Volume 2022, Issue 1 4845507
Research Article
Open Access

Boundedness of Multilinear Calderón-Zygmund Operators on Grand Variable Herz Spaces

Hammad Nafis

Hammad Nafis

Department of Mathematics and Statistics, Riphah International University, I-14 Islamabad, Pakistan riphah.edu.pk

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Humberto Rafeiro

Corresponding Author

Humberto Rafeiro

United Arab Emirates University, College of Sciences, Department of Mathematical Sciences, P.O. Box 15551, Al Ain, Abu Dhabi, UAE uaeu.ac.ae

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Muhammad Asad Zaighum

Muhammad Asad Zaighum

Department of Mathematics and Statistics, Riphah International University, I-14 Islamabad, Pakistan riphah.edu.pk

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First published: 28 March 2022
Citations: 3
Academic Editor: Douadi Drihem

Abstract

In this paper, we prove the boundedness of multilinear Calderón-Zygmund operators on product of grand variable Herz spaces. These results generalize the boundedness of multilinear Calderón-Zygmund operators on product of variable exponent Lebesgue spaces and variable Herz spaces.

1. Introduction

There has been increased interest in the study of multilinear singular integral operators in recent years. The class of multilinear singular integrals with standard Calderón-Zygmund kernels provides the foundation and starting point of research of the theory. Such a class of multilinear Calderón-Zygmund operators was introduced and first studied by Coifman and Meyer [13] and later by Grafakos and Torres [4]. For the boundedness and other properties of multilinear fractional integrals, we refer to, e.g., [58].

Variable Lebesgue spaces were introduced in [9], but stayed under the radar for a considerable amount of time. Apart from some previous sporadic episodes, the research boom on such spaces can be traced back to the foundational paper [10]. Since then, these spaces have attracted much attention of mathematicians, not only because of their connection with harmonic analysis but also due to their usefulness in application to a wide range of problems, see, e.g., [11]. The standard references to the general theory of variable Lebesgue spaces are [12, 13].

The classical definition of Herz spaces was introduced in [14]. Many studies can be found related to these spaces and its variations, which include variable Herz spaces, continual Herz spaces, and Herz spaces with variable smoothness and integrability. For details, see [1521] and references therein.

Grand Lebesgue spaces on bounded sets, which proved to be useful in application to partial differential equations, were introduced in [22, 23]. In the last years, various operators of harmonic analysis have been intensively studied on grand spaces, see for instance [20, 2432]. Grand Lebesgue sequence spaces were introduced recently in [33], where several operators of harmonic analysis were studied, e.g., maximal, convolutions, Hardy, Hilbert, and fractional operators.

In this paper, we prove the boundedness of multilinear Calderón-Zygmund operators on grand variable Herz spaces which were introduced in [34]. The present paper is organized in the following way. Apart from the introduction, in Section 2, we recall some definitions and results related to variable exponent spaces. Section 3 contains some details about multilinear Calderón-Zygmund kernels and the proof of the main result.

Notations.

  • (i)

    is the set of natural numbers and 0 ∪ {0}

  • (ii)

    is the set of integers

  • (iii)

    is the set of negative integers

  • (iv)

    for n, m and n < m

  • (v)

    B(x, r) is the ball of radius r center at the point x

  • (vi)

    Bk≔{xn : |x| ≤ 2k} for all k

  • (vii)

    Rt,τBτ\Bt = {x : t<∣x∣<τ} is a spherical layer

  • (viii)

    RkBk\Bk−1

  • (ix)

  • (x)

    fg means that fCg and fg means that fgf

  • (xi)

    constants (often different constant in the same chain of inequalities) will mainly be denoted by c or C

2. Function Spaces with Variable Exponent

In this section, we recall definitions and results related to variable exponent Lebesgue spaces, variable Herz spaces, and grand variable Herz spaces.

2.1. Lebesgue Space with Variable Exponent

For the current section, we refer to [1013, 35] unless and until stated otherwise. Let q be a real-valued measurable function on n with values in [1, ∞). For Xn, we suppose that
(1)
where q(X)≔essinfxXq(x)andq+(X)≔esssupxXq(x). By Lq(·)(n), we denote the space of measurable function f on n such that
(2)
It is a Banach space, see [13, 35], endowed with norm:
(3)

By q, we denote the conjugate exponent of q, defined by q(x) = q(x)/(q(x) − 1). In the sequel, we use

log condition:
(4)
where A = A(q) > 0 does not depend on x, y;
decay condition at 0:
(5)
holds for some q(0) ∈ (1, ∞); and
decay condition at ∞: there exists a number q(∞) ∈ (1, ∞), such that
(6)
where A = A(q) does not depend on x.
Given a function , the Hardy-Littlewood maximal operator M is defined by
(7)
We adopt the following notations:
  • (i)

  • (ii)

    consists of all measurable functions q satisfying q > 1 and q+ < ∞

  • (iii)

    and denote the classes of which satisfy (5) and (6), respectively

  • (iv)

    is the set of all for which M is bounded on Lq(·)(n)

For the following lemma, we refer to, e.g., [12].

