Journal of Operators

Volume 2015 (2015), Article ID 172754, 8 pages

http://dx.doi.org/10.1155/2015/172754

## On Some Algebraic and Operator-Theoretic Properties of -Toeplitz Operators

Department of Science and Mathematics, The American University of Afghanistan, Darulaman Road, Kabul, Afghanistan

Received 4 September 2014; Revised 16 December 2014; Accepted 17 December 2014

Academic Editor: Dashan Fan

Copyright © 2015 Mehdi Nikpour. 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.

#### Abstract

Based on a spectral problem raised by Barría and Halmos, a new class of Hardy-Hilbert space operators, containing the classical Toeplitz operators, is introduced, and some of their Toeplitz-like algebraic and operator-theoretic properties are studied and explored.

#### 1. Introduction

All of the work I am about to describe takes place in the Hardy-Hilbert space of the unit circle , denoted by , and consists of all square-integrable (with respect to the normalized arc-length measure ) functions, on , whose negative Fourier coefficients all vanish; that is, For more details and basic properties of Hardy spaces, the reader is referred to [1, Chapters 1 and 2] or [2, Chapter 17].

Two of the most intensely studied classes of bounded operators on are Toeplitz and Hankel operators. Originally, an infinite matrix is called* Toeplitz* (resp.,* Hankel*) if its entries depend just on the difference (resp., the sum) of their indices. Hence, Toeplitz matrices are the ones with constant diagonals, and Hankel matrices are those with constant skew-diagonals. They both play a decisive role in a very wide circle of problems in operator theory, -algebras, moment problems, interpolation by holomorphic or meromorphic functions, inverse spectral problems, orthogonal polynomials, prediction theory, Wiener-Hopf equations, boundary problems of function theory, the extension theory of symmetric operators, singular integral equations, models of statistical physics, and many others. Also, there exists a vast literature on the theory of Toeplitz and Hankel operators; see, for example, [3–8].

Maybe a naïve reason also for their importance is the fact that Toeplitz and Hankel operators are compressions of (bounded) multiplication operators and their flipped, respectively, to . Indeed, any essentially bounded function on induces, in a natural way, three bounded operators, one on and the two others on , as follows.(i)The* Multiplication operator * is given by , for .(ii)The* Toeplitz operator * is defined, in terms of the orthogonal projection from onto , as the compression of to ; , for .(iii)The* Hankel operator * is defined as the compression of the “flipped” onto ; , for , where is the unitary self-adjoint operator on (the so-called* flip operator*) defined by , for , mapping onto (the orthogonal complement of ) and onto . In each case, is called the* symbol* of the operator.

These classes of operators can also be considered as solutions to some linear operator-equations involving the Toeplitz operator , known as the* unilateral forward shift*, and its Hilbert-adjoint , usually called the* unilateral backward shift*. Indeed, it is well known that an operator is Hankel if and only if (*Hankel equation*) and that an operator is Toeplitz if and only if (*Toeplitz equation*).

Generalizations of such operator-equations have been studied and explored for some time. For instance, in [9] the operator-equation , for arbitrary contractions and acting on different Hilbert spaces, has been studied. Pták in [10] studied the solutions to the operator-equation , where and are contractions.

Here we study an operator-equation, on , which is a slight modification to the Toeplitz equation; namely, , for an arbitrary complex number . This operator-equation appeared in [11] and it was asked what its operator-solutions could be, what algebraic and operator-theoretic properties those solutions had, and how these operator-solutions relate to the case (Toeplitz operators). Fortunately, this problem is a spectral one; that is, its solutions are the eigen-operators of a bounded operator-valued linear transformation on , which have been found and characterized by Sun in [12].

In this paper we study and develop some algebraic and operator-theoretic properties of Toeplitz operators as the bounded operator-solutions to the operator-equation , for an arbitrary complex number . In most cases, it is shown that Toeplitz operators behave the same as the classical Toeplitz operators, on . We also introduce the classes of analytic and coanalytic Toeplitz operators (Definition 9), which generalize the most commonly considered classes of Toeplitz operators, and apply them to study the multiplicative properties of Toeplitz operators. In Theorem 14, we show that a product of two Toeplitz operators is again one precisely when each operator is either analytic or coanalytic, which generalizes [13, Theorem 8]. We then give an example (Example 16) to show that, unlike the Toeplitz case, two analytic (resp., coanalytic) Toeplitz operators need not commute, (which violates [13, Theorem 9] in the Toeplitz operators’ context). Though, we can still obtain some necessary and sufficient conditions for pairs of (co-)analytic Toeplitz operators to commute (Theorem 17). We also obtain an interesting result (Corollary 18) on the problem of invertibility for Toeplitz operators and its connection with our notions of analyticity and coanalyticity, which generalizes [13, Corollary 2]. Finally, we study the relation between analytic/coanalytic Toeplitz operators and the classical Hankel operators (Theorem 19). This work justifies Barría and Halmos’ suggestion, in [11], that the notion of Toeplitzness may be worthy of study.

