#### Abstract

A Stevi-Sharma operator denoted by is a generalization product of multiplication, differentiation, and composition operators. In this paper, we characterize the bounded and compact Stevi-Sharma operator from a general class of Banach function spaces into Zygmund-type spaces with some of the most convenient test functions on the open unit disk. Using several restrictive terms, we show that all bounded operators from into the little Zygmund-type spaces are compact. As an application, we show that our results hold up for some other domain spaces of , such as the Hardy space and the weighted Bergman space.

#### 1. Introduction

Meanwhile, in this work, is the open unit disk of the complex plane . We denote by the family of analytic functions in ; also, we denote by the family of analytic self-maps of and the set of conformal automorphisms of the disk . As is usual, the Banach space is the family of bounded functions , defined by the norm .

For any weighted function , the weighted Banach space is the space of functions such that

Moreover, the little weighted Banach space is the space of functions such that

The Bloch-type space is the Banach space of functions such that , with the norm . The little Bloch-type space is the closed subspace of consisting of functions such that .

The space of all functions is said to be a weighted Zygmund-type space , if all functions are such that . The space becomes a Banach space with the norm

The little weighted Zygmund-type space is the closed subspace of and consists of functions , such that .

Exactly as in the aforementioned spaces, when , we get the Zygmund-type space . In addition, the space is a Banach space with the norm , where . The Zygmund spaces also satisfy whenever . When , we get the classical Zygmund space denoted by . In fact, the motivation for the name and for studying the Zygmund-type spaces comes from the Zygmund class; see, for example, Chapter 5 of Duren’s book [1]. The Zygmund space and its subspace play an important role in connection to the theory of the Hardy spaces . Indeed, in [2], it was shown that can be viewed as the dual of the Hardy space . More generally, the spaces defined by replacing the function in the definition of with its th derivative, under the assumption for , can be viewed as duals of the Hardy spaces . A recent study of several operators on Zygmund-type spaces has attracted significant research attention; see, for example, [3–5].

For any and , we have the following linear operators: (1): the differentiation operator(2): the multiplication operator(3): the composition operator(4): the weighted composition operator

For a unified manner of treatment of these operators, many researchers seek to present the various products of multiplication, composition, and differentiation operators; see, for example, [3, 4, 6–8]. Stevi et al. was the first to introduce the operator in [7]. So, the operator is called the Stevi-Sharma operator, which is defined as

Over the past 10 years, the boundedness and compactness of this operator have been studied extensively in the most well-known spaces of analytic functions; for example, see [5, 9].

This paper proceeds as follows. In Section 2, we present basic facts and prerequisites required for the Banach spaces. During Section 3, we characterize the boundedness of the operators from a large class of Banach spaces into the Zygmund-type space , also we give their norm estimates. Throughout Section 4, we study the compactness of from a large class of Banach spaces into the Zygmund-type space , under specific conditions on class . The boundedness and compactness of from to are investigated in Section 5. Finally, in Section 6 we apply our results to the Hardy spaces , and the weighted Bergman space , ( and ).

##### 1.1. Suppositions

In this paper, always, we will suppose the following: (i) and , unless otherwise stated(ii)For any , we will do the following differentiation over and over again:

So, to simplify the formulation of our results, we will define the following quantities: (iii)For any two quantities and that are jointly dependent on , we stipulate that , meaning that there is a positive constant that fulfills . Thus, when , we hold that and the quantities and are said to be equivalent. If , then if and only if

#### 2. Some of the Prerequisites Required for Banach Spaces

Following popular terminology in functional analysis, we stipulate a Banach space whose elements are functions , with norm and whose functionals of point-evaluation are bounded. Always we will suppose through this paper that is a Banach space of functions .

Let indicate the functional of point-evaluation at . So, for any , we can define

Thus, for each and , we obtain

Now, under appropriate modifications, we will list the specific conditions given in [10], including the conditions used to formulate the results of the present work. (I)For all and , we get (II)With respect to the uniform convergence topology on compact subsets of , the unit ball of a Banach space is comparatively compact(III)By a positive constant in compact subsets of , the norm is bounded below and (IV)For , let where . Then, the linear map is a compact map on , also (V)For all and , we have and , where (VI)For all , let ; then , where

The following proposition also is proved in [10].