Lemma 1 (Generalized Hölder’s Inequality). Given , define by 1/r(x) = 1/p(x) + 1/q(x). Then, there exists a constant c such that for all fLp(·)(n) and gLq(·)(n), fgLr(·)(n) and

(8)

The following lemma appears in [21].

Lemma 2. Let D > 1 and Then

(9)
(10)
where c0 ≥ 1 and c ≥ 1 depend on D, but do not depend on r.

2.2. Herz Spaces with Variable Exponent

The classical Herz spaces were first introduced in [14]. We recall the definition of variable exponent Herz spaces.

Definition 3. Let 1 < p < ∞, α, and . The homogeneous Herz space is defined by

(11)
where
(12)

The nonhomogeneous Herz space is defined by

(13)
where
(14)

For the boundedness of integral operators on Herz type spaces, we refer to, e.g., [18, 19, 36].

2.3. Grand Lebesgue Sequence Space

In this section, we recall the definition of grand Lebesgue sequence space. For the following definition and statements, see [33]. In what follows, stands for one of the sets n, , , and 0.

Definition 4. Let 1 ≤ p < ∞ and θ > 0. The grand Lebesgue sequence space lp),θ is defined by the norm

(15)

2.4. Grand Variable Herz Space

Following [34], we now introduce the grand variable Herz spaces.

Definition 5. Let αL(), 1 ≤ p < ∞, q : n⟶[1, ∞), and θ > 0. The homogeneous grand variable Herz space is defined by

(16)
where
(17)

The following lemma, see [15], is helpful to estimate the norm of characteristics functions.

Lemma 6. Let and let R = B(0, r)\B(0, (r/2)). If |R| ≤ 2n, then

(18)
with the implicit constants independent of r and xR.

The left-hand side equivalence remains true for every |R| > 0 if we assume, additionally, that

3. Boundedness of Multilinear Calderón-Zygmund Operators

Consider the multilinear operator T of the form
(19)
where and , the space of compactly supported functions. Let K(x, y1, ⋯, ym) be a locally integrable function defined away from the diagonal , which satisfies the size estimate
(20)
for some c > 0 and all with xyj for some j. For smoothness, assume that for some ε > 0,
(21)
provided that |xx| ≤ 1/2max{|xy1|, ⋯, |xym|} and
(22)
whenever for all
Such kernels are called Calderón-Zygmund kernels and the class of all functions satisfying (20), (21), and (22) with parameters m, c, and ε will be denoted by m-CZK(c, ε), compare [4]. We say that T be as in (19) is an m-linear Calderón-Zygmund operator, if
  • (i)

    The related kernel belongs to m-CZK(c, ε) class

  • (ii)

    T is bounded from to Lq for some 1 < q1, q2, ⋯, qm < ∞, and 1/q = 1/q1 + ⋯+1/qm

Grafakos and Torres [4] proved the boundedness of T from to Lq,∞ for some 1 < q1, q2, ⋯, qm < ∞, and 1/q = 1/q1 + ⋯+1/qm, and from L1 × ⋯×L1 to L1/m,∞.

The boundedness of the multilinear Calderón-Zygmund operator T on variable exponent Lebesgue spaces was proved in [37], as stated below.

Lemma 7. Let , , with 1/q(x) = 1/q1(x) + ⋯+1/qm(x), and for some 0 < s < q. Then, the m-linear Calderón-Zygmund operator T is bounded on the product of variable exponent Lebesgue spaces. Moreover,

(23)
with the constant C independent of .

We now state and prove the boundedness of multilinear Calderón-Zygmund operator on grand variable Herz spaces.

Theorem 8. Let , , such that qi(0) ≤ qi(∞), 1/q(x) = 1/q1(x) + ⋯+1/qm(x), , and for some 0 < s < q. Let θ > 0, 1 < pi < ∞, and αiL() be log-Hölder continuous both at the origin and at infinity for with

(24)

Suppose that and Then, the m-linear Calderón-Zygmund operator T is bounded on the product of grand variable Herz spaces. Moreover,

(25)
with the constant C > 0 independent of .

Proof. We restrict ourselves to m = 2, the general case following in a similar manner. Defining and , we decompose the component functions of as

(26)

For future usage, we divide into the following sets

(27)
and, for X and Y arbitrary subsets of , we define
(28)

From Definition 5 and (26), we have

(29)
where
(30)
(31)

It is necessary to estimate I1, I2, I3, I5, I6, and I9, since I4, I7, and I8 can be obtained in a similar manner as I2, I3, and I6, respectively. Estimation for I1: splitting = 0 and by the asymptotic 2kα(x)≃2kα(0) (xRk and k < 0) and 2kα(x)≃2kα(∞) (xRk and k ≥ 0), we get

(32)

For φ, σLk, xRk, y1Rφ, and y2Rσ, we have

(33)
(34)
yielding
(35)

From estimate (35), Hölder’s inequality, 1/q(x) = 1/q1(x) + 1/q2(x), and Lemma 2, we obtain

(36)

Taking into account the previous estimate for νk(Lk, Lk), the equality 1p = 1p1 + 1p2, and Hölder’s inequality, we have

(37)

By Hölder’s inequality, Fubini’s theorem for series, , and defining b1n/q1(0) − n + α1(0) < 0, we obtain

(38)

The estimate is obtained, mutatis mutandis, via the estimation for I111. With this estimates at hand, we obtain

(39)

To estimate I12, we split as follow

(40)

The estimate I122 follows in similar manner as in I11 with simply replaced αi(0) by αi(∞) and used the fact qi(0) ≤ qi(∞).

