[
Abstract
Let be the class of bounded away from one and infinity functions such that the HardyLittlewood maximal operator is bounded on the variable Lebesgue space . We show that if belongs to the Hörmander class with , , then the pseudodifferential operator is bounded on the variable Lebesgue space provided that . Let be the class of variable exponents represented as where , , and . We prove that if slowly oscillates at infinity in the first variable, then the condition
is sufficient for the Fredholmness of on whenever . Both theorems generalize pioneering results by Rabinovich and Samko [23] obtained for globally logHölder continuous exponents , constituting a proper subset of .
PDO on Variable Lebesgue Spaces]
Pseudodifferential Operators
on Variable Lebesgue Spaces
A. Yu. Karlovich]Alexei Yu. Karlovich
I. M. Spitkovsky]Ilya M. Spitkovsky
[
To Professor Vladimir Rabinovich on the occasion of his 70th birthday
Mathematics Subject Classification (2000). Primary 47G30; Secondary 42B25, 46E30.
Keywords. Pseudodifferential operator, Hörmander symbol, slowly oscillating symbol, variable Lebesgue space, HardyLittlewood maximal operator, FeffermanStein sharp maximal operator, Fredholmness..
We denote the usual operators of first order partial differentiation on by . For every multiindex with nonnegative integers , we write . Further, , and for each vector , define and where stands for the Euclidean norm of .
Let denote the set of all infinitely differentiable functions with compact support. Recall that, given , a pseudodifferential operator is formally defined by the formula
where the symbol is assumed to be smooth in both the spatial variable and the frequency variable , and satisfies certain growth conditions (see e.g. [25, Chap. VI]). An example of symbols one might consider is the class , introduced by Hörmander [12], consisting of with
where and and the positive constants depend only on and .
The study of pseudodifferential operators with symbols in on socalled variable Lebesgue spaces was started by Rabinovich and Samko [23, 24].
Let be a measurable a.e. finite function. By we denote the set of all complexvalued functions on such that
for some . This set becomes a Banach space when equipped with the norm
It is easy to see that if is constant, then is nothing but the standard Lebesgue space . The space is referred to as a variable Lebesgue space.
Lemma 1.1.
We will always suppose that
(1.1) 
Under these conditions, the space is separable and reflexive, and its dual space is isomorphic to , where
Given , the HardyLittlewood maximal operator is defined by
where the supremum is taken over all cubes containing (here, and throughout, cubes will be assumed to have their sides parallel to the coordinate axes). By denote the set of all measurable functions such that (1.1) holds and the HardyLittlewood maximal operator is bounded on .
Assume that (1.1) is fulfilled. Diening [7] proved that if satisfies
(1.2) 
and is constant outside some ball, then . Further, the behavior of at infinity was relaxed by CruzUribe, Fiorenza, and Neugebauer [5, 6], where it was shown that if satisfies (1.2) and there exists a such that
(1.3) 
then . Following [9, Section 4.1], we will say that if conditions (1.2)–(1.3) are fulfilled, then is globally logHölder continuous.
Conditions (1.2) and (1.3) are optimal for the boundedness of in the pointwise sense; the corresponding examples are contained in [20] and [5]. However, neither (1.2) nor (1.3) is necessary for . Nekvinda [18] proved that if satisfies (1.1)–(1.2) and
(1.4) 
for some and , then . One can show that (1.3) implies (1.4), but the converse, in general, is not true. The corresponding example is constructed in [3]. Nekvinda further relaxed condition (1.4) in [19]. Lerner [15] (see also [9, Example 5.1.8]) showed that there exist discontinuous at zero or/and at infinity exponents, which nevertheless belong to . We refer to the recent monograph [9] for further discussions concerning the class .
Our first main result is the following theorem on the boundedness of pseudodifferential operators on variable Lebesgue spaces.
Theorem 1.2.
Let , , and . If , then extends to a bounded operator on the variable Lebesgue space .
The respective result for and satisfying (1.1)–(1.3) was proved by Rabinovich and Samko [23, Theorem 5.1].
