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Abstract and Applied Analysis

Volume 2012 (2012), Article ID 682436, 7 pages

http://dx.doi.org/10.1155/2012/682436

## A Regularity Criterion for the Navier-Stokes Equations in the Multiplier Spaces

^{1}College of Physics and Electronic Information Engineering, Wenzhou University, Zhejiang, Wenzhou 325035, China^{2}The Key Laboratory of Low-voltage Apparatus Intellectual Technology of Zhejiang, Wenzhou 325035, China

Received 16 February 2012; Accepted 23 April 2012

Academic Editor: Benchawan Wiwatanapataphee

Copyright © 2012 Xiang'ou Zhu. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

We exhibit a regularity condition concerning the pressure gradient for the Navier-Stokes equations in a special class. It is shown that if the pressure gradient belongs to , where is the multipliers between Sobolev spaces whose definition is given later for , then the Leray-Hopf weak solution to the Navier-Stokes equations is actually regular.

#### 1. Introduction

Consider the Navier-Stokes equations in : where is the velocity field, is the scalar pressure, and with div in the sense of distribution is the initial velocity field. For simplicity, we assume that the external force has a scalar potential and is included into the pressure gradient.

In the famous paper, Leray [1] and Hopf [2] constructed a weak solution of (1.1) for arbitrary with . The solution is called the Leray-Hopf weak solution. Regularity of such Leray-Hopf weak solutions is one of the most significant open problems in mathematical fluid mechanics. We note here that there are partial regularity results from Scheffer and from Caffarelli et al., see [3, 4] and references therein. Besides, more work was pioneered by Serrin [5] and extended and improved by Giga [6], Struwe [7, 8], and Zhou [9]. Further results can be found in [10–16] and references therein.

Introducing the class , Serrin [5] showed that if we have a Leray-Hopf weak solution belonging to with the exponents and satisfying , , , then the solution , while the limit case was shown much later by Sohr [17] (see also [18]).

Regularity results including assumptions on the pressure gradient have been given by Zhou [15], and it was extended later by Struwe [8] to any dimension . It is shown that if the gradient of pressure with , then the corresponding weak solution is actually strong. For the recent work on the regularity problem containing the pressure, velocity field, and the quotient of pressure-velocity, we refer to [19–21] for details.

The purpose of this short paper is to establish a regularity criterion in terms of the pressure gradient for weak solutions to the Navier-Stokes equations in the class . This work is motivated by the recent results [22, 23] on the Navier-Stokes equations. It is an unusual, larger space considered in the current paper than (the following Lemma 2.3) and possesses more information. Obviously, the present result extends some previous ones. For more facts concerning regularity of weak solutions, we refer the readers to the celebrated papers [24–30].

#### 2. Preliminaries

We recall the definition of the multiplier space, which was introduced in [31] (see also [32, 33]). The space of pointwise multipliers, which map into , is defined in the following way.

*Definition 2.1. *For , is a Banach space of all distributions on such that there exists a constant such that for all we have and
where we denote by the completion of the space with respect to the norm and denote by the Schwarz class.

The norm of is given by the operator norm of pointwise multiplication

*Remark 2.2. *Equivalently, we will say that if and only if the inequality
holds for all .

Lemma 2.3. *Let . Then the following embedding:
**
holds.*

* Proof. *Indeed, let . By using the following well-known Sobolev embedding:
with , we have by Hölder’s inequality
where . Then, it follows that
This completes the proof.

*Example 2.4. *Due to the well-known inequality
we see that .

Indeed, since the functions of class are dense in in the norm , suppose . Then by virtue of the Cauchy-Schwarz inequality we obtain and thus for , and

#### 3. Regularity Theorem

Now we state our result as following.

Theorem 3.1. *Let for some and in the sense of distributions. Suppose that is a Leray-Hopf solution of (1.1) in . If the pressure gradient satisfies
**
then is a regular solution in the sense that
*

* Proof. *In order to prove this result, we have to do a priori estimates for the Navier-Stokes equations and then show that the solution satisfies the well-known Serrin regularity condition. Multiply both sides of the first equation of (1.1) by and integrate by parts to obtain (see, e.g., [30])
for . Then we have
where we have used

Let us estimate the integral
on the right-hand side of (3.4). By the Hölder inequality and the Young inequality, we have
where ; we have used the inequality
and the Young inequality with :
for . Hence by (3.4) and the above inequality, we derive
Now by Gronwall’s lemma (see for instance in [28, Lemma 2]), we have
Due to the integrability of the pressure gradient, it follows that
Consequently falls into the well-known Serrin’s regularity framework. Therefore, the smoothness of follows immediately. This completes the proof of Theorem 3.1.

*Remark 3.2. *By a strong solution we mean a weak solution of the Navier-Stokes equation such that
It is wellknown that strong solutions are regular (we say classical) and unique in the class of weak solutions.

#### Acknowledgment

The author thanks the anonymous referee for his/her comments on this paper.

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