Abstract
Let \(T_{\Pi\vec {b}}\) be the commutator generated by a multilinear square function and Lipschitz functions with kernel satisfying Dini-type condition. We show that \(T_{\Pi\vec {b}}\) is bounded from product Lebesgue spaces into Lebesgue spaces, Lipschitz spaces, and Triebel–Lizorkin spaces.
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1 Introduction
Let \(A(x)\) be an elliptic \(n\times n\) matrix with complex-valued entries that are merely bounded and measurable, and let \(T=\operatorname{div}(A(x)\nabla)\). The well-known problem of Kato is to show the boundedness of \(T^{1/ 2}\) from the Sobolev space \(H^{1}(\mathbb{R}^{n})\) to \(L^{2}(\mathbb{R}^{n})\). Fabes et al. [6] studied a family of multilinear square functions and applied it to the Kato problem. In fact, they obtained a collection of multilinear Littlewood–Paley estimates and then applied them to two problems in partial differential equations. The first problem is the estimation of the square root of an elliptic operator in divergence form, and the second is the estimation of solutions to the Cauchy problem for nondivergence-form parabolic equations. Such a square function has important applications in PDEs and other fields’ we refer to [1–7, 9, 10, 13, 14, 17–19] and the references therein. We now give the definition of the multilinear square function of type \(\omega(t)\).
Suppose that \(\omega(t):[0,\infty)\mapsto[0,\infty)\) is a nondecreasing function with \(0<\omega(1)<\infty\). For \(a>0\), we say that ω∈ \(\operatorname{Dini} (a)\) if
Let \(K_{t}(x,y_{1},\ldots, y_{m})\) be a locally integrable function defined away from the diagonal \(x=y_{1}=\cdots=y_{m}\) in \((\mathbb{R}^{n})^{m+1}\). We say that \(K_{t}(x,y_{1},\ldots, y_{m})\) is a kernel of type \(\omega(t)\) if there is a positive constant A such that the following conditions hold.
Size condition:
Smoothness condition:
whenever \(|z-x|\leq\frac{1}{2} \max_{j=1}^{m}{|x-y_{j}|}\), and
whenever \(|y_{j}-y_{j}'|\leq\frac{1}{2} \max_{j=1}^{m}{|x-y_{j}|}\).
For any \(x\notin\bigcap_{j=1}^{m} \mathtt{supp}\,\, f_{j}\) and \(f_{j}\in C_{c}^{\infty}(\mathbb{R}^{n})\), we say T is a multilinear square function of type \(\omega(t)\) if
In this paper, we always assume that T can be extended to bounded operators from \(L^{q_{1}}\times\cdots\times L^{q_{m}}\) to \(L^{q}\) for some \(1< q,q_{1},\ldots,q_{m}<\infty\) with \(\frac{1}{q_{1}}+\cdots+\frac {1}{q_{m}}=\frac{1}{q}\).
Remark 1.1
When \(\omega(x)=x^{\gamma}\) for some \(\gamma>0\), the boundedness of a multilinear square function was studied by Xue et al. [18].
Definition 1.2
(Iterated commutators)
Given a collection of locally integrable functions \(\vec{b}=(b_{1},\ldots,b_{m})\), the iterated commutator of a multilinear square function is defined by
Definition 1.3
(Commutators in the jth entry)
Given a collection of locally integrable functions \(\vec{b}=(b_{1},\ldots,b_{m})\), we define the commutator of a multilinear square function T as
where each term is the commutator of \(b_{j}\), and T in the jth entry of T, that is,
For the commutators generated by the multilinear Calderón–Zygmud-type singular integrals and Lipschitz functions with the kernel of standard estimates, Wang and Xu [16] and Mo and Lu [11] obtained the boundedness from a product of Lebesgue spaces to the Lebesgue space, to the homogenous Triebel–Lizorkin space, and to Lipschitz spaces, respectively. Motivated by these results, we study the boundedness of commutators generated by the multilinear square functions and Lipschitz functions. The main results of this paper are as follows.