Following [23, Definition 4.5], a symbol is said to be slowly oscillating at infinity in the first variable if
where
(1.5) 
for all multiindices and . We denote by the class of all symbols slowly oscillating at infinity. Finally, we denote by the set of all symbols , for which (1.5) holds for all multiindices and . The classes and were introduced by Grushin [11].
We denote by the set of all variable exponents for which there exist constants , , and a variable exponent such that
for almost all . Rabinovich and Samko observed in the proof of [23, Theorem 6.1] that if satisfies (1.1)–(1.3), then . It turns out that the class contains many interesting exponents which are not globally logHölder continuous (see [13]). In particular, there exists such that for every satisfying the function
belongs to .
As usual, we denote by the identity operator on a Banach space. Recall that a bounded linear operator on a Banach space is said to be Fredholm if there is an (also bounded linear) operator such that the operators and are compact. In that case the operator is called a regularizer for the operator .
Our second main result is the following sufficient condition for the Fredholmness of pseudodifferential operators on variable Lebesgue spaces.
Theorem 1.3.
Suppose and . If
(1.6) 
then the operator is Fredholm on the variable Lebesgue space .
As it was the case with Theorem \@setrefth:boundedness, for satisfying (1.1)–(1.3) this result was established by Rabinovich and Samko [23, Theorem 6.1]. Notice that for such condition (1.6) is also necessary for the Fredholmness (see [23, Theorems 6.2 and 6.5]). Whether or not the necessity holds in the setting of Theorem \@setrefth:sufficiency, remains an open question.
The paper is organized as follows. In Section \@setrefsec:sharp, the DieningRůžička generalization (see [10]) of the FeffermanStein sharp maximal theorem to the variable exponent setting is stated. Further, Diening’s results [8] on the duality and leftopenness of the class are formulated. In Section \@setrefsec:pointwise we discuss a pointwise estimate relating the FeffermanStein sharp maximal operator of and for and . Such an estimate for the range of parameters , , and as in Theorem \@setrefth:boundedness was recently obtained by Michalowski, Rule, and Staubach [16]. Combining this key pointwise estimate with the sharp maximal theorem and taking into account that is bounded on for some whenever , we give the proof of Theorem \@setrefth:boundedness in Section \@setrefsec:proof.
Section \@setrefsec:Fredholmness is devoted to the proof of the sufficient condition for the Fredholmness of a pseudodifferentail operator with slowly oscillating symbol. In Section \@setrefsec:interpolation, we state analogues of the RieszThorin and Krasnoselskii interpolation theorems for variable Lebesgue spaces. Section \@setrefsec:calculus contains the composition formula for pseudodifferential operators with slowly oscillating symbols and the compactness result for pseudodifferential operators with symbols in . Both results are essentially due to Grushin [11]. Section \@setrefsec:sufficiency contains the proof of Theorem \@setrefth:sufficiency. Its outline is as follows. From (1.6) it follows that there exist symbols and such that . Since , the operator is compact on all standard Lebesgue spaces. Its compactness on the variable Lebesgue space is proved by interpolation, since it is bounded on the variable Lebesgue space , where is the variable exponent from the definition of the class . Actually, the class is introduced exactly for the purpose to perform this step. Therefore is a right regularizer for on . In the same fashion it can be shown that is a left regularizer for . Thus is Fredholm.
We start with the following simple but important property of variable Lebesgue spaces. Usually it is called the lattice property or the ideal property.
Lemma 2.1.
(see e.g. [9, Theorem 2.3.17]) Let be a measurable a.e. finite function. If , is a measurable function, and for a.e. , then and .
Let . For a cube , put
The FeffermanStein sharp maximal function is defined by
where the supremum is taken over all cubes containing .
It is obvious that is pointwise dominated by . Hence, by Lemma \@setrefle:lattice,
whenever . The converse is also true. For constant this fact goes back to Fefferman and Stein (see e.g. [25, Chap. IV, Section 2.2]). The variable exponent analogue of the FeffermanStein theorem was proved by Diening and Růžička [10].
