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Sobolev’s inequality for Riesz potentials of functions in generalized Morrey spaces with variable exponent attaining the value 1 over non-doubling measure spaces
Journal of Inequalities and Applications volume 2013, Article number: 12 (2013)
Abstract
Our aim in this paper is to give Sobolev’s inequality for Riesz potentials of functions in generalized Morrey spaces with variable exponent attaining the value 1 over non-doubling measure spaces. The main result is oriented to the outrange of the well-known Adams theorem.
MSC: 31B15, 46E35, 26A33.
1 Introduction
The boundedness of fractional integral operators on Morrey spaces is known as the Adams theorem. Recently, many endpoint results have been obtained for this theorem, and in this paper we extend them to generalized Morrey spaces with variable exponent attaining the value 1 over non-doubling measure spaces.
In 1938 Morrey observed that a weaker regularity sufficed in order that the solutions in elliptic differential equations were smooth [1]. This observation grew up to be a useful tool for partial differential equations in general. Nowadays, his technique turned out to be a wide theory of function spaces called Morrey spaces; see also [2]. The (original) Morrey space with and is a normed space whose norm is given by for .
We are oriented to building up a theory of a metric measure space for which the notion of dimension is not equipped with μ, where d is a distance function and μ is a Borel measure. For example, we encounter the situation where more than one dimension comes into play.
1. In consider the set , where and . Denote by the s-dimensional Hausdorff measure in for . Consider . Then μ has two different dimensions.
2. The above example is a little artificial. Consider the Cantor dust, which is given by , where is given recursively by
Then the Hausdorff dimension of E is 1 while each is a union of closed cubes in .
As the second example shows, when we consider the subsets in , it is natural to approximate them with sets having different dimension. Faced with such an inconvenience, we work on a separable metric space X equipped with a nonnegative Radon measure μ, where the notion of dimensions of X and μ does not come into play.
We assume that
for all . By we denote the open ball centered at x of radius . We assume
for and for simplicity. We write for the distance of the points x and y in X. We are interested in the operator of the form , where f is a μ-measurable function with a certain μ-integrability. Note that (1.1) is indispensable because we envisage examples in which has singularity at the diagonal. Meanwhile, (1.2) does not lose any generality; it is just a matter of restricting X to .
Let G be a bounded open set in X. Our argument to follow heavily depends upon its diameter . Let , and . Define the Morrey norm by
for μ-measurable functions f, where runs over all elements in . The Morrey space is the set of all μ-measurable functions f for which the norm is finite. When , is denoted by .
In this paper, we aim to understand how the fractional integral operators behave in generalized Morrey spaces with variable exponent attaining the value 1 over non-doubling measure spaces. We obtain the following constant exponent case as a corollary of a more general theorem in non-doubling and variable exponent setting (see Theorem 2.1 below). The result is new even for a constant exponent case.
Theorem 1.1 Let α, β, ν be constants. Define an index by , and let . Define
for a positive μ-measurable function f. Then there exists a constant such that
for all and , whenever f is a nonnegative μ-measurable function on G satisfying .
We remark that extends naturally the fractional integral operator defined by Kokilashvili on [3]. We also remark that an extension to quasi-metric spaces was performed in [4].
Theorem 1.1 describes the missing part of the Adams inequality, which we recall now.
Proposition 1.2 (Adams, [5])
Let , and . Assume . Then there exists a constant such that for all .
In the endpoint case, when , two natural questions about Proposition 1.2 arise.
1. Can we prove a similar result by enlarging slightly?
2. Can we prove a similar result by shrinking slightly?
The second question is considered in [6]. We aim to consider the first question in the present paper.
It is the condition on μ that counts in the present paper; we do not postulate on μ the so-called doubling condition. Recall that a Radon measure μ is said to be doubling if there exists a constant such that for all (=X) and . Otherwise, μ is said to be non-doubling. In connection with the 5r-covering lemma, the doubling condition had been a key condition in harmonic analysis. However, Nazarov, Treil and Volberg showed that the doubling condition was not necessary by using the modified maximal operator [7, 8]. The idea of replacing with in (1.3) originates from these papers. In the present paper, we show that this idea works even for Riesz potentials.
