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Estimates for Marcinkiewicz integral operators and extrapolation
Journal of Inequalities and Applications volume 2014, Article number: 269 (2014)
Abstract
In this article, we establish estimates for parametric Marcinkiewicz integral operators with rough kernels. These estimates and extrapolation arguments improve and extend some known results on Marcinkiewicz integrals.
MSC:40B20, 40B15, 40B25.
1 Introduction
Throughout this article, let , be the unit sphere in which is equipped with the normalized Lebesgue surface measure . Also, we let for and denote the exponent conjugate to p; that is .
Let , where ( with ), h is a measurable function on and Ω is a function on with and
For a suitable mapping , a measurable function h on and an Ω satisfying (1.1), we define the Marcinkiewicz integral operator for by
If , we denote by . The operators have their roots in the classical Marcinkiewicz integral operators which were introduced by Stein in [1] in which he studied the Boundedness of when (). More precisely, he proved that is of type for and of weak type .
The Marcinkiewicz integral operators play an important role in many fields in mathematics such as Poisson integrals, singular integrals and singular Radon transforms. They have received much attention from many authors (we refer the readers to [1–6], as well as [7], and the references therein).
Before introducing our results, let us recall the definition of the space and the definition of the block space , which are related to our work. For , let denote the class of all measurable functions Ω on that satisfy
The special class of block spaces (for and ) was introduced by Jiang and Lu in the study of the singular integral operators (see [8]), and it is defined as follows: A q-block on is an function that satisfies (i) , (ii) , where and is a cap on for some and . The block space is defined by
where each is a complex number; each is a q-block supported on a cap on , and
Define , where the infimum is taken over the whole q-block decomposition of Ω, then is a norm on the space , and the space is a Banach space.
Employing the ideas of [9], Wu [10] pointed out that for and for ,
The study of parametric Marcinkiewicz integral operator was initiated by Hörmander in [11] in which he showed that is bounded on for when and with . However, the authors of [12] proved that is bounded on for when and with . This result was improved in [13] in which the authors established that is bounded on if and , where is the collection of all measurable functions satisfying .
On the other hand, Al-Qassem and Al-Salman in [2] found that if with , then is bounded on for . Furthermore, they proved that is sharp on .
Walsh in [7] found that is bounded on if , and the exponent is the best possible. However, under the same conditions, Al-Salman et al. in [4] improved this result for any .
Recently, it was proved in [14] that if for some and for some , then is bounded on for any p satisfying , where ϕ is , a convex and increasing function with . Very recently, Al-Qassem and Pan established in [15] that if for some and for some , then is bounded on for any p satisfying , where is a polynomial mapping and each is a real valued polynomial on .
Our main concern in this work is in dealing with Marcinkiewicz operators under very weak conditions on the singular kernels. In fact, we establish certain estimates for , and then we apply an extrapolation argument to obtain and improve some results on Marcinkiewicz integrals. Our approach in this work provides an alternative way in dealing with such kind of operators. Our main result is described in the following theorem.
Theorem 1.1 Let for some , for some . Suppose that ϕ is , a convex and increasing function with . Then for any with p satisfying , there exists a constant (independent of Ω, h, γ, and q) such that
where
Throughout this paper, the letter C denotes a bounded positive constant that may vary at each occurrence but independent of the essential variables.
2 Definitions and lemmas
In this section, we present and establish some lemmas used in the sequel. Let us start this section by introducing the following.
Definition 2.1 Let . For a suitable function ϕ defined on , a measurable function and , we define the family of measures and the corresponding maximal operators and on by
where is defined in the same way as , but with replacing Ω, h by , , respectively. We write for the total variation of σ.
In order to prove Theorem 1.1, it suffices to prove the following lemmas.
Lemma 2.2 Let , for some and for some . Suppose that ϕ is , a convex and increasing function with . Then there are constants C and α with such that
hold for all . The constant C is independent of k, ξ and ϕ.