Finally, it should be mentioned that this work has its roots in [13] and been inspired by [12, 14].

We close this section by setting up the notations required for what is to follow.

##### 1.1. Notations

(i) is the -algebra of all bounded linear operators on .(ii) stands for the two-sided ideal of all compact operators in .(iii)For , denotes the spectrum of ; denotes the set of eigenvalues for .(iv)The standard tensor notation will be used for operators of rank one: for and vectors in , the operator is defined by .(v)The standard orthonormal basis for , where are functions in defined as , for .(vi)For and , stands for the th-Fourier coefficient of ; that is, .(vii) consists of all boundary functions of bounded holomorphic functions on .

#### 2. -Toeplitzness

Here we give two approaches for defining the concept of “Toeplitzness”: matricial and operator-theoretic approaches.

*Definition 1. *One calls a singly infinite matrix a *Toeplitz matrix* if, on each diagonal (parallel to the main diagonal), the entries are in continued proportion; that is, the matrix is a Toeplitz matrix if there exists a such that , for .

For a fixed complex number , a typical example of a singly infinite Toeplitz matrix is
In [13], it is shown that Toeplitz operators, on , are the solutions of the operator-equation , on . Equivalently, this means that they are the eigen-operators of the following operator-valued linear transformation (let us call it* Toeplitz mapping on *):
corresponding to the eigenvalue 1. This suggests a general context in which Toeplitz operators can be embedded.

*Definition 2. *One calls an operator, in , a *Toeplitz operator* if it is an eigen-operator of the Toeplitz mapping corresponding to one of its eigenvalues.

More precisely, for , the set consists of Toeplitz operators corresponding to , or, equivalently, It can be easily checked that the Toeplitz mapping is a contraction; moreover, since , we should have . Thus, there is no Toeplitz operator for ; that is, . But every diagonal operator with diagonal , for , is a solution for . Thus, . Therefore, ; that is, the only eigen-operators for are the ones corresponding to the eigenvalues living in .

*Observation 1. *Bounded operator-solutions to , on , exist if and only if . For more details on the form of a Toeplitz operator, see [12].

Theorem 3 (Sun [12]). *Let . The operator-equation has bounded solutions if and only if . One then has the following. *(i)*If , all solutions are of the form , where is a Toeplitz operator and is the diagonal unitary operator defined as for all .*(ii)*If , all solutions are compact operators of the form
* *for some and .*

*For convenience, let us divide Toeplitz operators into two main classes: unimodular Toeplitz operators and nonunimodular Toeplitz operators, which are the ones corresponding to the eigenvalues of the Toeplitz mapping on the unit circle and the unit disk, respectively.*

*Remark 4. *Some immediate consequences of Sun’s Theorem are that the nonunimodular Toeplitz operators are compact (so are not invertible) and unimodular Toeplitz operators are not. Indeed, in the latter case, the only compact unimodular Toeplitz operator is the zero operator.

*Remark 5. *By Definition 2, if , for some , then . Hence, the entries of its matrix representation , with respect to the monomial basis for , satisfy
This yields
where

*3. Basic Properties of -Toeplitz Operators*

*Recall that consists of all (classical) Toeplitz operators, and it turns out, as we will see later, that other Toeplitz operators behave like them. Also, notice that, for each , forms a complex vector subspace of .*

*Here we look at the following straightforward properties of the Toeplitz operators. First, from Definition 2, we observe that, for each , is topologically well behaved. Indeed, for , since is weakly continuous in its middle factor, is weakly closed, and, therefore, a fortiori, it is strongly and uniformly closed.*

*The next result, inspired by [14, Theorem 4.5], states that self-adjointness only exists among real Toeplitz operators; that is, Toeplitz operators correspond to real eigenvalues for .*

*Proposition 6. For and , one has the following. (i).(ii)If and , then .*

* Proof. *(i) Since , we have , from which, by taking adjoints, we get . So .

(ii) If is a nonzero self-adjoint element of , then . But this means that
which implies , or, equivalently, .

*Remark 7. *As a consequence of Sun’s Theorem, every nonunimodular Toeplitz operator is compact. This, in turn, states that they are not only noninvertible, but also nonessentially invertible. But the situation is different for unimodular ones. Indeed, the only compact unimodular one is the zero operator: for , letting , and be nonnegative integers, we have
Now, if is a compact operator, then , as ; it follows that , for all nonnegative .

And, if we apply the same procedure for , we obtain , for all nonnegative . Therefore, .