Theorem 1.1
Let T be a multilinear square function of type \(\omega(t)\) with \(\omega\in\operatorname{Dini}(1)\). Suppose \(b_{j}\in\dot{\wedge}_{\beta_{j}}\) with \(0 < \beta _{j} < 1\) for \(j = 1, \ldots,m\) and \(\beta= \beta_{1} + \cdots+ \beta_{m}\). If \(1 < p_{1}, \ldots, p_{m} <\infty\), \(0< q <\infty\), and \(1/p_{j} > \beta _{j}/n\) with \(1/q = 1/p_{1}+\cdots+1/p_{m}-\beta/n\), then \(T_{\Pi\vec {b}}\) can be extended to a bounded operator from \(L^{p_{1}}\times\cdots \times L^{p_{m}}\) into \(L^{q}\).
Theorem 1.2
Let T be a multilinear square function of type \(\omega(t)\) with \(\omega\in\operatorname{Dini}(1)\). Suppose \(b_{j}\in\dot{\wedge}_{\beta_{j}}\) with \(0 < \beta _{j} < 1\) for \(j = 1, \ldots,m\) and \(\beta= \beta_{1} + \cdots+ \beta_{m}\). Let \(1 < p_{1}, \ldots, p_{m} <\infty\), \(0<1/p_{j} < \beta_{j}/n\), and \(0<\beta -n/ p<1\) with \(1/p = 1/p_{1}+\cdots+1/p_{m}\). If ω satisfies
then \(T_{\Pi\vec {b}}\) can be extended to a bounded operator from \(L^{p_{1}}\times\cdots \times L^{p_{m}}\) into Lipschitz space \(\dot{\wedge}_{\beta-n/ p}\).
Theorem 1.3
Let T be a multilinear square function of type \(\omega(t)\) with \(\omega\in\operatorname{Dini}(1)\). Suppose \(b_{j}\in\dot{\wedge}_{\beta_{j}}\) with \(0 < \beta _{j} < 1\) for \(j = 1, \ldots,m\) and \(\beta= \beta_{1} + \cdots+ \beta_{m}\). If \(1 < p_{1}, \ldots, p_{m} <\infty\) with \(1/p = 1/p_{1}+\cdots+1/p_{m}\) and ω satisfies
then \(T_{\Pi\vec {b}}\) can be extended to a bounded operator from \(L^{p_{1}}\times\cdots \times L^{p_{m}}\) into the Triebel–Lizorkin space \(\dot{F}_{p}^{\beta,\infty}\).
The paper is organized as follows. Some definitions and preliminaries are given in Sect. 2. In Sect. 3, we focus ourselves on a key lemma, which will be used in the proof of Theorem 1.1. The proofs of Theorems 1.2 and 1.3 are given in Sect. 4.
2 Preliminaries
Definition 2.1
For \(\delta>0\), \(M_{\delta}\) is the maximal function defined by
In addition, \(M^{\sharp}\) is the sharp maximal function of Feffeman and Stein,
and
Given a locally integrable function f, for \(0\leq\beta< n\), we define the fractional maximal function
If \(\beta=0\) and \(r=1\), then \(M_{0,1}f=Mf\) denotes the usual Hardy–Littlewood maximal function. When \(\beta=0\), we denote \(M_{r,\beta}\) simply by \(M_{r}\).
Chanillo [1] proved that if \(0 <\beta< n, 0 < r < p< n/\beta\), and \(1/q = 1/p-\beta/n\), then
Definition 2.2
([12])
For \(\beta>0\), the homogenous Lipschitz space \(\dot{\wedge}_{\beta}(\mathbb{R}^{n})\) is the space of functions f such that
where \(\Delta_{h}^{k}\) denotes the kth difference operator.
To prove our theorem, we need the following lemmas.