Proof As for , it is enough to prove this lemma for . By Hölder’s inequality, we get
Let us first consider the case . By a change of variable, we obtain
where . Write , where
By the conditions on ϕ and the mean value theorem we have
Hence, by Van der Corput’s lemma, , and then by integration by parts, we conclude
Combining the last estimate with the trivial estimate , and choosing , we get
which leads to
By the assumption of ϕ, and since the last integral is finite, we obtain
For the case , we use Hölder’s inequality to obtain
By this, Van der Corput’s lemma, and the above procedure, we obtain
and therefore
The estimate in (2.3) can be proved by using the cancellation property of Ω. By a change of variable, we have
Since is increasing and , we obtain
which when combined with the trivial estimate , we derive
The proof is complete. □
Following a similar argument to the one used in [[16], Lemma 2.7], we achieve the following lemma.
Lemma 2.3 Suppose that ϕ is given as in Lemma 2.2. Let be the maximal function of f in the direction y defined by
Then there exists a constant such that
for any with .
Proof By a change of variable, we get
Since the function is non-negative, decreasing and its integral over is equal to , then by [[16], Lemma 2.6] we obtain
where is the Hardy-Littlewood maximal function of f in the direction of y. By this, and since is bounded in with bounded independent of y, we obtain our desired result. □
Lemma 2.4 Let for some and for some . Assume that and ϕ are given as in Definition 2.1 and Lemma 2.2, respectively. Then for any with , there exists a constant (independent of Ω, h and f) such that
Proof By Hölder’s inequality, we have
Using Minkowski’s inequality for integrals gives
By using Hölder’s inequality plus Lemma 2.3, we finish the proof. □
Lemma 2.5 Let for some , for some and . Assume that and ϕ are given as in Definition 2.1 and Lemma 2.2, respectively. Then for any p satisfying , there is a positive constant such that
holds for arbitrary functions on .
Proof We employ some ideas from [2, 15], and [17]. By Schwarz’s inequality, we obtain
Let us first prove this lemma for the case . By duality, there is a non-negative function with such that
By this, (2.5), and a change of variable we derive
Since , then , and since , then by Lemma 2.4, Hölder’s inequality, and the same arguments that Stein and Wainger used in [18], we obtain
For the case , by the duality, there are functions defined on with such that
where
As , we obtain, by applying the above procedure,
where ς is a function in with . Thus, by (2.6) and (2.7), our estimate holds for ; and therefore the proof of Lemma 2.5 is complete. □
In the same manner, we prove the following lemma.
Lemma 2.6 Let for some , for some and . Assume that and ϕ are given as in Definition 2.1 and Lemma 2.2, respectively. Then for any p satisfying , there exists a constant such that
holds for arbitrary functions on .
3 Proof of the main result
We prove Theorem 1.1 by applying the same approaches that Al-Qassem and Al-Salman [2] as well as Fan and Pan [17] used. Let us first assume that for some ; and ϕ is , a convex and increasing function with . By Minkowski’s inequality, we get
Take ; and for , let be a smooth partition of unity in adapted to the interval . More precisely, we require the following:
where is independent of θ. Let . Decompose , where
Define . Then for any ,
Let us first compute the -norm of . By using Plancherel’s theorem and Lemma 2.2, we obtain
where . Thus,
Applying the Littlewood-Paley theory and Theorem 3 along with the remark that follows its statement in [[19], p.96], plus using Lemma 2.5, we see that
holds for . By interpolation between (3.3) and (3.4) we obtain
Consequently, by (3.2) and (3.5), we get our result for the case for some .
The proof of our theorem for the case for some is obtained by following the above argument, except that we need to invoke Lemma 2.6 instead of Lemma 2.5. Therefore, the proof of Theorem 1.1 is complete.
4 Further results
The power of our theorem is in applying the extrapolation method on it (see [16]). In particular, Theorem 1.1 and extrapolation lead to the following theorem.
Theorem 4.1 Suppose that for some and Ω satisfies (1.1). Let ϕ be , a convex and increasing function with .
-
(i)
If for some , then
for .
-
(ii)
If , then
for , where
We point out that the boundedness of was obtained in [14] if for some , and the boundedness () of was investigated in [4] if .
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Acknowledgements
The author would like to thank Dr. Hussain Al-Qassem for his suggestions and comments on this note.
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Ali, M. Estimates for Marcinkiewicz integral operators and extrapolation. J Inequal Appl 2014, 269 (2014). https://doi.org/10.1186/1029-242X-2014-269
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DOI: https://doi.org/10.1186/1029-242X-2014-269