Research Article Brézis-Wainger Inequality on Riemannian Manifolds

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1 Hindawi Publishing Corporation Journal of Inequalities and Applications Volume 2008, Article ID 7596, 6 pages doi:0.55/2008/7596 Research Article Brézis-Wainger Inequality on Riemannian anifolds Przemysław Górka Department of athematics and Information Sciences, Warsaw University of Technology, Pl. Politechniki, 0066 Warsaw, Poland Correspondence should be addressed to Przemysław Górka, pgorka@mini.pw.edu.pl Received 0 November 2007; Accepted 29 April 2008 Recommended by Shusen Ding The Brézis-Wainger inequality on a compact Riemannian manifold without boundary is shown. For this purpose, the oser-trudinger inequality and the Sobolev embedding theorem are applied. Copyright q 2008Przemysław Górka. 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.. Introduction There is no doubt that the Brézis-Wainger inequality see is a very useful tool in the examination of partial differential equations. Namely, a lot of estimates to a solution of PDE are obtained with the help of the Brézis-Wainger inequality. Especially, the inequality is often applied in the theory of wave maps. In this paper, we extend the Brézis-Wainger result onto a compact Riemannian manifold. We show the following theorem. Theorem.. Let,g be an n-dimensional compact Riemannian manifold, and u H k,p, udv g 0, forn > k > n/p,wherek is a positive integer and p is a real number. Then, u L and u L u H k,n/k C log u ) H k,p,. u H k,n/k where C C k, is a positive constant. The proof relies on the application of a oser-trudinger inequality see Theorem 2.2 and the Sobolev embedding theorem see Theorem 2.. oreover, we will use the integral representation of a smooth function via the Green function see 2.

2 2 Journal of Inequalities and Applications 2. Preliminaries In order to make this paper more readable, we recall some definitions and facts from the theory of Sobolev spaces on Riemannian manifolds. In particular, we present useful inequalities and embeddings. Let,g be a smooth, compact Riemannian manifold without boundary. We will denote by C a space of smooth real functions. For φ C and integer m, wedenote by m φ the mth covariant derivative of φ. Next, for φ C and for a fixed integer m and arealp, we set φ H m,p m i 0 i φ p dv g ) /p, where by V g we have denoted the Riemannian measure on the manifold,g. We define the Sobolev space H m,p as a completion of C with respect to H m,p. We close this section stating the following results, which will be used in the proof of the main result. Theorem 2. Sobolev embedding theorem 3, 4. Let,g be a smooth, compact Riemannian n- manifold. Then, for any real numbers p<qand any integers 0 m<k,if /q /p k m /n, then H k,p H m,q. oreover, there exists a constant C such that for all u H k,p, the following inequality holds: 2. u H m,q C u H k,p. 2.2 Theorem 2.2 oser-trudinger inequality 5. Let,g be a smooth, compact Riemannian n- manifold and k a positive integer, strictly smaller than n. There exist a constant C C k, and λ k, n such that for all u C n with udv g 0 and k u n/k dv g, the following inequality holds: e λ k,n u/ u H k,n/k n/ n k dv g C. 2.3 Let us stress that this inequality is a generalization of the oser and Trudinger result see Proof of the main result In this section, we will prove the main result, that is, Theorem.. Proof. Let us notice that by the assumptions we have an embedding H k,p H k,n/k. First of all, we show the following lemma. Lemma 3.. If u C, udv g 0, k u n/k dv g, andd,, thene u L d. oreover, the following estimate holds: e u L d C u H k,n/k,d, ). 3.

3 Przemysław Górka 3 Proof. Let us put ) n n k /n u a λ k, n, n k u H k,n/k ) n n/ n k b d λ k, n u n k H k,n/k 3.2 in Young s inequality ab ap p bq q, 3.3 where /p /q. Then, we obtain an estimate ud λ k, n u u H k,n/k ) n/ n k k n dn/k λ k, n n n k ) n k /n u n/k H k,n/k, where λ k, n is a constant from the oser-trudinger inequality see Theorem 2.2. Next, we can estimate e ud dv g C ) d, u H k,n/k e λ k,n u/ u H k,n/k n/ n k dv g Subsequently, we can apply the oser-trudinger inequality to the right-hand side of the above inequality: e λ k,n u/ u H k,n/k n/ n k dv g c. 3.6 From this, the proof of Lemma 3. follows. Now, we can go back to the proof of Theorem.. First, we prove the theorem for u C such that u H k,n/k and udv g 0. Replacing u by u if necessary, we may suppose that u L u x 0 ), 3.7 for x 0. Let us recall see 2 that for,g, a compact Riemannian n-manifold, there exists a Green function G : R such that for any φ C and any z, φ z φdv g G z, x Δ g φ x dv g, Vol 3.8 where Vol is a Riemannian volume of the manifold,g,andδ g is the Laplace- Beltrami operator on a manifold;