Lemma 2.1
([12])
Let \(b\in\dot{\wedge}_{\beta}\), \(0<\beta<1\). For any cubes \(Q'\), Q in \(\mathbb{R}^{n}\) such that \(Q'\subset Q\), we have
Lemma 2.2
([12])
-
(1)
For \(0 <\beta< 1\) and \(1 \leq q <\infty\), we have
$$\|f\|_{\dot{\wedge}_{\beta}}\approx\sup_{Q} \frac{1}{|Q|^{1+n/ \beta }} \int_{Q} |f-f_{Q}|\approx\sup _{Q} \frac{1}{|Q|^{n/ \beta}} \biggl( \int _{Q} |f-f_{Q}|^{q} \biggr)^{\frac{1}{q}}. $$ -
(2)
For \(0 <\beta< 1\) and \(1 \leq p<\infty\), we have
$$\|f\|_{\dot{F}^{\beta,\infty}_{p}}\approx \biggl\Vert \sup_{Q} \frac {1}{|Q|^{1+n/ \beta}} \int_{Q} |f-f_{Q}| \biggr\Vert _{L^{p}}. $$
Lemma 2.3
([15])
Let \(\frac{1}{p}=\frac {1}{p_{1}}+\cdots+\frac{1}{p_{m}}\) and \(\vec{\omega}\in A_{\vec{p}}\). Let T be a multilinear square function of type \(\omega(t)\) with \(\omega\in \operatorname{Dini}(1)\).
-
(1)
If \(1< p_{1}, \ldots, p_{m}<\infty\), then
$$\|T\vec{f}\|_{L^{p}(\nu_{\vec{\omega}})}\leq C\prod_{i=1}^{m} \|f_{i}\|_{L^{p_{i}}(\omega_{i})}. $$ -
(2)
If \(1\leq p_{1}, \ldots, p_{m}<\infty\), then
$$\|T\vec{f}\|_{L^{p,\infty}(\nu_{\vec{\omega}})}\leq C\prod_{i=1}^{m} \|f_{i}\|_{L^{p_{i}}(\omega_{i})}. $$
3 Proof of Theorem 1.1
To prove Theorem 1.1, we need the following estimates for \(T_{\Pi\vec {b}}\) and \(T^{j}_{ \vec{b}}\). We just consider the case \(m=2\) for simplicity; our method still holds for general m with little modifications.
Lemma 3.1
Let \(0 <\delta<\epsilon< 1/ 2\), and let T be a bilinear square function of type \(\omega(t)\) with ω∈ \(\operatorname{Dini}(1)\).
-
(i)
If \(b_{1}\in\dot{\wedge}_{\beta_{1}} \) and \(b_{2}\in\dot{\wedge }_{\beta_{2}} \) with \(0<\beta_{1}, \beta_{2}<1\) such that \(\beta_{1}+\beta_{2}=\beta\), then
$$\begin{aligned} M^{\sharp}_{\delta}T_{\Pi\vec{b}}(f_{1},f_{2}) (x) \leq& C \Biggl\{ \prod_{i=1}^{2} \|b_{i}\|_{\dot{\wedge}_{\beta_{i}}} M_{\epsilon ,\beta} \bigl(T(f_{1},f_{2}) \bigr) (x) \\ &{}+\|b_{1}\|_{\dot{\wedge}_{\beta_{1}}} M_{\epsilon,\beta_{1}} \bigl(T^{2}_{\vec {b}}(f_{1},f_{2}) \bigr) (x) \\ &{}+\|b_{2}\|_{\dot{\wedge}_{\beta_{1}}} M_{\epsilon,\beta_{2}} \bigl(T^{1}_{\vec {b}}(f_{1},f_{2}) \bigr) (x) \\ &{}+\prod_{i=1}^{2}\|b_{i} \|_{\dot{\wedge}_{\beta_{i}}}M_{1,\beta _{1}}(f_{1}) (x) M_{1,\beta_{2}}(f_{2}) (x) \Biggr\} . \end{aligned}$$(3.1) -
(ii)
If \(b_{j}\in\dot{\wedge}_{\beta}\), \(j=1,2 \), and \(0<\beta<1\), then
$$ M^{\sharp}_{\delta}T^{j}_{\vec{b}}(f_{1},f_{2}) (x)\leq C\|b_{j}\|_{\dot{\wedge}_{\beta}} \bigl\{ M_{\epsilon,\beta} \bigl(T(f_{1},f_{2})\bigr) (x)+ M_{1,\beta}(f_{j}) (x)M (f_{k}) (x) \bigr\} , $$(3.2)where \(k\neq j\), \(k=1,2\).