*4. Analyticity and Coanalyticity of -Toeplitz Operators*

*4. Analyticity and Coanalyticity of -Toeplitz Operators*

*In [13, p. 96], analyticity and coanalyticity of a (classical) Toeplitz operator are defined and characterized in terms of its commutativity with and , respectively [13, Theorem 7]. Here, we define and give an analogous characterization of these two properties in the Toeplitz operators’ setting. But let us first assign them a “symbol” (similar to one in the classical case) as a generating function.*

*Definition 8. *For , let . The* symbol of * is defined to be
and is denoted by , in which is called the* analytic symbol* and is called the* coanalytic symbol* of .

*Observation 2. *For a Toeplitz operator , is the function whose nonnegative Fourier coefficients are the terms of the -column of its matrix representation, with respect to the monomial basis for , and whose nonpositive Fourier coefficients are the terms of the -row of that matrix.

*Definition 8 determines two -functions, namely, and , by which we may characterize the properties of analyticity and coanalyticity, for such operators, as follows.*

*Definition 9. *A Toeplitz operator, , is called (i)*analytic* if is an analytic function (i.e., is the constant function );(ii)*coanalytic* if is a coanalytic function (i.e., is the constant function ).

*This definition makes the following remark obvious.*

*Remark 10. *For , letting (i) is analytic if and only if , for all (i.e., ),(ii) is coanalytic if and only if , for all (i.e., ). Hence, is analytic if and only if is coanalytic.

*Observation 3. *Notice that for, and , , where is the diagonal operator with diagonal . Indeed,
This observation provides us with two classes of typical examples of analytic and coanalytic Toeplitz operators. (1)For and , is an analytic Toeplitz operator. Indeed for ,
(2)For and , is a coanalytic Toeplitz operator. Indeed for ,

Note that if is analytic, its matrix representation with respect to the monomial basis for is lower triangular. Indeed, for with
With the same reasoning one can show that coanalytic Toeplitz operators correspond to upper triangular Toeplitz matrices.

*Before stating the first result of this section, we need to introduce some terms. For a Hilbert space bounded operator , consider the operator-equation
for some complex number . If there is a nonzero (bounded) operator and a scalar as above that satisfy (16), according to [15], it is said that -commutes with and that is an extended eigenvalue and is an extended eigen-operator of .*

*Equation (16) has been studied in [16] and, independently, in [17]. These works provided extensions of Lomonosov’s classic result [18].*

*Now, we use these terms to state our next result which characterizes analyticity and coanalyticity of Toeplitz operators in terms of their -commutativity with and .*

*Theorem 11. Let and . A necessary and sufficient condition that is an analytic (coanalytic) Toeplitz operator in is that it -commutes with -commutes with ; that is, .*

*Proof. *(i) Let be an analytic Toeplitz operator in ; that is, . Hence,
Now, let be such that . Just by multiplying both sides by from the left, one can easily see that . To show it is analytic, we need to prove , for . So,
which means is an analytic Toeplitz operator.

(ii) If is a coanalytic Toeplitz operator in , that is, , then
Now, let -commute with ; that is, . Multiplying both sides by from the right shows that . To prove coanalyticity, we need to show . So, we have

*Remark 12. *Using the terms aforementioned, Theorem 11 can also be restated as follows.

Theorem (i)* Let **. A necessary and sufficient condition in which ** is analytic is that ** is an extended eigen-operator of ** corresponding to the extended eigenvalue *.(ii)* Let **. A necessary and sufficient condition in which ** is coanalytic is that ** is an extended eigen-operator of ** corresponding to the extended eigenvalue **.*

*Remark 13. *Notice that if , it can be represented by the finite-rank operator . Thus,(1) is analytic if and only if is the constant function and, in this case, .(2)And is coanalytic if and only if is the constant function and, in this case, .

*5. Multiplicative Properties of -Toeplitz Operators*

*5. Multiplicative Properties of -Toeplitz Operators*

*Although, for a fixed , is closed under finite summation of its elements, the corresponding result rarely holds for products. As an application of Theorem 11, we will see that Toeplitzness is preserved under multiplication, on the right, by analytic Toeplitz operators and, on the left, by coanalytic ones.*

*Theorem 14. For , let and .A necessary and sufficient condition that the product is a Toeplitz operator in is that either is coanalytic or is analytic.*

*Proof. *Let us first assume . Hence,
So, for holding the equality, we should have either or ; that is, either is coanalytic or is analytic.

Let us suppose, for now, is a coanalytic Toeplitz operator in and . To show that , we apply Theorem 11 to write
which proves is a Toeplitz operator in .

And if is an analytic Toeplitz operator in and , again using Theorem 11 results in
which proves the same thing.

