# A PRIORI ESTIMATES FOR SEMISTABLE SOLUTIONS OF SEMILINEAR ELLIPTIC EQUATIONS. In memory of Rou-Huai Wang

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1 A PRIORI ESTIMATES FOR SEMISTABLE SOLUTIONS OF SEMILINEAR ELLIPTIC EQUATIONS XAVIER CABRÉ, MANEL SANCHÓN, AND JOEL SPRUCK In memory of Rou-Huai Wang 1. Introduction In this note we consider semistable solutions of the boundary value problem { Lu + f(u) = 0 in, (1.1) u = 0 on, where R n is a smooth bounded domain with n 2, f C 2, and Lu := i (a ij (x)u j ) is uniformly elliptic. More precisely, we assume that (a ij (x)) is a symmetric n n matrix with bounded measurable coefficients, i.e., a ij = a ji L (), for which there exist positive constants c 0 and C 0 satisfying (1.2) c 0 ξ 2 a ij (x)ξ i ξ j C 0 ξ 2 for all ξ R n, x. By semistability of the solution u, we mean that the lowest Dirichlet eigenvalue of the linearized operator at u is nonnegative. That is, we have the semistability inequality (1.3) f (u)η 2 dx a ij (x)η i η j dx for all η H0(). 1 There is a large literature on a priori estimates, beginning with the seminal paper of Crandall and Rabinowitz [4]. In [4] and subsequent works, a basic and standard assumption is that u is positive in and f C 2 is positive, nondecreasing, and superlinear at infinity: (1.4) f(0) > 0, f f(t) 0 and lim =. t + t Note that, under these assumptions and with f(u) replaced by λf(u) with λ 0, semistable solutions do exist for an interval of parameters λ (0, λ ); see [4]. Research of the first and second authors supported by MTM C04-01 (Spain) and 2009SGR345 (Catalunya). The second author is also supported by ERC grant (ANGEOM project). Research of the third author supported in part by the NSF and Simons Foundation. 1

2 2 XAVIER CABRÉ, MANEL SANCHÓN, AND JOEL SPRUCK In recent years there have been strong efforts to obtain a priori bounds under minimal assumptions on f (essentially (1.4)), mainly after Brezis and Vázquez [1] raised several open questions. The following are the main results in this direction. The important paper of Nedev [5] obtains the L bound for n = 2 and 3 if f satisfies (1.4) and in addition f is convex. Nedev states his result for L = but it is equally valid for general L. When 2 n 4 and L =, Cabré [2] established that the L bound holds for arbitrary f if in addition convex. Villegas [9] replaced the condition that is convex in Cabré s result assuming instead that f is convex. For the radial case, Cabré and Capella [3] proved the L bound when n 9. On the other hand, it is well known that there exist unbounded semistable solutions when n 10 (for instance, for the exponential nonlinearity e u ). For convex nonlinearities f and under extra assumptions involving the two numbers (1.5) τ := lim inf t f(t)f (t) f (t) 2 τ + := lim sup t f(t)f (t) f (t) 2 much more is known (see more detailed comments after Corollary 1.3). For instance, Sanchón [6] proved that u L () whenever τ = τ + 0 and n 9. This hypothesis is satisfied by f(u) = e u, as well as by f(u) = (1 + u) m, m > 1. It is still an open problem to establish an L estimate in general domains when n 9 under (1.4) as the only assumption on f. Our purpose here is to prove the following results: Theorem 1.1. Let f C 2 be convex and satisfy (1.4). Assume in addition that for every ε > 0, there exist T = T (ε) and C = C(ε) such that (1.6) f (t) Cf(t) 1+ε for all t > T. Then if u is a positive semistable solution of (1.1), we have f (u) L p () for all p < 3 and n 2, while f(u) L p () for all p < n and n 6. n 4 As a consequence, we deduce respectively: (a) If n 5, then u L (). (b) If n 6, then u W 1,p 0 () for all p < n and u n 5 Lp () for all p < particular, if n 9 then u H0(). 1 n. In n 6 Theorem 1.1 establishes the L bound up to dimension 5 when (1.6) holds. If we assume more about f we can obtain an L bound up to dimension n = 6.

