Electronic Journal of Differential Equations, Vol. 2024 (2024), No. 32, pp. 1–9. ISSN: 1072-6691. URL: https://ejde.math.txstate.edu, https://ejde.math.unt.edu DOI: 10.58997/ejde.2024.32 EXISTENCE OF SEMI-NODAL SOLUTIONS FOR ELLIPTIC SYSTEMS RELATED TO GROSS-PITAEVSKII EQUATIONS JOÃO PABLO PINHEIRO DA SILVA, EDCARLOS DOMINGOS DA SILVA Communicated by Claudianor O. Alves Abstract. In this work we consider existence of semi-nodal solutions, i.e., solutions of the form (u, v) with u > 0 and v± := max{0,±v} ̸≡ 0 for a class of elliptic systems related to the Gross-Pitaevskii equation. 1. Introduction This work concerns the elliptic system −∆u = λ1u+ µ1|u|2p−2u+ β|u|p−2u|v|q, in Ω −∆v = λ2v + µ2|v|2q−2v + β|u|p|v|q−2v, in Ω u = v = 0, on ∂Ω . (1.1) For p = q = 2 the cubic system (1.1) arises in mathematical models for various physics problems, especially in nonlinear optics and Bose-Einstein condensation, see [14, 17]. In those works present information on the physical significance of non- cubic nonlinearities and on the existence and multiplicity of solutions. Furthermore, when λi < 0, system (1.1) comes from the study of solitary wave solutions of the coupled Gross-Pitaevskii equations, −i ∂ ∂t Φ1 = ∆Φ1 + µ1|Φ1|2Φ2 + βΦ2 2Φ1, x ∈ Ω, t > 0 −i ∂ ∂t Φ2 = ∆Φ2 + µ2|Φ2|2Φ1 + βΦ2 1Φ2, x ∈ Ω, t > 0 Φj = Φj(x, t) ∈ C, j = 1, 2 Φj(x, t) = 0, x ∈ ∂Ω, t > 0, j = 1, 2. (1.2) When Φ1(x, t) = e−iλ1tu and Φ2(x, t) = e−iλ2tv, system (1.2) reduces to (1.1). In the Kerr-like photorefractive media, the solution Φj represents the jth element of the beam (see [2]). The self-focusing in the jth component of the beam is related to the positive constant µj , whereas the coupling constant β > 0 signifies the interaction between the two beam components. When µj = 0, the self-focusing 2020 Mathematics Subject Classification. 35J47, 35J50. Key words and phrases. Elliptic systems; variational methods; semi-nodal solutions; Gross-Pitaevskii equation. ©2024. This work is licensed under a CC BY 4.0 license. Submitted June 10, 2023. Published April 25, 2024. 1 2 J. P. P. D. SILVA, E. D. SILVA EJDE-2024/32 has been suppressed, and this type of situation is also relevant in optics (see for example [15, 16, 20]). The problem denoted by system (1.2) is also encountered in the Hartree-Fock theory for a binary mixture of Bose-Einstein condensates in two different hyperfine states |1⟩ and |2⟩ (see for example [13]). In this context, each Φj represents the corresponding condensate amplitude, while µj and β denote the intra and interspecies scattering lengths. The self-interactions of the single state |j⟩ are represented by the sign of µj , with µj > 0 indicating the focusing case and µj < 0 corresponding to the defocusing case. When the intraspecies scattering length µj is zero, it means that the interaction between particles of the same species is extremely weak or nonexistent (see for example [22]). In addition, the sign of β plays a crucial role in determining whether the interactions between states |1⟩ and |2⟩ are attractive or repulsive. Specifically, if β > 0, the