*From the proof of Theorem 14, along with considering Remark 7, one may deduce the following property for unimodular Toeplitz operators.*

*Proposition 15. For , let and . If for some , then . Moreover, either is coanalytic or is analytic.*

* Proof. *Considering the assumptions, we have
which implies
Since and is a nonzero Toeplitz operator in , cannot be of finite rank (see Remark 7). Hence, both sides in (25) should be zero. Therefore, , and this in turn implies that either should be coanalytic or is analytic.

*Recall that if the symbols of two (classical) Toeplitz operators are either analytic or coanalytic, they necessarily commute [13, Theorem 9]. But, surprisingly, this is not the case among Toeplitz operators; for, let us look at an example.*

*Example 16. *For some , (1)let be analytic such that is arbitrary and is given by
where is the Kronecker delta. Note that can also be represented as
Hence, we have
Therefore, as analytic Toeplitz operators, and do not commute.(2)In the other direction, consider two coanalytic Toeplitz operators , such that is arbitrary and , the Hilbert space adjoint of in the previous case; that is,
Again, note that can also be represented as
Hence, we have
Therefore, as coanalytic Toeplitz operators, and do not commute.

*Though, we can still obtain some necessary and sufficient conditions for pairs of (co-)analytic Toeplitz operators to commute.*

*Theorem 17. For , let and . (i)If both and are analytic, then if and only if , for almost all .(ii)If both and are coanalytic, then if and only if , for almost all .*

*Proof. *(i) Assume that and are both analytic Toeplitz operators such that , for almost all . So, by Theorem 14, . Also, analyticity of and reveals that
which in turn implies
But since is an analytic Toeplitz operator, we just need to consider the Fourier coefficients of , which can be obtained from the finite sum in (33) by letting , which gives us the th-Fourier coefficient of ; that is,
which is nothing but the th-Fourier coefficient of , for almost all , since
where , for and .

By the assumption , hence the finite sum in (34) is also equal to the th-Fourier coefficient of ; that is,
which is the th-Fourier coefficient of . What we already showed is that the th-Fourier coefficients of and are equal. Therefore, . This proves the sufficiency condition in (37).

Let us assume that . This assumption, along with analyticity of and , implies
for , such that . Now, letting in (37), we obtain
for almost all . This proves the necessity condition in (37).

(ii) Assume that and are both coanalytic Toeplitz operators such that
for almost all . Their coanalyticity implies that and are analytic, which satisfy (39). Therefore, by the sufficiency condition in (37), they commute. This in turn implies that and commute. This proves the necessity condition in (37).

Now, suppose coanalytic Toeplitz operators and commute, which means analytic Toeplitz operators and commute. Hence, by the necessity condition in (37), we should have
which proves the sufficiency condition in (37).

*Another consequence of Theorem 11 characterizes the Toeplitz operators having Toeplitz operator inverses.*

*Corollary 18. For , let . If is invertible, then a necessary and sufficient condition that is a Toeplitz operator is that is either analytic or coanalytic.*

* Proof. *Suppose that is invertible. If is analytic, then, by Theorem 11, -commutes with ; that is,
from which follows
But, on one hand, (42) implies that is a Toeplitz operator in and, on the other hand, that it is an analytic Toeplitz operator, using Theorem 11.

The case for coanalyticity walks through the same steps as the latter case.

Suppose now that is known to be a Toeplitz operator in , for some . Having the following operator-equations,
we obtain
each of which implies ; that is, , and, in this case, from the first equation follows either is coanalytic or is analytic. And the second one also implies either is coanalytic or is analytic.

If is not coanalytic, then is analytic and not constant; this implies that is not coanalytic and hence that is analytic. The same reasoning also works when it is assumed that is not analytic.

*6. -Toeplitzness versus Hankelness*

*6. -Toeplitzness versus Hankelness*

*One of the properties of Hankelness is that it is preserved under multiplication, on the right, by analytic Toeplitz operators or, on the left, by coanalytic Toeplitz operators. Indeed, if such that is a Hankel operator and is an analytic Toeplitz operator, then
which states that satisfies the Hankel equation, so is a Hankel operator. A similar way shows that is a Hankel operator, where is a coanalytic Toeplitz operator.*

*It turns out that Hankel operators behave in a similar manner when they meet analytic/coanalytic Toeplitz operators.*

*Theorem 19. Let be a Hankel operator and . If is an analytic and is a coanalytic Toeplitz operator in , then and satisfy the Hankel equation in the sense that and .*

* Proof. *As it is well known, is a Hankel operator if and only if . Then, simply, we have
proving the assertion.

*Conflict of Interests*

*Conflict of Interests*

*The author declares that there is no conflict of interests regarding the publication of this paper.*

*Acknowledgment*

*Acknowledgment*

*The author would like to express his sincere gratitude to the anonymous referee for his/her helpful comments that will help improve the quality of the paper.*

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