Electronic Journal of Differential Equations, Vol. 2025 (2025), No. 46, pp. 1–8. ISSN: 1072-6691. URL: https://ejde.math.txstate.edu, https://ejde.math.unt.edu DOI: 10.58997/ejde.2025.46 NONLOCAL CRITICAL KIRCHHOFF PROBLEMS IN HIGH DIMENSION GIOVANNI ANELLO Abstract. We study the nonlocal critical Kirchhoff problem − ( a+ b ∫ Ω |∇u|2dx ) ∆u = |u|2 ∗−2u+ λf(x, u), in Ω, u = 0, on ∂Ω, where Ω is a bounded smooth domain in RN , N > 4, a, b > 0, λ ∈ R, 2∗ := 2N N−2 is the critical exponent for the Sobolev embedding, and f : Ω × R → R is a Carathéodory function with subcritical growth. We establish the existence of global minimizers for the energy functional associated to this problem. In particular, we improve a recent result proved by Faraci and Silva [3] under more strict conditions on the nonlinearity f and under additional conditions on a and b. 1. Introduction Very recently, Faraci and Silva [3] considered the problem − ( a+ b ∫ Ω |∇u|2dx ) ∆u = |u|2 ∗−2u+ λf(x, u), in Ω, u = 0, on ∂Ω, (1.1) (1.1) where Ω is a bounded smooth domain in RN , N > 4, a, b > 0, λ ∈ R, and f : Ω × R → R is a Carathéodory function, with f(x, 0) = 0, for a.a. x ∈ Ω, satisfying the subcritical growth condition ess sup x∈Ω sup t∈R |f(x, t)| 1 + |t|p−1 < +∞, for some p ∈ (2, 2∗), (1.2) where 2∗ := 2N N−2 is the critical exponent for the embedding W 1,2 0 (Ω) ↪→ Lm(Ω), m ≥ 1. They investigated the existence of local and global minimizers as well as the existence of saddle points of the energy functional Φλ : W 1,2 0 (Ω) → R associated to (1.1), which is defined by Φλ(u) = a 2 ∥u∥2 + b 4 ∥u∥4 − 1 2∗ ∥u∥2 ∗ 2∗ − λJ(u), for each u ∈ W 1,2 0 (Ω), where ∥u∥ := (∫ Ω |∇u(x)|2dx )1/2 is the standard norm of W 1,2 0 (Ω), ∥u∥2∗ := (∫ Ω u(x)|2 ∗ dx )1/2∗ 2020 Mathematics Subject Classification. 35J20, 35J25. Key words and phrases. Nonlocal problem; Kirchhoff equation; weak solution; critical growth; approximation; variational methods. ©2025. This work is licensed under a CC BY 4.0 license. Submitted January 27, 2025. Published May 6, 2025. 1 2 G. ANELLO EJDE-2025/46 is the standard norm of L2∗(Ω), and J(u) = ∫ Ω (∫ u(x) 0 f(x, t)dt ) dx. (1.3) In particular, in [3], the existence of a nonzero global minimizer uλ of Φλ, with Φλ(uλ) ≤ 0, is proved for λ > 0 large and under the additional conditions (I) a N−4 2 b ≥ C1(N) := 4(N − 4) N−4 2 N N−2 2 S N 2 N , where SN = inf u∈W 1,2 0 (Ω)\{0} ∥u∥2 ∥u∥22∗ ; (II) limt→0 f(x, t)/t = 0 uniformly for a.a. x ∈ Ω, (III) for a.a. x ∈ Ω, f(x, t)t > 0, for all t ∈ R \ {0}; (IV) ess infx∈Ω inft∈A f(x, t) > 0, for an open interval A ⊂ (0,∞). Saddle points of Φλ with positive