3 A PRIORI ESTIMATES FOR SEMISTABLE SOLUTIONS 3 Theorem 1.2. Let f C 2 be convex and satisfy (1.4). Assume in addition that there exist ε (0, 1) and T = T (ε) such that (1.7) f (t) Cf(t) 1 ε for all t > T. Then if u is a positive semistable solution of (1.1), we have f (u) L 3 ε 1 ε () for all n 2, while f(u) L p () for all p < (1 ε)n 4ε and n 6 +. (1 ε)n 4+2ε 1 ε As a consequence, we deduce respectively: (a) If n < 6 + 4ε, then u 1 ε L (). (b) If n 6 + 4ε 1,p (1 ε)n, then u W 1 ε 0 () for all p < and u (1 ε)n 5+3ε Lp () for all p < (1 ε)n 4ε. In particular, if n < 10 + then u (1 ε)n 6+4ε 1 ε H1 0(). The main novelty of our results are twofold. On the one hand, we do not assume any lower bound on f to obtain our estimates, nor any bound on f as in [4] or [6] (as commented below). On the other hand, we obtain L p estimates for f (u). To our knowledge such estimates do not exist in the literature. In fact, using the L p estimate for f(u) established in Theorem 1.2 and standard regularity results for uniformly elliptic equations, it follows that u is bounded in L () whenever n < 6 + 2ε. Note 1 ε that the range of dimensions obtained in Theorem 1.2 (a), n < 6 + 4ε, is bigger than 1 ε this one. This will follow from the L p estimate on f (u). Of course, in both results (and also in the rest of the paper), u L p or u W 1,p mean that u is bounded in L p or in W 1,p by a constant independent of u. Our assumptions (1.6) and (1.7) in Theorems 1.1 and 1.2 are related to the hypothesis τ + 1 (recall (1.5)). Indeed, by the definition of τ +, for every δ > 0 there exists T = T (δ) such that f(t)f (t) (τ + + δ)f (t) 2 for all t > T, or equivalently, d f (t) 0 for all t > T. Thus, dt f(t) τ + +δ f (t) f(t) f (T ) τ ++δ f(t ) τ ++δ = C for all t > T. From this, it is clear that if τ + 1, then assumption (1.6) holds choosing δ = ε. Note that 0 τ 1 always holds since f is a continuous function defined in [0, + ). If instead τ + < 1, then (1.7) is satisfied with ε = 1 τ + δ, where δ > 0 is arbitrarily small. Therefore as an immediate consequence of part (i) of Theorems 1.1 and 1.2 we obtain the following. Corollary 1.3. Let f C 2 be convex and satisfy (1.4). Let u be a positive semistable solution of (1.1). The following assertions hold:

4 4 XAVIER CABRÉ, MANEL SANCHÓN, AND JOEL SPRUCK (a) If τ + = 1 and n < 6 then u L (). (b) If τ + < 1 and n < τ + then u L (). If τ + < 1, then for every ε (0, 1 τ + ), there exists a positive constant C such 1 1 τ that f(t) C(1 + t) + ε for all t 0 (this can be easily seen integrating twice in the definition of τ + ). Thus under this hypothesis, f has at most polynomial growth. All the results in the literature considering τ and τ + (defined in (1.5)) assume τ > 0. Instead in Corollary 1.3, no assumption is made on τ. Crandall and Rabinowitz [4] proved an a priori L bound for semistable solutions when 0 < τ τ + < 2+τ +2 τ and n < 4+2τ +4 τ. Note that for nonlinearities f such that τ = 1 and τ + < 5 one obtains the L bound if n 9 (a dimension which is optimal). This is the case for many exponential type nonlinearities, as for instance f(u) = e uα for any α R +. The results in [4] were improved in [6] establishing that u L () whenever τ > 0 and n < τ (remember that τ 1). Moreover, if 0 < τ τ + < 1, then using an iteration argument in [4], one has that u L () ( whenever n < ) τ τ. Note that Corollary 1.3 coincides with these results in the case where τ = 0. Let us make some further comments on conditions (1.6) and (1.7) in Theorems 1.1 and 1.2, respectively. Remark 1.4. (i) Condition (1.6) is equivalent to since (1.6) holds if and only if logf (t) logf(t) lim sup t + (1 + ε) + C logf (t) logf(t) 1, logf(t) for all t > T ; note that f(t) + as t + by (1.4). Many nonlinearities f satisfy this condition (like exponential or power type nonlinearities). (ii) Setting s = f(t) and t = γ(s), (1.6) is equivalent to the condition γ (s) θs 1 ε for some θ > 0 and for all s sufficiently large. This clearly shows that (1.6) does not follow from the convexity of f alone (which is equivalent to γ being nonincreasing). Instead, condition (1.7) is equivalent to γ (s) θs 1+ε for some θ > 0 and for all s sufficiently large. In particular, γ(s) θs ε for s large enough, or equivalently, f(t) C(1 + t) 1 ε for some constant C > 0 and for all t.