Here, we summarize the structure of the space in Proposition 1.3 and Remark 1.4 below. Note that the Morrey space does depend upon the parameter k, which is illustrated by the following proposition.
Proposition 1.3 ([9])
There does exist a bounded separable metric space such that and do not coincide as sets.
About the modified Morrey norm, we have the following remarks.
Remark 1.4 Let be a μ-measurable function and G be a bounded open subset of X.
-
1.
From the definition of the norms, we learn for all and .
-
2.
If , and , then by the Hölder inequality.
-
3.
If μ is a doubling measure, then and are equivalent for all , and .
Our result can be readily translated into the Morrey space , where is the set of all functions f such that
Keeping our fundamental result, Theorem 1.1, in mind, let us describe our results in full generality. We seek to establish Sobolev’s inequality for Riesz potentials of functions in generalized Morrey spaces with variable exponent attaining the value 1 over non-doubling measure spaces, which will extend the results in our earlier papers [10–17]. In the present paper, we show that a modification enables us to obtain boundedness results in generalized Morrey spaces with variable exponent attaining the value 1 over non-doubling measure spaces even when
We consider variable exponents and on X such that
(P1) ;
(P2) whenever and ;
(Q1) ;
(Q2) whenever and .
In general, if satisfies (P2) (resp. satisfies (Q2)), then (resp. ) is said to satisfy the log-Hölder (resp. loglog-Hölder) condition. Observe that these conditions are much weaker than Lipschitz continuity.
Next, we redefine the Riesz potential. Recall that we are working on a bounded subset G of X. For a bounded μ-measurable function and a constant , we define the Riesz potential of (variable) order for a nonnegative μ-measurable function f on G by
Here and in what follows, we tacitly assume that outside G. Observe that this naturally extends the Riesz potential operator
when is the N-dimensional Euclidean space and .
We also assume
for appearing in the definition of the operator .
Now, we are going to formulate our results in full generality. First of all, we set
here we assume
(Φ) is convex on for every .
Note that (Φ) holds for some if and only if there is a positive constant K such that
(see [[18], Theorem 5.1]). Observe from (Φ) that the function is nondecreasing on for fixed .
Now, we redefine and extend the definition of Morrey norms adapted to the function Φ. Let be a fixed parameter and let G be a bounded subset of X. Let us denote by the diameter of G. For bounded μ-measurable functions and , we introduce the family of all μ-measurable functions f on G such that for some ,
Denote by the smallest value of λ satisfying (1.6). Note that ν and β need not be continuous.
The space is a further generalization of generalized Morrey spaces with non-doubling measures on , which are taken up in [19, 20]. Nowadays, generalized Morrey spaces are not generalization for its own sake. Note that generalized Morrey spaces occur naturally when we consider the limiting case as Proposition 1.5 below shows.
Proposition 1.5 ([[21], Theorem 5.1])
Let and . Then there exists a positive constant such that
holds for all with and for all balls B.
In view of the integral kernel of (see [22]) and the Adams theorem, we have
is bounded as long as
However, if , the number q not being finite, the boundedness assertion (1.7) is no longer true. Hence, Proposition 1.5 can be considered as a substitute of (1.7). Proposition 1.5 shows that the price to pay is the factor . We refer to [21] for a counterexample showing that (1.7) is no longer true for .
Remark that if , the parameter k is not essential as long as as Proposition 1.6 below shows.
Proposition 1.6 ([9])
Let and be the Euclidean space. Suppose that G is a bounded open set. Assume in addition that ν and β satisfy the log-Hölder continuity (P2) and the loglog-Hölder continuity (Q2), respectively. Then the spaces and coincide as sets and their norms are equivalent.