energy are also proved to exist provided that a, b satisfy the more restrictive condition (I’) a N−4 2 b ≥ 1 2 ( N N−2 )N−2 2 C1(N). In [3] the method of proof is essentially based on [4, Lemma 2.1], which ensures the sequential weak lower semicontinuity of the functional Φλ under condition (I), and on [4, Lemma 2.2] which ensures that Φλ satisfies the Palais-Smale condition under condition (I’). In this article, we show that a nonzero global minimizer for Φλ exists for λ > 0 large without any assumption on a, b except their positivity and under much less restrictive conditions on the nonlinearity f . More precisely, we will prove the following result Theorem 1.1. Assume that f satisfies (1.2) and that the functional J : W 1,2 0 (Ω) → R, defined in (1.3), has no global maximizer in W 1,2 0 (Ω). Then, there exists λ∗ ∈]0,+∞[, such that for each λ > λ∗, Φλ admits a global minimizers uλ such that Φλ(uλ) < 0. In particular, uλ is a non-zero weak solution of Problem (1.1). The proof follows by approximating Φλ with appropriate sequentially weakly lower semicon- tinuous functionals. It is an easy matter to see that if f satisfies condition (III) then J cannot have global maximizers. Thus, our existence result improves in several directions [3, Theorem 1.1]. Another simple condition on f which guarantees that the functional J has no global maximizer in W 1,2 0 (Ω) will be stated later. The reader is referred to [1, 2, 6, 5, 7, 9, 10] for other papers dealing with the Kirchhoff equation in high dimension (N ≥ 4). See also [8] and references therein for an overview of papers devoted to the Kirchhoff problem. 2. Proof of the main result In what follows, for each m ≥ 1, we denote by ∥ · ∥m the standard norm of the space Lm(Ω), and if 1 ≤ m ≤ 2∗, we denote by cm the best constant for the embedding W 1,2 0 (Ω) ↪→ Lm(Ω), that is cm := sup u∈W 1,2 0 (Ω)\{0} ∥u∥m ∥u∥ . Finally, for each r > 0, we denote by Br the closed ball in W 1,2 0 (Ω) centered at 0 with radius r. Proof of Theorem 1.1. Under the subcritical condition (1.2), it is well known that J is (well de- fined) C1 and sequentially weakly continuous in W 1,2 0 (Ω). This implies that, for each r > 0, there exists ur ∈ Br such that sup u∈Br J(u) = J(ur). Moreover, since, by assumption, J has no global maximizer on W 1,2 0 (Ω), the following strict inequality holds sup u∈Br J(u) < sup u∈W 1,2 0 (Ω) J(u). (2.1) EJDE-2025/?? NONLOCAL KIRCHHOFF PROBLEMS 3 In addiction, since N > 4, one has 2∗ < 4, and so we can fix l0 ∈ R such that l0 > S − N 2(N−4) N b− N−2 2(N−4) , a 2 t2 + b 4 t4 − 1 2∗ S − 2∗ 2 N t2 ∗ > 0, for each t ≥ l0. Next, in