5 A PRIORI ESTIMATES FOR SEMISTABLE SOLUTIONS 5 On the other hand, f(t)f (t)/f (t) 2 = sγ (s)/γ (s), a second derivative condition on γ, in contrast with the first derivative conditions of (1.6) and (1.7). Therefore, for most nonlinearities satisfying (1.6) (or (1.7)), the limit ff /(f ) 2 at infinity does not exist (i.e., τ < τ + ) and in addition, it may happen that τ = 0. (iii) Note that by convexity, εf (t) f(t + ε) f(t) f(t + ε) for all t. Therefore, (1.6) holds if f(t + ε) Cf(t) 1+ε for all t sufficiently large. 2. Preliminary estimates We start by recalling the following standard regularity result for uniformly elliptic equations. Proposition 2.1. Let a ij = a ji, 1 i, j n, be measurable functions on a bounded domain. Assume that there exist positive constants c 0 and C 0 such that (1.2) holds. Let u H 1 0() be a weak solution of { Lu + c(x)u = g(x) in, u = 0 on, with c, g L p () for some p 1. Then, there exists a positive constant C independent of u such that the following assertions hold: (i) If p > n/2 then u L () C( u L 1 () + g L p ()). (ii) Assume c 0. If 1 p < n/2 then u L r () C g L p () for every 1 r < np/(n 2p). Moreover, u W 1,r 0 () C for every 1 r < np/(n p). Part (i) of Proposition 2.1 is established in Theorem 3 of [7] with the L 2 -norm of u instead of the L 1 -norm. However, an immediate interpolation argument shows that the result also holds with u L 1 (). Note also that in the right hand side of this estimate, u L () C( u L 1 () + g L p ()), some dependence of u must appear (think on the equation with g 0 satisfied by the eigenfunctions of the Laplacian). For part (ii) we refer to Theorems 4.1 and 4.3 of [8]. As an easy consequence of Proposition 2.1 (i) we obtain the following: Corollary 2.2. Let u H 1 0() be a nonnegative weak solution of (1.1) with f nondecreasing and convex. Assume p > n/2. If there exists a positive constant C independent of u such that u L 1 () C and f (u) L p () C, then u L () C for some positive constant C independent of u.

6 6 XAVIER CABRÉ, MANEL SANCHÓN, AND JOEL SPRUCK Proof. Rewrite equation (1.1) as Lu + c(x)u = f(0) where c(x) = (f(u) f(0))/u. Then by convexity, 0 c(x) f (u) and the result follows by Proposition 2.1 (i). The following estimates involving f(u) := f(u) f(0) are due to Nedev [5] when L =. We give here a new proof of the estimates consistent with our own approach. Note that assumptions (1.6) and (1.7) in Theorems 1.1 and 1.2, respectively, also hold replacing f by f on their right hand side, since f(t) 2(f(t) f(0)) = 2 f(t) for t large enough. We will use this fact in the proof of both results. Lemma 2.3. Let f C 2 be convex and satisfy (1.4). If u is a positive semistable solution of (1.1), then there exists a positive constant C independent of u such that (2.1) f(u)f (u) dx C and f(u)f (u)a ij (x)u i u j dx C. Proof. Let u be a semistable solution of (1.1) and f(u) = f(u) f(0). Note that f(u) satisfies L( f(u)) + f (u) f(u) = f(0)f (u) + f (u)a ij (x)u i u j. Multiplying the previous identity by f(u) and using the semistability condition (1.3), we obtain 0 (a ij ( f(u)) i ( f(u)) j f (u) f(u) 2 ) dx = f(0) f (u) f(u) dx f(u)f (u)a ij u i u j dx, or equivalently, (2.2) f(u)f (u)a ij (x)u i u j dx f(0) f(u)f (u) dx. As a consequence, the second estimate in (2.1) follows by the first one. Multiplying the equation (1.1) by the test functions ζ = f (u) f (0) and we find (2.3) { ζ = 0 if u M f (u) f (M) if u > M, f (u)a ij (x)u i u j dx = f(u)(f (u) f (0)) dx