4 4 Journal of Inequalities and Applications 2 there exists a constant C such that for any x, y \Δ, x G x, y C dg x, y ) n, 3.9 where Δ is a diagonal: Δ { x, y : x y }, 3.0 and d g x, y is a Riemannian distance from x to y. Let us define the map ψ e u. Next, we apply the first property of the Green function to the map ψ and to the point x 0. Namely, 0 ψ ) x 0 Vol Vol e u ) dv g G x 0,x ) Δ g ψ x dv g e u L x G x 0,x ) x ψ x dvg. 3. Subsequently, by the second property of the Green function we can estimate ψ x 0 as follows: ψ ) x 0 e u Vol L C dg x0,x )) n u eu dv g. 3.2 Now, we will try to estimate the last term in the inequality 3.2. Let us notice that if u H k,p, then u H k,p. Next, by the Sobolev theorem see Theorem 2., H k,p L q, for q p k n, 3.3 and we have that u L q. Using elementary calculations, one can easily show the lemma. Lemma 3.2. There exist r<n/ n and a finite d such that the following equality holds: r q, d 3.4 where q is the exponent from the Sobolev theorem. By Hölder s inequality with exponents r, q, d, we can estimate the inequality 3.2 as follows: ψ ) x 0 e u Vol L C u L q e u L d, 3.5

5 Przemysław Górka 5 where C sup x0 C/ d g x 0, n L r and r is the exponent from Lemma 3.2. Finally, by Lemma 3. and the Sobolev theorem, we obtain ψ x 0 ) C u H k,p ). 3.6 Let us stress that since u H k,n/k, the constant C does not depend on u. Wecanrewrite the inequality 3.6 as follows: Hence, e u x 0 C u H k,p ). 3.7 u L u x 0 ) log C u H k,p )). 3.8 Taking into account u H k,n/k c u H k,p,weobtain u L C log u H k,p. 3.9 This finishes the proof of the inequality in the case u C such that u H k,n/k and udv g 0. Subsequently, one can easily obtain the inequality for u C such that udv g 0. Now, we prove the theorem for an arbitrary u H k,p such that udv g 0. We apply the density argument. Namely, for any ε>0, there exists ũ ε C such that ũ ε u H k,p ε Next, we define u ε ũ ε ũ ε dv g. Vol Such u ε has zero-mean value. oreover, the following inequality holds: uε u H k,p 2ε Indeed, u ε u H k,p ũ ε u H k,p u ) H dv g Vol ũε k,p ε ũ ε u dv g Vol H k,p ε ũ ε u ) /p p dv g 2ε Hence, u L u u L ε u H ) ε u H ε C log k,p k,n/k u H ε k,n/k Finally, we take ε 0. This completes the proof.

6 6 Journal of Inequalities and Applications Acknowledgments The author wishes to thank Professor Yuxiang Li for pointing out the paper 5. oreover, the author thanks the referees for comments and invaluable suggestions. References H. Brézis and S. Wainger, A note on limiting cases of Sobolev embeddings and convolution inequalities, Communications in Partial Differential Equations, vol. 5, no. 7, pp , T. Aubin, Nonlinear Analysis on anifolds. onge-ampère Equations, vol. 252 of Grundlehren der athematischen Wissenschaften, Springer, New York, NY, USA, E. Hebey, Sobolev Spaces on Riemannian anifolds, vol. 635 of Lecture Notes in athematics, Springer, Berlin, Germany, E. Hebey, Nonlinear Analysis on anifolds: Sobolev Spaces and Inequalities, vol.5ofcourant Lecture Notes in athematics, American athematical Society, Providence, RI, USA, L. Fontana, Sharp borderline Sobolev inequalities on compact Riemannian manifolds, Commentarii athematici Helvetici, vol. 68, no., pp , D. R. Adams, A sharp inequality of J. oser for higher order derivatives, The Annals of athematics, vol. 28, no. 2, pp , J. oser, A sharp form of an inequality by N. Trudinger, Indiana University athematics Journal, vol. 20, pp , N. S. Trudinger, On imbeddings into Orlicz spaces and some applications, Journal of athematics and echanics, vol. 7, pp , 967.