view of (2.1), we can also fix u0 ∈ W 1,2 0 (Ω), with ∥u0∥ > l0, such that J(u0) > sup u∈Bl0 J(u) ≥ J(0) = 0. (2.2) Now, consider the number λ∗ defined by λ∗ = a 2∥u0∥2 + b 4∥u0∥4 − 1 2∗ ∥u0∥2 ∗ 2∗ − inft∈[0,l0] ( a 2 t 2 + b 4 t 4 − 1 2∗S − 2∗ 2 N t2 ∗) J(u0)− sup∥u∥≤l0 J(u) . We will show that for each λ ∈]λ∗,+∞[, Φλ admits a global minimizer uλ such that Φλ(uλ) < 0. Let λ > λ∗. First of all, observe that, being J(u0) > 0, one has Φλ(u0) < a 2 ∥u0∥2 + b 4 ∥u0∥4 − 1 2∗ ∥u0∥2 ∗ 2∗ − λ∗J(u0), (2.3) and, since ∥u0∥ > l0, by the choice of l0 one also has a 2 ∥u0∥2 + b 4 ∥u0∥4 − 1 2∗ ∥u0∥2 ∗ 2∗ ≥ a 2 ∥u0∥2 + b 4 ∥u0∥4 − 1 2∗ S − 2∗ 2 N ∥u0∥2 > 0, from which one infers λ∗ ≥ a 2∥u0∥2 + b 4∥u0∥4 − 1 2∗ ∥u0∥2 ∗ 2∗ J(u0) > 0. Therefore, by (2.2) and (2.3), one has Φλ(u0) < 0. (2.4) Now, fix a sequence of positive numbers {εn}n∈N such that εn < 2∗ − 2, for each n ∈ N, and lim n→+∞ εn = 0. For each n ∈ N, consider the functional Φλ,n : W 1,2 0 (Ω) → R, defined by Φλ,n(u) = a 2 ∥u∥2 + b 4 ∥u∥4 − 1 2∗ − εn ∥u∥2 ∗−εn 2∗−εn − λJ(u), for each u ∈ W 1,2 0 (Ω). Since 2∗ − εn < 2∗, the functional u ∈ W 1,2 0 (Ω) → 1 2∗ − εn ∥u∥2 ∗−εn 2∗−εn − λJ(u) is C1 and sequentially weakly continuous. Consequently, the functional Φλ,n is C1 and sequentially lower weakly semicontinuous. Moreover, recalling that 2∗ < 4, it turns out lim ∥u∥→+∞ Φλ,n(u) = +∞. Therefore, Φλ,n admits a global minimizer uλ,n ∈ W 1,2 0 (Ω). Note also that, thanks to (1.2), we can find a constant C > 0 such that 0 = Φλ,n(0) ≥ Φλ,n(uλ,n) ≥ a 2 ∥uλ,n∥2 + b 4 ∥uλ,n∥4 − |Ω| εn 2∗ 2∗ − εn ∥uλ,n∥2 ∗−εn 2∗ − λC(1 + ∥uλ,n∥pp) ≥ b 4 ∥uλ,n∥4 − (1 + |Ω|)(2∗−2)/2∗ 2 (1 + S − 2∗ 2 N ∥uλ,n∥2 ∗ )− λC(1 + cpp∥uλ,n∥p). Hence, since p < 2∗ < 4, we infer that sup n∈N ∥uλ,n∥ < +∞. Consequently, there exist l ∈ [0,+∞) and uλ ∈ W 1,2 0 (Ω) such that, up to a subsequence, 4 G. ANELLO EJDE-2025/46 (i) ∥uλ,n∥ → l ∈ [0,+∞[; (ii) un → uλ ∈ W 1,2 0 (Ω), weakly in W 1,2 0 (Ω); (iii) un → uλ, strongly in Lq(Ω) and there exists g ∈ L1(Ω) such that |un|q ≤ g a.e. in Ω, for each q ∈ [1, 2∗); (iv) un → uλ, a.e. in Ω. Moreover, by the Concentration Compactness Principle, we know that |∇un|2 → dµ, |un|2 ∗ → dν, (2.5) weakly-∗ in the sense of measures, with dµ ≥ |∇uλ|2 + ∑ k∈Ñ µkδxk ; dν = |uλ|2 ∗ + ∑ k∈Ñ νkδxk ; (µkS −1 N ) N N−2 ≥ νk > 0, for each k ∈ Ñ, (2.6) where Ñ ⊆ N is at most countable, and xk ∈ Ω. We claim that Ñ = ∅. Indeed, assume, on the contrary, that there is some k ∈ Ñ, and, for each r > 0, choose a C1-function φr : RN → [0, 1] such that φr(x) = 0 if |x− xk| ≥ 2r, φr(x) = 1 if |x− xk| ≤ r, |∇φr(x)| ≤ 2 r if x ∈ RN . Since uλ,n is a critical point