interactions are attractive, while β < 0 implies that the interactions are repulsive. This feature is important in understanding the competition between different states and can have a significant impact on the behavior of the system as a whole. Recently, there has been growing interest in studying systems of the form (1.1) that are related to the system (1.2) in the cubic case p = q = 2. This is evidenced by the increasing number of research papers published on the topic, among which we highlight [1, 3, 8, 9, 10, 18, 19, 21, 25] and references therein. On this subject, we also refer the interested reader to [23]. In this work, we investigate the existence of semi-nodal solutions for system (1.1), that is, solutions where u > 0 in Ω and v± := max{0,±v} ̸≡ 0 in Ω, which has also received attention in recent studies, in particular, we are interested in the case where µ1 = µ2 = 0 and p + q < 2∗, β > 0 and N ≤ 5. Clapp and Soares [11] dealt with the case where p = q < 2∗/2 = N/(N − 2), λj = −1, µj = 0 and Ω = RN with N ≥ 4, among other results, they showed the existence of semi-nodal solutions subject to the mentioned conditions. Chen, Lin & Zou [5, 6] dealt with the case p = q = 2, λj < 0, µj > 0, β > 0, and Ω ⊂ RN bounded with N ∈ {1, 2, 3}, they showed existence and multiplicity results of nodal solutions for (1.1). In [7], the same authors provided the existence of semi-nodal solutions for (1.1) for the critical case p = q = 2∗/2 with Ω ⊂ RN bounded, N ≥ 6, µj > 0, λj ∈ (0, λ1(Ω)) and β < 0, here λ1(Ω) is the first eigenvalue of (−∆, H1 0 (Ω)). In this work, we are interested in the case where µj = 0, λj < λ1(Ω), β > 0, p > 1, q > 2 with p + q < 2∗. In particular, 3 < p + q < 2∗ which implies that 3 < 2∗. Hence our main result applies only for the cases N ∈ {3, 4, 5} where p+ q < 2∗. Furthermore, assuming that N ∈ {1, 2}, it suffices that p > 1 and q > 2 because 2∗ = +∞. For the sake of convenience, we will change the notation of system (1.1) to this case, more specifically, we will consider the system −∆u = λu+ ξup−1|v|q, in Ω −∆v = µv + τup|v|q−2v, in Ω u = v = 0, on ∂Ω u > 0, v± ̸≡ 0 in Ω. (1.3) Our main result reads as follows. Theorem 1.1. Assume that Ω ⊂ RN is a smooth bounded domain, p > 1, q > 2 with p + q < 2∗ = 2N/(N − 2) for N ∈ {3, 4, 5}, and p + q < +∞ for N ∈ {1, 2}, EJDE-2024/32 SOLUTIONS TO SEMI-NODAL SOLUTIONS 3 λ, µ < λ1(Ω), where λ1(Ω) is the first eigenvalue of (−∆, H1 0 (Ω)). Then there exists a pair of solution u, v ∈ C2(Ω) to (1.3). Our approach is based on minimization arguments presented in [4, 24] with the necessary technical modifications. The main difficulties in our approach are to avoid semi-trivial solutions (i.e. solution of the form (u, 0) or (0, v)) and to construct Palais-Smale sequence that converges to the infimum of the functional associated with system (2.21) restricted to a certain subset of the Nehari manifold. 