Proof
Fix a point x and a cube \(Q(x_{Q},l)\) containing x with side-length l, and set \(Q^{*}=8\sqrt{n}Q=Q(x_{Q},8\sqrt{n}l)\). We split \(f_{j}\) as \(f_{j}=f^{0}_{j}+f^{\infty}_{j}\), where \(f^{0}_{j}=f_{j}\chi_{Q^{*}}\) and \(f^{\infty}_{j}=f_{j}\chi_{\mathbb{R}^{n} \setminus Q^{*}}\) for \(j=1,2\). As is well known, to obtain (3.1), it suffices to show that
for some constant c to be determined.
Let \(\lambda_{1}=(b_{1})_{Q^{*}}\) and \(\lambda_{2}=(b_{2})_{Q^{*}}\). The sublinearity of \(T_{\Pi\vec{b}}\) leads to
Thus we have
We now observe the elementary inequality
which follows from the fact \(z\in Q\) and \(b\in\dot{\wedge}_{\beta}\). From Hölder’s inequality and the assumption \(\beta_{1}+\beta _{2}=\beta\), for \(0 <\delta<\epsilon< 1/ 2\), we have
Similarly, we have
and
Now we deal with \(T_{4}\). Set \(c= T((b_{1}-\lambda_{1})f_{1}^{\infty },(b_{2}-\lambda_{2})f_{2}^{\infty})(x)\). We may bound \(T_{4}\) as
where
and
For \(T_{41}\), by Kolmogorov’s inequality and Lemma 2.3 we get
For any \(y\in\mathbb{R}^{n} \setminus Q^{*}\) and \(b\in\dot{\wedge }_{\beta}\), there exists \(Q'\) such that \(Q^{*}\subset Q'\) and \(|y-x_{Q}|\sim |Q'|^{1/ n}\). Then, from Lemma 2.1 we have
For any \(y_{2}\in(Q^{*})^{c}\) and \(z\in Q\), we have \(|z-y_{2}|\sim|y_{2}-x_{Q}|\). By Minkowski’s inequality and the size condition (1.1) we get
By using the same technique we get \(T_{42}\leq C\|b_{1}\|_{\dot{\wedge }_{\beta_{1}}} \|b_{2}\|_{\dot{\wedge}_{\beta_{2}}}M_{1,\beta _{1}}(f_{1})(x)M_{1,\beta_{2}}(f_{2})(x)\).
To estimate \(T_{44}\), we use Minkowski’s inequality and (1.2) and (3.3). Since \((\mathbb{R}^{n} \setminus Q^{*})^{2}\subseteq \mathbb{R}^{2n}\setminus(Q^{*})^{2}\subseteq\bigcup_{k=1}^{\infty}(2^{k+3 }\sqrt{n}Q)^{2}\setminus(2^{k+2 }\sqrt{n}Q)^{2}\), we deduce that
Combing all our estimates together, we obtain (3.1).
Now we are in the position to prove (3.2). It is sufficient to consider the operator with only one symbol. Fix \(b\in\dot{\wedge}_{\beta}\) and consider the operator
We have to prove that
Let \(\lambda=b_{Q^{*}}\). We can control \(T_{b}(\vec{f})(x)\) as
Then, for any constant c, we obtain that
By Hölder’s inequality we get
We bound the second part as
where
and
By Kolmogorov’s inequality and Lemma 2.3 we get
By using the Minkowski inequality and (1.1) and (3.3) we obtain that
Similarly, we deduce that
Since \((\mathbb{R}^{n} \setminus Q^{*})^{2}\subseteq\mathbb {R}^{2n}\setminus(Q^{*})^{2}\subseteq\bigcup_{k=1}^{\infty}(2^{k+3 }\sqrt {n}Q)^{2}\setminus(2^{k+2 }\sqrt{n}Q)^{2}\), we can use Minkowski’s inequality and (1.2) and (3.3) to get
Thus we finish the proof of (3.2). Then Lemma 3.1 is proved. □
Proofs of Theorem 1.1
By using Lemma 3.1 and modifying the proof of Theorem 1.1 in [8] we can finish the proof of Theorem 1.1. We omit the proof. □
4 Proof of Theorems 1.2 and 1.3
For simplicity, we just consider the case \(m=2\); our method still holds for general m with little modifications.