of Φλ,n, one has 0 = Φ′ λ,n(uλ,n)(φ) = (a+ b∥uλ,n∥2) ∫ Ω ∇uλ,n(x)∇φ(x)dx − ∫ Ω |uλ,n(x)|2 ∗−εn−1φ(x)dx− λ ∫ Ω f(x, uλ,n(x))φ(x)dx. for each φ ∈ W 1,2 0 (Ω). In particular, choosing φ = uλ,nφr, by the Hölder inequality and 0 ≤ φr(x) ≤ 1, we obtain 0 = (a+ b∥un∥2−εn) [ ∫ Ω |∇uλ,n(x)|2φr(x) + ∫ Ω uλ,n(x)∇uλ,n(x)∇φr(x)dx ] − ∫ Ω |uλ,n(x)|2 ∗−εnφr(x)dx− λ ∫ Ω f(x, uλ,n(x))uλ,n(x)φr(x)dx ≥ (a+ b∥un∥2−εn) [ ∫ Ω |∇uλ,n(x)|2φr(x)dx+ ∥uλ,n∥∥uλ,n∇φr∥2 ] − |Ω| εn 2∗ (∫ Ω |un,λ(x)|2 ∗ φr(x)dx )(2∗−εn)/2 ∗ − λ ∫ Ω f(x, uλ,n(x))uλ,n(x)φr(x)dx. (2.7) In addiction, by (2.5) and (2.6), one has lim n→+∞ ∫ Ω |∇uλ,n(x)|2φr(x)dx = ∫ Ω φr(x)dµ ≥ ∫ Ω |∇uλ(x)|2φr(x)dx+ ∑ j∈Ñ µjφρ(xj), lim n→+∞ ∫ Ω |uλ,n(x)|2 ∗ φr(x)dx = ∫ Ω |uλ(x)|2 ∗ φr(x)dx+ ∑ j∈Ñ νjφr(xj), and, by (i)–(iv), one has lim n→+∞ ∥uλ,n∥ (∫ Ω |uλ,n(x)|2|∇φr(x)|2dx )1/2 = l (∫ Ω |uλ(x)|2|∇φr(x)|2dx )1/2 , EJDE-2025/?? NONLOCAL KIRCHHOFF PROBLEMS 5 lim n→+∞ ∫ Ω |uλ,n(x)|2|∇φr(x)|2dx = ∫ Ω |uλ(x)|2|∇φr(x)|2dx lim n→+∞ ∫ Ω f(x, uλ,n(x))uλ,n(x)φr(x)dx = ∫ Ω f(x, uλ(x))uλ(x)φr(x)dx. Taking the above into account and passing to the limit as n → +∞ in (2.7), one obtains 0 ≥ (a+ bl2) [ ∫ Ω |∇uλ(x)|2φr(x)dx+ ∑ j∈Ñ µjφr(xj) + ∫ Ω |uλ(x)|2|∇φr(x)|2dx ] − ∫ Ω |uλ(x)|2 ∗ φr(x)dx− ∑ j∈J νjφr(xj)− ∫ Ω f(x, uλ(x))uλ(x)φr(x)dx. (2.8) Now, it is straightforward to check that∫ Ω |∇uλ(x)|2φr(x)dx → 0, ∫ Ω |uλ(x)|2 ∗ φr(x)dx → 0,∫ Ω f(x, uλ(x))uλ(x)φr(x)dx → 0 as r → 0. Furthermore, recalling that |∇φr(x)| ≤ 2 r for each x ∈ RN , one also has∫ Ω |uλ(x)|2|∇φr(x)|2dx = ∫ Ω∩Br(xk) |uλ(x)|2|∇φr(x)|2dx ≤ (∫ Ω∩Br(xk) |uλ(x)|2 ∗ dx )2/2∗(∫ Ω∩Br(xk) |∇φr(x)| 2·2∗ 2∗−2 dx )(2∗−2)/2∗ ≤ 4 r2 (∫ Ω∩Br(xk) |uλ(x)|2dx )2/2∗(∫ Br(xk) dx )(2∗−2)/2∗ = 4ω (2∗−2)/2∗ N (∫ Ω∩Br(xk) |uλ(x)|2dx )2/2∗ , where ωN is the volume of the unit sphere in RN . Then, since∫ Ω∩Br(xk) |uλ(x)|2dx → 0 as r → 0, one has ∫ Ω |uλ(x)|2|∇φr(x)|2dx → 0 as r → 0. Consequently, passing to the limit as r → 0 in (2.8), it follows that 0 ≥ (a+ bl2)µk − νk. By the above inequality and (2.6), we obtain (a+ bl2)S N N−2 N ≤ µ 2 N−2 k . Then, since by (2.5) and (2.6) one has l2 = lim n→+∞ ∥uλ,n∥2 ≥ ∥uλ∥2 + ∑ j∈Ñ µj ≥ µk, we finally infer that bS N N−2 N l2 < (a+ bl2)S N N−2 N ≤ l 4 N−2 from which ∥uλ∥ ≤ l < S − N 2(N−4) N b− N−2 2(N−4) < l0 (2.9) Now, observe that, since uλ,n is a global minimum point for Φλ,n, one has Φλ,n(uλ,n) ≤ Φλ,n(u0), 6 G. ANELLO EJDE-2025/46 where u0 is as in (2.2). By the previous inequality it follows that a 2 ∥uλ,n∥2 + b 4 ∥uλ,n∥4 − |Ω| εn 2∗ 2∗ − εn S − 2∗−εn 2 N ∥uλ,n∥2 ∗−εn − λJ(uλ,n) ≤ a 2 ∥u0∥2 + b 4 ∥u0∥4 − 1 2∗ ∥u0∥2 ∗−εn 2∗−εn − λJ(u0). A straightforward application of