7 and (2.4) A PRIORI ESTIMATES FOR SEMISTABLE SOLUTIONS 7 f (u)a ij (x)u i u j dx = respectively. Combining (2.2) and (2.3), we obtain (2.5) (f(u) 2f(0))f (u)a ij (x)u i u j dx f(0)f (0) f(u)(f (u) f (M)) dx, f(u) dx f(0) 2 f (u) dx. Choose M (depending on f) such that f(t) > 2f(0) + 2 for all t M. On the one hand, using (2.4), the convexity of f, and that (a ij ) is a positive definite matrix, we obtain 2 f(u)(f (u) f (M)) dx = 2 f (u)a ij (x)u i u j dx (f(u) 2f(0))f (u)a ij (x)u i u j dx. On the other hand, for some constant C depending only on f (and M), there holds (f(u) 2f(0))f (u)a ij (x)u i u j dx C a ij (x)u i u j dx {u M} {u M} CM f(u) dx, where the last inequality follows from multiplying equation (1.1) by min{u, M}. Combining the previous bounds with (2.5), it follows that (2.6) 2 f(u)(f (u) f (M)) dx f(0)f (0) f(u) dx + CM f(u) dx. Finally, choose M > M (depending only on f) such that f (M) < f (t) if t > M. 2 Then (2.6) implies f(u)f (u) dx C f(u) dx, and using that f (t) + at infinity (see Remark 2.4 below), we conclude f(u)f (u) dx C, where C is independent of u.

8 8 XAVIER CABRÉ, MANEL SANCHÓN, AND JOEL SPRUCK Remark 2.4. Note that f(t)/t f (t) for all t 0 since f is convex. In particular, by condition (1.4), we obtain lim t f (t) =. Therefore, as a consequence of estimate (2.1) we obtain (2.7) f(u)dx C, where C is a constant independent of u. As in [5], from this and Proposition 2.1 (ii), one deduces (2.8) u in L q () for all q < n/(n 2). Our results improve this estimate under the additional assumptions on f of Theorems 1.1 and 1.2. The following is a sufficient condition on f to guarantee u H0(). 1 Note that by convexity of f, tf (t) f(t) 0 for all t 0. If we further assume that for some ε > 0, tf (t) f(t) εt for t > T (ε), then u is bounded in H0() 1 by a constant independent of u. Indeed, noting that ε a ij (x)u i u j dx = ε f(u)u dx C + f(u)(uf (u) f(u)) dx = C + a ij (x)u i (uf (u) f(u)) j dx = C + uf (u)a ij (x)u i u j dx C, where in the last inequality we used the superlinearity of f and the second estimate in (2.1). 3. Proof of Theorems 1.1 and 1.2 Proof of Theorem 1.1. Assume (1.6). In fact, as we said before Lemma 2.3 we may assume that (1.6) holds replacing f by f: for every ε > 0, there exist T = T (ε) and C = C(ε) such that (3.1) f (t) C f(t) 1+ε for all t > T. In the following, the constants C may depend on ε and T but are independent of u. We start by proving that f (u) L p () for all p < 3 and as a consequence the statement in part (a). Let α = 3+ε (with ε as in (3.1)). Multiplying (1.1) by 1+ε

9 (f (u) f (0)) α 1+f(u) A PRIORI ESTIMATES FOR SEMISTABLE SOLUTIONS 9 and integrating by parts we obtain (3.2) f(u) 1 + f(u) (f (u) f (0)) α dx + f (u) = α 1 + f(u) (f (u) f (0)) α 1 a ij (x)u i u j dx f (u) α 1 f (u) α a ij (x)u i u j dx {u T } 1 + f(u) + C f(u) (1+ε)(α 1) 1 f (u)a ij (x)u i u j dx { C C { T {u>t } a ij (x)u i u j dx + {u<t } f(u) dx + f (u) (1 + f(u)) 2 (f (u) f (0)) α a ij (x)u i u j dx } f(u)f (u)a ij (x)u i u j dx } f(u)f (u)a ij (x)u i u j dx. In particular, by Lemma 2.3 and the bound (2.7), we obtain f (u) α dx C where α = 3 + ε 1 + ε. Therefore, by the arbitrariness of ε > 0, we obtain f (u) L p () for all p < 3. As a consequence, by Corollary 2.2 and since u L 1 () (see Remark 2.4), we obtain the L estimate established in part (a), i.e., if n < 6 then u L () C. In the following, we may assume n 6. Let us prove now that f(u) L p () for all p < n/(n 4), and as a consequence, the statement in part (b). Now we take α = Multiplying (1.1) by (f (u) f (0)) α and using (3.1) and Lemma 2.3, 1+ε we obtain f(u)(f (u) f (0)) α dx = α (f (u) f (0)) α 1 f (u)a ij u i u j dx (3.3) C f(u)f (u)a ij u i u j dx C. Hence, using the convexity of f and that f (0) f (t)/2 for t large, we obtain (3.4) f(u) α+1 u α dx C for all α (1, 2).