For estimate I121, by Hölder’s inequality, Lemma 2, and the inequality qi(0) ≤ qi(∞), we obtain

(41)

From the estimate of νk(, ), the equality 1/p = 1/p1 + 1/p2, and Hölder’s inequality, we have

(42)

Invoking the Hölder inequality and defining ξ1n/q1(0) − n + α1(∞) < 0, we have

(43)

Similar estimate, with the corresponding changes, is obtained for A2, from which we obtain Hence

(44)

Estimation for I2: as in the case of I1, we obtain the following estimate

(45)

Notice that, for xRk, y1Rφ, y2Rσ, φLk, and σMk, we have

(46)
from which, taking Lemma 2 into consideration and elementary computations, we obtain
(47)

From the estimate for νk(Lk, Mk) and Hölder’s inequality, we get

(48)

Notice that I211 = I111. For the estimate I212, we reason as follows

(49)

The term I22 is estimated by

(50)

To estimate I221, by Hölder’s inequality, Lemma 2, and the inequality q1(0) ⩽ q1(∞), we have

(51)

Thus using (51) and by Hölder’s inequality, we have

(52)

The term B1 is equal to A1 and for B2 we use similar arguments as for I212, replacing α2(0) with α2(∞).

For the term I222, by Hölder’s inequality and Lemma 2, we have

(53)

Taking into consideration (53) and applying Hölder’s inequality, we get

(54)

Noting that and estimating in a similar fashion as I212, we obtain , from which we get

(55)

Estimation for I3: we have

(56)

For φLk, σNk, xRk, y1Rφ, and y2Rσ, we get

(57)
from which, taking into account the elementary inequality , it follows
(58)

By Hölder’s inequality, Lemma 2, q2(0)≲q2(∞), and k, we obtain

(59)

To estimate the term I31, using (59) and the Hölder inequality, we obtain

(60)

We have , since I311 = I111.

As for I312, we split as follow

(61)

By Hölder’s inequality, Fubini’s theorem for series, the inequality n/q2(∞) + α2(0) > 0 and , we obtain

(62)

For D2, using the same argument as above and the inequality α2(0) ⩽ α2(∞), we obtain

(63)

So

The term I32 can be estimated as

(64)

For k0, φ, σNk, q1(0)≲q1(∞), and Lemma 2, we obtain

(65)

By (65) and Hölder’s inequality, we get

(66)

Note that The estimate for can be obtained in a similar way as for I312, by replacing α2(0) with α2(∞).

For estimate I322, by Hölder’s inequality and Lemma 2, we have

(67)

Using (67) and Hölder’s inequality, we obtain

(68)

We have and the estimate for the term is similar to Taking all the estimates into account yields

(69)

Estimation for I5: We have

(70)

By the Lq(·)-boundedness of T, see Lemma 7, we obtain that

(71)

By Hölder’s inequality, we have

(72)

Similarly, we can obtain similar estimate for I52, replacing αi(0) by αi(∞) with k0. Therefore

(73)

Estimation for I6: we have

(74)

For k, φMk, σNk, and xRk, we have

(75)
yielding
(76)

By Hölder’s inequality, Lemma 2, k, and inequality q2(0) ⩽ q2(∞), we obtain

(77)

By (77) and invoking Hölder’s inequality, we get

(78)

Note that the estimate I611 is equal to that of I51 and I612 is obtained by similar argument used in I312.

For k0, we have

(79)

Using (79) and Hölder’s inequality, we get

(80)

The estimate I621 is similar to I52 and Hence

(81)

Estimation for I9: we estimate I9 as usual

(82)

For φNk, y1Rφ, and xRk, we have ∣xy1 | 2φ, whereas for σNk, y2Rσ, and xRk, we have ∣xy2 | 2σ. Under the assumptions for the obtained inequalities, we have

(83)

When k, by Hölder’s inequality, Lemma 2, and qj(0) ⩽ qj(∞), we obtain

(84)

Thus

(85)

The estimates for the terms I911 and I912 are obtained in the same way as for I312. Therefore

(86)

Finally, for the term I92, we have

(87)

The terms I921 and I922 can be estimated in a similar manner as for that I322. Thus

(88)

Taking into account the estimates (44), (55), (69), (73), (81), and (88), we get

(89)
which completes the proof.

Conflicts of Interest

The authors declare that there is no conflict of interest.

Authors’ Contributions

All the authors have contributed equally in preparation and finalization of the manuscript.

Acknowledgments

We gratefully acknowledge the constructive comments of the anonymous referees.

    Data Availability

    No data were used to support this study.

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