What is different from the classical setting endowed with Lebesgue measure is that we need to take an attentive care of the parameters k appearing in (1.6). For example, unlike the doubling measure spaces, we need delicate geometric observations (see (2.19) for example).
Among many other function spaces, Morrey spaces with variable exponents are worth investigating because our result Theorem 2.1 shows that the smoothing effect that the fractional integral operators enjoy is local. It is believed that p in the definition of measures the local integrability, while ν seems to control the global integrability.
Here we make a historical remark about the research of this field. The case of Theorem 2.1 is covered in [[12], Theorem 1.2] and [[13], Corollary 4.2]. Roughly speaking, we seek to discuss the boundedness of the operator from the Morrey space to another Morrey space with suitable , which extends the results in [10–17]. When , the maximal functions play a crucial tool by Hedberg’s trick (see [23]). In view of [7, 8, 24, 25], the modified maximal operator is bounded and this was useful for the case when . In case , our strategy is to give an estimate of by the use of another Riesz-type potentials of order 0, which plays a role of the maximal functions. For related results, see [26–28].
Finally, we explain some notations used in the present paper. The function denotes the characteristic function of E. Throughout the present paper, let C denote various constants independent of the variables in question. In analogy with and , we define , and .
2 Sobolev’s inequality in the case
Recall that , and are bounded μ-measurable functions and . Throughout this section, we additionally assume that
Note that for . Thus, from and (1.4), we have
Our first aim is to give the following Morrey version of Sobolev-type inequality for Riesz potentials
We consider the Sobolev exponent
and the new modular function
Theorem 2.1 Assume that satisfies (P1) with , (P2) and (2.1). Then, for each , there exists a constant such that
for all and , whenever f is a nonnegative μ-measurable function on G satisfying .
Remark 2.2 In Theorem 2.1, it is known that we cannot take (see [[14], Remark 3.3] and O’Neil [[28], Theorem 5.2]).
Here we outline the proof of Theorem 2.1.
For and , and setting
we consider the logarithmic potential (of order 0)
Decompose
Following the Hedberg trick [23], we plan to control by not by maximal functions (Lemma 2.3). Next, we estimate by the use of Young’s inequality (Lemma 2.4). Finally, an optimization (see (2.22) below), together with the boundedness property of (Lemma 2.5), yields the desired estimate in terms of .
Lemma 2.3 For , and a nonnegative μ-measurable function f on G, set
Let be fixed. Then there exists a constant such that
whenever f is a nonnegative μ-measurable function on G satisfying .
Proof Recall that μ does not charge a point x (see (1.1) above) and that by virtue of (2.2). We thus have
Consequently,
From (Φ), observe that the function is nondecreasing on for fixed . With this in mind, for , which we specify in (2.10), we decompose and estimate the integral defining according to the level set :
The first term is estimated by (2.9):
Recall also that is defined by (2.6). Inserting them, we have
where . We set
For , from (P2) and the boundedness of and ν, we deduce
so that
Similarly, by (Q2) we have
Consequently, it follows from (2.11) and (2.12) that
Recall now that is given by (2.7). Observe also that
since , and p, q, β are all bounded. Thus,
Now, the result follows. □
In our earlier work, we obtained the following lemma, whose proof is simple; we do not recall the proof.
The quantity will be taken care of by using the next lemma.
Lemma 2.4 ([[9], Lemma 5.4])
Let f be a nonnegative μ-measurable function on G such that
Then
for and .
What remains to show for the proof of Theorem 2.1 is to give a boundedness property on Morrey spaces for .
Lemma 2.5 There exists a constant such that
, and nonnegative μ-measurable functions f satisfying .
Proof For and , write
where . Then we have
by the Fubini theorem and a geometric observation. Since we are assuming (1.1), we can use a routine dyadic decomposition to obtain
We now insert the definition of (see (2.6) above) and we have
A geometric observation shows that provided . Thus, we have
Now, we invoke the assumption that and that . From these conditions, (2.13) and the definition of g, we deduce
so that
Next, we claim that and imply that
and that
Here we check only (2.19). When , we have
We use (1.1), (2.18) and (2.19) for to obtain
Consequently, from (2.2), (2.13) and the assumption that β is a bounded function, we obtain
Recall that and that β is bounded. Therefore, it follows that
for . Hence, we see that
From (2.16), (2.17) and (2.20), we conclude (2.15). □
Now we are ready to prove Theorem 2.1.