Proposition 1. *For , the map is bounded on compact subsets of . Further, if a Banach space is reflexive, then is compact and shows relatively of uniform convergence, with respect to the topology on a compact subset of .*

The Lemma 2 helps to distinguish the properties of the operator concerned with studying in this article.

Lemma 2. *Let there be a Banach space satisfying the above conditions (V) and (VI). Suppose that , then for any and , such that , there is a set of functions defined in , for , such that .
**Moreover, the sequences converge to uniformly on compact subsets of , when .*

*Proof. *Let , which is the automorphism of that changes zero and , that is, , for any . Note that
Now, fix , such that , and consider the functions , for all and . Let in condition (V); then for all , we have and
Also, it is not difficult to prove that
Finally, the uniform convergence to of the sequences is self-evident, when .

#### 3. Boundedness of

In this section, we characterize the boundedness of the operators from a large class of Banach spaces into the Zygmund-type space ; also we give their norm estimates.

First of all, to simplify the formulation of the next results, for as in (4), we will define the following:

Now, we introduce the main results of this section.

Theorem 3. *Let there be a Banach space satisfying the above conditions (V) and (VI). Then, is a bounded operator if and only if all the quantities are finite, for . Moreover, in which case
*

*Proof. *First, suppose that is bounded. Then,
Set in Lemma 2, we observe that
Also, we have and .

Then, we obtain
For any with , by (5), we have
Next, we prove that is finite. So, set in Lemma 2; we observe that , but
Also, we have and .

Then, we obtain
Using the triangle inequality, we get
Condition (VI) for shows that
Thus, by (16), we have
Using (15), we obtain
For the third time, we go back to Lemma 2 with ; we know , also , and .

Then,
We repeat using the triangle inequality; we get
Repeat condition (VI) for and since , we obtain
Using (15) and (21), we obtain
Finally, we go to prove that is finite. For fix , such that . Then, we have
Since is bounded, by condition (VI) for , we observe that
Now, using (15), (21), and (26), we have
Because of this, all the quantities are finite, where , and
On the other hand, suppose all the quantities are finite, where . For fix we let , such that . For , by conditions (VI) and (4), we obtain
In addition to that,
We can choose a sufficiently large , the highest estimate
Moreover, from (1) and (33), we have
That ends the proof of Theorem 3.

In the case of and in Theorem 3, we define the following quantities:

Now we deduce the following result as a corollary of Theorem 3.

Corollary 4 ([4], Theorem 3.1). *Let a Banach space be satisfying the above conditions (V) and (VI). Then, is a bounded operator if and only if all the quantities are finite, for . Moreover, in which case , where
*

#### 4. Compactness of

Here, we study the compactness of from a large class of Banach spaces into the Zygmund-type space , under specific conditions on class . Before that, we mention the Lemma 5, which can be proved in a standard way; for example, see [6, 10].

Lemma 5. *Let a Banach space be satisfying condition (II) and whose the functionals of point-evaluation are continuous. Then, the bounded operator from into Zygmund-type space is compact if and only if for any bounded sequence in which converges uniformly to as on compact subsets of , .**Moreover, to simplify the formulation of the main results in this section, for as in (4) and , we set
*

Now, we introduce the main results of this section.

Theorem 6. *Let a Banach space be satisfy the above conditions (II), (V), and (VI). Suppose that is a bounded operator, and let
**Then, is a compact operator if and only if all the quantities , for .*

*Proof. *It is clear that condition (38) holds if the constants contain in , that is . First, assume that is a compact operator. Obviously, the result is intuitive if . So from now, we suppose . Also from now to the end of the proof, we let a sequence in , be such that , for ,
Since for each and , then, for each and fix , there exists such that and
Clearly, is a norm-bounded sequence; also is a bounded map on compact sets in . Using the fact , then is a uniformly bounded sequence on compact sets. For all and , set the functions
Then, we see that
Clearly, also converges uniformly to on compact subsets of . By the condition (V), we obtain and .

Since , by the functions defined in Lemma 2 for , we can consider the functions
By Lemma 2, we observe that
Also, we have and , for all .

Using the hypothesis that condition (II) holds, by applying Lemma 5, for all we obtain
In the case of in (45) and (47), we get
Thence,
From (40) and (43), we have
Thus, by (38), we have
We know from Theorem 3 and the boundedness of the operator , that all the quantities are finite, where . Therefore, we obtain from (50) and (51):