the Lebesgue Dominated Convergence Theorem shows that ∥u0∥2−εn 2−εn → ∥u0∥2 ∗ 2∗ . Hence, passing to the limit as n → +∞ in the above inequality, we ob- tain a 2 l2 + b 4 l4 − 1 2∗ S − 2∗ 2 N l2 ∗ − λJ(uλ) ≤ a 2 ∥u0∥2 + b 4 ∥u0∥4 − 1 2∗ ∥u0∥2 ∗ 2∗ − λJ(u0). This inequality and (2.9) imply that inf t∈[0,l0] (a 2 t2 + b 4 t4 − 1 2∗ S − 2∗ 2 N t2 ∗ ) − λ sup ∥u∥≤l0 J(u) ≤ a 2 ∥u0∥2 + b 4 ∥u0∥4 − 1 2∗ ∥u0∥2 ∗ 2∗ − λJ(u0) from which, in view of (2.2), λ ≤ a 2∥u0∥2 + b 4∥u0∥4 − 1 2∗ ∥u0∥2 ∗ 2∗ − inft∈[0,l0] ( a 2 t 2 + b 4 t 4 − 1 2∗S − 2∗ 2 N t2 ∗) J(u0)− sup∥u∥≤l0 J(u) = λ∗ against the choice of λ. Therefore, it must be Ñ = ∅. This fact and (2.5) and (2.6) imply lim n→+∞ ∥uλ,n∥2∗ = ∥uλ∥2∗ , which, together with (i)–(iv) and the Brezis-Lieb Lemma, implies in turn that uλ,n → uλ, strongly in L2∗(Ω). In particular, one infers that lim n→+∞ ∥uλ,n∥2∗−εn 2∗−εn = ∥uλ∥2∗2∗ . (2.10) Now, since uλ,n is a global minimizer of Φλ,n, one has Φλ,n(uλ,n) ≤ Φλ,n(uλ) for each n ∈ N, Then, passing to the limit as n → +∞ in the above inequality and recalling (2.9) and (2.10), it follows that a 2 l2 + b 4 l4 ≤ a 2 ∥uλ∥2 + b 4 ∥uλ∥4 ≤ a 2 l2 + b 4 l4, that is lim n→+∞ ∥uλ,n∥ = l = ∥uλ∥, Since the norm of W 1,2 0 (Ω) is uniformly convex and uλ,n → uλ weakly in W 1,2 0 (Ω), the above limit implies that uλ,n → uλ, strongly in W 1,2 0 (Ω), Consequently, Φλ,n(uλ,n) → Φλ(uλ). Finally, recalling again that uλ,n is a global minimizer of Φλ,n, one has Φλ(uλ) = lim n→+∞ Φλ,n(uλ,n) ≤ lim n→+∞ Φλ,n(u) = Φλ(u) for each u ∈ W 1,2 0 (Ω). Therefore, uλ is a global minimizer of Φλ and, moreover, taking (2.4) into account, it also turns out Φλ(uλ) ≤ Φλ(u0) < 0. This completes the proof. □ The key assumption in Theorem 1.1 is that J has no global maximizer. The following proposi- tion shows a simple situation in which this assumption is satisfied. EJDE-2025/?? NONLOCAL KIRCHHOFF PROBLEMS 7 Proposition 2.1. Assume f satisfying (1.2) and suppose that for some δ > 0 and ϕ ∈ Lp(Ω) it holds ∫ ϕ(x) 0 f(x, t)dt > sup |ξ|≤δ ∫ ξ 0 f(x, t)dt, for a.a. x ∈ Ω. (2.11) Then, J has no global maximizer in W 1,2 0 (Ω). Proof. First of all, observe that thanks to (1.2), the functional J is well defined in the entire space Lp(Ω) and it is (strongly) continuous in this space. In particular, being W 1,2 0 (Ω) dense in Lp(Ω), one has sup W 1,2 0 (Ω) J = sup Lp(Ω) J. (2.12) Now, arguing by contradiction, assume that there exists a global maximizer v ∈ W 1,2 0 (Ω) for J in W 1,2 0 (Ω). Since v ∈ W 1,2 0 (Ω), the set A := {x ∈ Ω : |v(x)| ≤ δ} has positive measure. Hence, in view (2.11),∫ A (∫ v(x) 0 f(x, t)dt ) dx < ∫ A (∫ ϕ(x) 0 f(x, t)dt ) dx. (2.13) At this point, define the function w : Ω → R as follows w(x) = { ϕ(x), if x ∈ A; v(x), if x ∈ Ω \A. Then, w ∈ Lp(Ω) and, by (2.12) and (2.13), one has J(w) = ∫ Ω (∫ w(x) 0 f(x, t)dt ) dx > J(v) = sup W 1,2 0 (Ω) J = sup Lp(Ω) J, which is absurd. □ To exhibit an example of function satisfying the assumptions of Proposition 2.1, it is sufficient to consider any Carathéodory function f : Ω × R → R satisfying the growth condition (1.2) and such that inf x∈Ω ∫ ξ0 0 f(x, t)dt =: c > 0, for some ξ0 ∈ R. Indeed, if f satisfies the above condition, since lim δ→0 sup |ξ|≤δ ∫ ξ 0 f(x, t)dt = 0, we can choose δ > 0 such that sup |ξ|≤δ ∫ ξ 0 f(x, t)dt < c, and if we define ϕ : Ω → R as ϕ(x) = ξ0, for all x ∈ Ω, we have ϕ ∈ Lp(Ω) and∫ ϕ(x) 0 f(x, t)dt = ∫ ξ0 0 f(x, t)dt ≥ c > sup |ξ|≤δ ∫ ξ 0 f(x, t)dt. 8 G. ANELLO EJDE-2025/46 References [1] G. Anello, F. Cammaroto, L. Vilasi; Non-negative solutions and strong maximum principle for a resonant quasilinear problem, Rev. Mat. Complut. 37 (3) (2024), 801-818. [2] S. Deng, X. Tian; Nondegeneracy of solutions to the critical p -Laplace Kirchhoff equation, Bull. Lond. Math. Soc. 55 (5) (2023), 2112-2128. [3] F. Faraci, K. Silva; On the Brezis-Nirenberg problem for a Kirchhoff type equation in high dimension, Calc. Var. 60 (1) (2021), Paper no. 22, 33 p. [4] F. Faraci, Cs. Farkas; On an open question of Ricceri concerning a Kirchhoff-type problem, Minimax Theory and its Applications, 4 (2019) 271-280. [5] Y. Y. Lan, B. Y. Tang; Multiplicity of solutions for the Kirchhoff equation with critical nonlinearity in high dimension, Math. Methods Appl. Sci. 44 (17) (2021), 13133-13145. [6] L. Kong, H. Chen; Normalized solutions for nonlinear Kirchhoff type equations in high dimensions, Electron. Res. Arch. 30 (4) (2022), 1282-1295. [7] D. Naimen, M. Shibata; Two positive solutions for the Kirchhoff type elliptic problem with critical nonlinearity in high dimension, Nonlinear Anal., 186 (2019) 187-208. [8] P. Pucci, V. D. Radulescu; Progress in nonlinear Kirchhoff problems, Nonlinear Anal. 186 (2019) , 1-5. [9] B. Ricceri; Energy functionals of Kirchhoff-type problems having multiple global minima, Nonlinear Anal., 115 (2015), 130-136. [10] Q. Xie, J. Yu; Bounded state solutions of Kirchhoff type problems with a critical exponent in high dimension, Commun. Pure Appl. Anal. 18 (1) (2019), 129-158. Giovanni Anello Department of Mathematics and Computer Science, Physical Science and Earth Science, University of Messina, Viale F. Stagno d’Alcontres 31, Italy Email address: ganello@unime.it 1. Introduction 2. Proof of the main result References