Proof of Theorem 2.1 We may assume that . For , write
according to (2.8). In view of Lemma 2.3, we find
with . Moreover, Lemma 2.4 yields
so that
Now, letting
we obtain
By Lemma 2.5, we obtain
for and , which completes the proof of Theorem 2.1. □
Remark 2.6 [[14], Example 2.11] shows that Theorem 2.1 is best possible to exponents appearing in the Morrey condition.
3 Sobolev’s inequality in the case and
Let and . In this section, we assume that there exists a constant such that
whenever and . A direct consequence of (3.1) is that
for . Let and Ψ be as in (2.4) and (2.5), respectively. Under this assumption, Theorem 2.1 is shown to be valid even for .
Theorem 3.1 Let and . Define Ψ by (2.5). Assume that , , and satisfy (2.1) and (3.1). Then there exists a constant such that
for all and , whenever f is a nonnegative μ-measurable function on G satisfying .
For and , we let
as before.
Note that this definition extends naturally (2.6). For a nonnegative μ-measurable function f on G, we define the (non-linear) logarithmic potential
To prove Theorem 3.1, we modify Lemmas 2.3 and 2.5 in the following manner, respectively.
Lemma 3.2 Let and define
for and a nonnegative μ-measurable function f on G. Then there exists a constant such that
Proof We abbreviate to E. For a constant , which will be specified in (3.6), we let
On , using (1.1) and the dyadic decomposition, we estimate f very crudely to obtain
If we pass to a continuous variable and keep in mind that , then we obtain
In summary, we obtained
On , we use (3.2) and .
for all , where . Hence, from (3.5), we deduce
We set
Then we have for ,
and
as we did in (2.11) and (2.12). Consequently, it follows that
Now the result follows. □
The next lemma is a counterpart for Lemma 2.5.
Lemma 3.3 Let f be a nonnegative μ-measurable function satisfying and define by (3.4). If Φ is given by (1.5), then there exists a constant independent of , and f such that
Proof Let f be a nonnegative μ-measurable function on G satisfying . Write
where . Let us estimate
For each , we decompose
Then, using (3.3), (3.9) and a crude estimate of the integrand, we obtain
From this and the fact that , we deduce
so that
Note that and imply that
and that
For , we insert (3.3) directly to obtain a pointwise estimate for :
As before, we decompose dyadically to estimate from above. The result is
From this and the fact that , we deduce
for . Hence, we see that
Thus, putting (3.8), (3.10) and (3.11) together, we obtain (3.7). □
Proof of Theorem 3.1 We may assume that by considering if necessary. Unlike Theorem 2.1, we have freedom to choose ε, so that we define . Let us obtain a pointwise estimate of for .
For , where δ is specified in (3.13) below, we decompose
As in the proof of (2.9), we have
since .
Note that . In view of Lemma 3.2, we find
with . Moreover, Lemma 2.4 yields
so that
Now, letting
we obtain
In view of Lemma 3.3, we find
which completes the proof of Theorem 3.1. □
Remark 3.4 [[14], Example 3.6] shows that Theorem 3.1 is best possible to exponents appearing in the Morrey condition.
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The authors are thankful to the anonymous referees for their valuable comments about the history of fractional integral operators on non-doubling measure spaces.
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Sawano, Y., Shimomura, T. Sobolev’s inequality for Riesz potentials of functions in generalized Morrey spaces with variable exponent attaining the value 1 over non-doubling measure spaces. J Inequal Appl 2013, 12 (2013). https://doi.org/10.1186/1029-242X-2013-12
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DOI: https://doi.org/10.1186/1029-242X-2013-12