for a useful discussion.

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#### Copyright

Copyright © 2012 Rasoul Eskandari et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.