10 10 XAVIER CABRÉ, MANEL SANCHÓN, AND JOEL SPRUCK We now repeat the iteration argument of Nedev [5]. Assume that u L p () for all 1 p < p 0. Given any positive number β, set 1 := {x : By (3.4), we have (3.5) Moreover, (3.6) f(u) α+1 2 u α > f(u) α+1 β }, 2 := \ 1 = {x : f(u) u α β }. 1 f(u) α+1 β dx C. f(u) p dx u α β p dx C for all p < β 2 α p 0. Choose β such that α + 1 β = β p α 0, i.e., β = (α + 1)/(1 + p 0 ). Then, by (3.5), (3.6), α and letting α 2, we obtain f(u) L p () for all 1 < p < 3p 0 2+p 0. Hence, by elliptic regularity theory (see Proposition 2.1 (ii)), u L p () for all 1 < p < p 1 := n 3p 0 2+p 0 n 2 3p 0 2+p 0 = 3np 0 2n + (n 6)p 0. By (2.8) we can start the iteration process with p 0 = n/(n 2). Set p k+1 := 3np k 2n+(n 6)p k for k 1. Note that p k n/(n 6) for all k 1 (by induction and since p 0 = n/(n 2) n/(n 6)). Moreover p k+1 > p k (in fact, this is equivalent to p k < n/(n 6)), and hence, lim k p k = n/(n 6) =: p. Therefore, we obtain f(u) L p () for all 1 < p < 3p 2+p = n. The remainder n 4 of the statements of Theorem 1.1 follow from standard elliptic regularity theory (see Proposition 2.1 (ii)). Proof of Theorem 1.2. The proof of Theorem 1.2 is essentially the same. Using assumption (1.7) in (3.2), the first part of the proof gives f (u) L α () with α = 3 ε. 1 ε Therefore, by Corollary 2.2 and since u L 1 () (see Remark 2.4), u L () when 3 ε > n 4ε, or equivalently, when n < ε 2 1 ε Assume n 6 + 4ε. To obtain the estimate on f(u), we deduce (3.3) with 1 ε α = using now (1.7) instead of (1.6). In particular, 1 ε f(u) α+1 dx C for α = u α 1 ε.

11 A PRIORI ESTIMATES FOR SEMISTABLE SOLUTIONS 11 At this point, we repeat the previous iteration argument to obtain the increasing sequence p 0 = n n 2, p k+1 = (1 ε)n (3 2ε)np k (2 ε + (1 ε)p k )n 2(3 2ε)p k, for all k 0, with limit p =. As a consequence, f(u) (1 ε)n 6+4ε Lp () for all p < β p α where β = (α + 1)/(1 + p ), i.e., f(u) α Lp (1 ε)n () for all p <. The remainder of (1 ε)n 4+2ε the statements of Theorem 1.2 follow from standard elliptic regularity theory (see Proposition 2.1 (ii)). References [1] H. Brezis and J.L. Vázquez, Blow-up solutions of some nonlinear elliptic problems, Rev. Mat. Univ. Complut. Madrid 10 (1997), [2] X. Cabré, Regularity of minimizers of semilinear elliptic problems up to dimension 4, Comm. Pure Appl. Math. 63 (2010), [3] X. Cabré and A. Capella, Regularity of radial minimizers and extremal solutions of semilinear elliptic equations, J. Funct. Anal. 238 (2006), [4] M. G. Crandall and P. H. Rabinowitz, Some continuation and variational methods for positive solutions of nonlinear elliptic eigenvalue problems, Arch. Ration. Mech. Anal. 58 (1975), [5] G. Nedev, Regularity of the extremal solution of semilinear elliptic equations, C. R. Acad. Sci. Paris 330 (2000), [6] M. Sanchón, Boundedness of the extremal solution of some p-laplacian problems, Nonlinear Analysis 67 (2007), [7] J. Serrin, Local behavior of solutions of quasilinear elliptic equations, Acta Math. 111 (1964), [8] N.S. Trudinger, Linear elliptic operators with measurable coefficients, Ann. Scuola Norm. Sup. Pisa (3) 27 (1973), [9] S. Villegas, Boundedness of extremal solutions in dimension 4, Adv. Math. 235 (2013), ICREA and Departament de Matemàtica Aplicada I, Universitat Politècnica de Catalunya, Barcelona, Spain address: Departament de Matemàtiques, Universitat Autònoma de Barcelona, Bellaterra, Spain address: Department of Mathematics, Johns Hopkins University, Baltimore, MD address:

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