Proof of Theorem 1.2
The theorem will be proved if we show that
Let \(c=c_{1}+c_{2}+c_{3}\), which will be determined later. Then we have
We can choose \(1< q, q_{j}<\infty\), \(q_{j}< n/\beta_{j} < p_{j}\), \(j=1,2\), with \(1/q=1/q_{1}+1/q_{2}-(\beta_{1}+\beta_{2})/n\). By Hölder’s inequality and Theorem 1.1 we obtain
For the second term, we take \(c_{1}=T((b_{1}-\lambda_{1})f_{1}^{0},f_{2}^{\infty})(x_{Q})\). Then
By Minkowski’s inequality and the size condition (1.1) we have
We now proceed as in the estimate of \(M_{21}\):
because of \(-1-1/p_{2} +\beta_{2}/n<0\).
Similarly,
By Minkowski’s inequality and (1.2) we have
where we have used the fact \(1-\beta_{2}/n +1/p_{2}>0\).
Thus
Similarly,
We deal with \(M_{4}\) as follows:
By Minkowski’s inequality and the size condition (1.1) we have
By Minkowski’s inequality and the smooth condition (1.2) we have
Similarly,
Now we estimate \(M_{44}\):
Combing the estimates for \(M_{1}\), \(M_{2}\), \(M_{3}\), \(M_{4}\), we get (4.1). Thus the proof of Theorem 1.2 is completed. □
Proof of Theorem 1.3
Let \(c=c_{1}+c_{2}+c_{3}\), which will be determined later. Then we have
In what follows, we estimate each term separately. For \(1< r< p\), by the Hölder inequality we have
Observe that
Let
Then
By the Hölder inequality we have
Take \(1< q_{1}< p_{1}\), \(1< q_{2}< p_{2}\), and \(1< q<\infty\) such that \(1/q=1/q_{1}+1/q_{2}\). Then by the Hölder inequality and Lemma 2.3 we have
For \(y_{2}\in(Q^{*})^{c}\), \(|y_{2}-x_{Q}|\sim|y_{2}-z|\), and \(|z-x_{Q}|\leq\frac {|y_{2}-z|}{2}\leq\frac{1}{2} \max{\{|z-y_{1}|,|z-y_{2}|\}}\), by Minkowski’s inequality and the smooth condition (1.2) we get
Similarly,
For \(y_{1},y_{2}\in(Q^{*})^{c}\), \(|y_{1}-x_{Q}|\sim|y_{1}-z|\), and \(|y_{2}-x_{Q}|\sim |y_{2}-z|\), by Minkowski’s inequality and the smooth condition (1.2) we get
Combining the estimates for \(N_{21}\), \(N_{22}\), \(N_{23}\), \(N_{24}\), \(N_{25}\), we get
Similarly, we have
and
Thus we deduce that
By the Hölder inequality, Lemma 2.3, and the normal inequalities for the maximal operators, we arrive at
where we have used that facts \(1 < r < p\), \(1 < q_{1} < p_{1}\), and \(1 < q_{2} < p_{2}\). This finishes the proof of Theorem 1.3. □
5 Conclusions
In this paper,we studied the boundedness properties of the commutator generated by a multilinear square function and Lipschitz functions with kernel satisfying Dini-type condition. We showed that such commutators are bounded from product Lebesgue spaces into the Lebesgue spaces, Lipschitz spaces, and Triebel–Lizorkin spaces.
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Acknowledgements
The authors are most grateful to the editor for careful reading the manuscript and valuable suggestions, which helped in improving an earlier version of this paper.
Funding
The first author was supported partly by the Key Research Project for Higher Education in Henan Province (No. 19A110017). The second author was supported partly by NSFC (Nos. 11471041, 11671039) and NSFC-DFG (No. 11761131002).
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Si, Z., Xue, Q. Estimates for iterated commutators of multilinear square fucntions with Dini-type kernels. J Inequal Appl 2018, 188 (2018). https://doi.org/10.1186/s13660-018-1778-8
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DOI: https://doi.org/10.1186/s13660-018-1778-8