Theorem 2.2.
Let . Given , the th maximal operator is defined by
where the supremum is taken over all cubes containing . For this is the usual HardyLittlewood maximal operator. Diening [8] established the following deep duality and leftopenness result for the class .
Theorem 2.3.
One of the main steps in the proof of Theorem \@setrefth:boundedness is the following pointwise estimate.
Theorem 2.4.
(see [16, Theorem 3.3]) Let and with , , and . For every ,
where is a positive constant depending only on and the symbol .
This theorem generalizes the pointwise estimate by Miller [17, Theorem 2.8] for and by Álvarez and Hounie [1, Theorem 4.1] for with the parameters satisfying and .
Let . One of the main steps in the Rabinovich and Samko’s proof [23] of the boundedness on of the operator with is another pointwise estimate
for all , where is a positive constant independent of . It was proved in [23, Corollary 3.4] following the ideas of Álvarez and Pérez [2], where the same estimate is obtained for the CalderónZygmund singular integral operator in place of the pseudodifferential operator .
Suppose . Then, by Theorem \@setrefth:Diening, and there exists a number such that is bounded on . In other words, there exists a positive constant depending only on and such that for all ,
(2.1) 
From Theorem \@setrefth:sharp it follows that there exists a constant such that for all ,
(2.2) 
On the other hand, from Theorem \@setrefth:pointwise and Lemma \@setrefle:lattice we obtain that there exists a positive constant , depending only on and , such that
(2.3) 
Combining (2.1)–(2.3), we arrive at
for all . It remains to recall that is dense in (see Lemma \@setrefle:density). ∎
For a Banach space , let and denote the Banach algebra of all bounded linear operators and its ideal of all compact operators on , respectively.
Theorem 3.1.
Let , , be a.e. finite measurable functions, and let be defined for by
Suppose is a linear operator defined on .

If for , then for all and

If and , then for all .
Part (a) is proved in [9, Corollary 7.1.4] under the more general assumption that may take infinite values on sets of positive measure (and in the setting of arbitrary measure spaces). Part (b) was proved in [23, Proposition 2.2] under the additional assumptions that satisfy (1.1)–(1.3). It follows without these assumptions from a general interpolation theorem by Cobos, Kühn, and Schonbeck [4, Theorem 3.2] for the complex interpolation method for Banach lattices satisfying the Fatou property. Indeed, the complex interpolation space is isomorphic to the variable Lebesgue space (see [9, Theorem 7.1.2]), and have the Fatou property (see [9, p. 77]).
Let and be the class of all pseudodifferential operators with . By analogy with [11, Section 2] one can get the following composition formula (see also [21, Theorem 6.2.1] and [22, Chap. 4]).
Proposition 3.2.
If and , then their product belongs to and its symbol is given by
where .
Proposition 3.3.
Let . If , then .
Proof.
From Theorem \@setrefth:boundedness it follows that for all constant exponents . By [11, Theorem 3.2], . Hence, by the Krasnoselskii interpolation theorem (Theorem \@setrefth:interpolation(b) for constant with ), for all . ∎
The idea of the proof is borrowed from [11, Theorem 3.4] and [23, Theorem 6.1]. Let be such that if and if . For , put
From (1.6) it follows that there exists an such that
Then it is not difficult to check that
belongs to . It is also clear that .
From Proposition \@setrefpr:composition it follows that there exists a function such that
(3.1) 
On the other hand, since
we have
(3.2) 
(3.3) 
Since , there exist , , and such that
From Theorem \@setrefth:boundedness we conclude that all pseudodifferential operators considered above are bounded on , , and . Since , from Proposition \@setrefpr:compactness it follows that . Then, by Theorem \@setrefth:interpolation(b), . Therefore, from (3.3) it follows that is a right regularizer for . Analogously it can be shown that is also a left regularizer for . Thus is Fredholm on . ∎
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