2. Main result To present our main result, we use the following notation: H := H1 0 (Ω)×H1 0 (Ω), ∥u∥2λ := ∥u∥2 − λ|u|22, and ∥v∥2µ := ∥v∥2 − µ|v|22, where ∥f∥ := ( ∫ Ω |∇f |2dx )1/2 is the norm of H1 0 (Ω), we will write |f |s as the norm of Ls(Ω) and f±(x) := max{0,±f(x)}. Given the condition λ, µ < λ1(Ω), we see that there exists Cλµ := Cλµ(λ1(Ω)) such that Cλµ∥u∥ ≤ ∥u∥λ ≤ C−1 λµ ∥u∥ and Cλµ∥v∥ ≤ ∥v∥µ ≤ C−1 λµ ∥v∥ for all u, v ∈ H1 0 (Ω) (2.1) To obtain solutions for system (1.3), we define the functional Iλµ ∈ C1(H,R) given by Iλµ(u, v) = 1 2 ∥u∥2λ + 1 2 ∥v∥2µ − 1 p+ q ∫ Ω |u|p|v|q dx Here we shall follow same ideas from [4] which allows us to minimize the functional Iλµ over the following subsets of the Nehari manifold Nλ := {(u, v) ∈ H : I ′λµ(u, v)(u, 0) = 0, u ̸≡ 0, v ̸≡ 0}, N± µ := {(u, v) ∈ H : I ′λµ(u, v)(0, v ±) = 0, u ̸≡ 0, v± ̸≡ 0}, Mλµ := Nλ ∩N+ µ ∩N− µ The following result is of fundamental importance for constructing a Palais-Smale sequence at the level where we obtain solutions to our problem. This approach is based on an idea presented in the work [24]. Lemma 2.1. Let (u, v) ∈ Mλµ and z, w ∈ H1 0 (Ω) \ {0} then for all δ > 0 there are unique positive numbers t = t(δ), r = r(δ), and s = s(δ) such that( t(u− δz), r(v − δw)+ − s(v − δw)− ) ∈ Mλµ. Moreover if ∥z∥, ∥w∥ ≤ 1 and ∥u∥, ∥v∥ ≤ M1, then there are constants M0 = M0(p, q, λ, µ, λ1(Ω),Ω,M1, N) > 0, and Ci = Ci(p, q, λ, µ, λ1(Ω),Ω,M1, N) > 0 such that |t′(0)|, |r′(0)|, |s′(0)| ∈ [0, C2] and |t(0)|, |r(0)|, |s(0)| ∈ [C1, C2], (2.2) ∥u∥, ∥v±∥ ≥ M0 (2.3) Proof. Firstly, we mention that (ϕ, φ) ∈ Mλµ if and only if ∥ϕ∥2λ = p p+ q ∫ |ϕ|p|φ|qdx and ∥φ±∥2µ = q p+ q ∫ |ϕ|p|φ±|dx. 4 J. P. P. D. SILVA, E. D. SILVA EJDE-2024/32 Therefore, for each (t(u− δz), r(v − δw)+ − s(v − δw)−) ∈ Mλµ, we obtain that ∥t(u− δz)∥2λ = p p+ q ∫ |t(u− δz)|p|r(v − δw)+ − s(v − δw)−|q dx ∥r(v − δw)+∥2µ = q p+ q ∫ |t(u− δz)|p|r(v − δw)+| dx ∥s(v − δw)−∥2µ = q p+ q ∫ |t(u− δz)|p|s(v − δw)−| dx (2.4) To make the presentation clear, we define the following functions: f1(δ) = ∥u− δz∥2λ, f2(δ) = ∫ Ω |u− δz|p[(v − δw)+]q, f3(δ) = ∫ Ω |u− δz|p[(v − δw)−]q, f4(δ) = ∥(v − δw)+∥2µ, f5(δ) = ∥(v − δw)−∥2µ. It follows from (2.4) that t(δ), r(δ), and s(δ) are precisely the solutions for the system t2f1(δ) = p p+ q tprqf2(δ) + p p+ q tpsqf3(δ), (2.5) r2f4(δ) = q p+ q tprqf2(δ), (2.6) s2f5(δ) = q p+ q tpsqf3(δ), (2.7) Here we observe that the solution t(δ) is given explicitly by t(δ) = ( 1 + p q ) 1 p+q−2 [f1(δ)] − q−2 2(p+q−2) { [f4(δ)] q q−2 [f2(δ)] 2 q−2 + [f5(δ)] q q−2 [f3(δ)] 2 q−2 } q−2 2(p+q−2) . (2.8) Recall that f1(0) = ∥u∥2λ, f2(0) = ∫ Ω |u|p|v+|q = p+ q q ∥v+∥2µ, f3(0) = ∫ Ω |u|p|v−|q = p+ q q ∥v−∥2µ, f4(0) = ∥v+∥2µ, f5(0) = ∥v−∥2µ Here we used that (u, v) ∈ Mλµ to determine the values of f2(0) and f3(0). Since 0 < µ < λ1(Ω), H 1 0 (Ω) ↪→ Lp(Ω), and H1 0 (Ω) ↪→ Lq(Ω) by Hölder inequality’s there exists Cpq = Cpq(Ω) > 0 such that( 1− µ λ1(Ω) ) ∥v±∥2 ≤ ∥v±∥2µ = q p+ q ∫ Ω |u|p|v±|q ≤ Cpq∥u∥p∥v±∥q. (2.9) Since q > 2 and ∥u∥, ∥v∥ ≤ M1, expression (2.9) yields a constant M0 > 0 such that ∥u∥, ∥v±∥2 ≥ M0, hence (2.3) is proved. As ∥u∥, ∥v±∥ ∈ [M0,M1], we easily see from the expressions of fi(0) that there existK1 = K1(a, b, p, q, λ, µ, λ1(Ω),M1, N) > 0 and K2 = K2(a, b, p, q, λ, µ, λ1(Ω),M1, N) > 0 such that K1 ≤ fi(0) ≤ K2, 1 ≤ i ≤ 5. (2.10) A standard calculation shows that f ′ 1(0) = − (∫ Ω ∇u∇z − λuz ) , f ′ 2(0) = −p ∫ Ω |u|p−2uz|v+|q − q ∫ |u|p|v+|q−1w, EJDE-2024/32 SOLUTIONS TO SEMI-NODAL SOLUTIONS 5 f ′ 3(0) = −p ∫ Ω |u|p−2uz|v−|q − q ∫ |u|p|v−|q−1w, f ′ 4(0) = − ∫ Ω ∇v+∇w − µ ∫ v+w, f ′ 5(0) = − ∫ Ω ∇v−∇w − µ ∫ v−w. It is important to observe that ∥z∥, ∥w∥ ≤ 1 and ∥u∥, ∥v∥ ≤ M1. By the Sobolev embedding theorems, there exists a constant K3 = K3(p, q, λ, µ, λ1(Ω),M1, N) > 0 such that |f ′ i(0)| ≤ K3, 1 ≤ i ≤ 5. (2.11) From the explicit expression of t(δ) given by in (2.8) it follows that for a certain Ψ ∈ C1(R5 +) (and Ψ /∈ C1(R5 +)) we can write t(δ) = Ψ(f1(δ), . . . , f5(δ)). Therefore t′(0) = ∑ f ′ i(0)Ψxi(f1(0), . . . , f5(0)), and from this equality, together with (2.10) and (2.11) we conclude that there exist Ci = Ci(p, q, λ, µ, λ1(Ω),Ω,M1, N) > 0 such that |t′(0)| ≤ C2 and C1 ≤ t(0) ≤ C2. The other inequalities can be derived from combining the last estimates with (2.6) and (2.7). □ The following proposition shows the existence of a Palais-Smale sequence that converges to the infimum of Iλµ over Mλµ. Notice also that p+ q > 2 and Iλµ(u, v) = (1 2 − 1 p+ q ) ( ∥u∥2λ + ∥v∥2µ ) for all (u, v) ∈ Mλµ. (2.12) Hence, inf(u,v)∈Mλµ Iλµ(u, v) > −∞. In what follows, we will use the notation ∥(φ, ϕ)∥ := ∥φ∥+ ∥ϕ∥ for all φ, ϕ ∈ H1 0 (Ω). Proposition 2.2. Let cλµ := inf(u,v)∈Mλµ Iλµ(u, v). Then there exists a sequence (un, vn) ∈ Mλµ such that Iλµ(un, vn) → cλµ and I ′λµ(un, vn) → 0. Moreover, there exist M0,M1 > 0 such that ∥un∥, ∥v±n ∥ ∈ [M0,M1] for all n ∈ N. Proof. By applying the Ekeland’s variational principle [12], we construct a sequence (un, vn) ∈ Mλµ such that Iλµ(un, vn) → cλµ, Iλµ(un, vn) < Iλµ(φ, ϕ) + 1 n ∥(un − u, vn − v)∥, ∀(φ, ϕ) ∈ Mλµ. (2.13) As (un, vn) ∈ Mλµ, then ∥un∥2λ + ∥vn∥2µ = ∫ Ω |un|p|vn|q which leads us to p+ q − 1 2(p+ q) ( ∥un∥2λ + ∥vn∥2µ ) = 1 2 ( ∥un∥2λ + ∥vn∥2µ ) − 1 p+ q ∫ Ω |un|p|vn|q = Iλµ(un, vn) = cλµ + on(1). The above expression and (2.1) gives us 2Cλµ ( ∥un∥2 + ∥vn∥2 ) ≤ ( ∥un∥2λ + ∥vn∥2µ ) = 2(p+ q)cλµ p+ q − 1 + on(1); therefore (un, vn) is a bounded sequence, i.e, there exists M1 > 0 such that ∥(un, vn)∥ ≤ M1 for all n ∈ N (2.14) By the Riesz Representation Theorem, it follows that for every fixed n ∈ N there exist zn, wn ∈ H1 0 (Ω) such that (zn, wn) ∼= I ′λµ(un, vn)/∥I ′λµ(un, vn)∥; moreover ∥(zn, wn)∥ = 1 and I ′λµ(un, vn)(zn, wn) = ∥I ′λµ(un, vn)∥. (2.15) 6 J. P. P. D. SILVA, E. D. SILVA EJDE-2024/32 From now on, we will assume that n ∈ N is fixed. Let t(δ) := tn(δ), r(δ) := rn(δ), s(δ) := sn(δ) be given as stated in Lemma 2.1 and u := un, v := vn, z := zn, w := wn we will define (u(δ), v(δ)) := (un(δ), vn(δ)) by (u(δ), v(δ)) := ( t(δ)[u− δz], r(δ)[v − δw]+ − s(δ)[v − δw]− ) ∈ Mλµ. Recall also that Iλµ ∈ C1(H,R), where H = H1 0 (Ω)×H1 0 (Ω). Setting R(X−Y ) := Iλµ(X) − Iλµ(Y ) − I ′λµ(X)(X − Y ) for any X,Y ∈ H, we have limX→Y (R(X − Y )/∥X − Y ∥ ) = 0. Since limδ→0+(u(δ), v(δ)) = (u, v), it is not difficult to see that (∥u(δ)− u∥+ ∥v(δ)− v∥) /δ → ∥t′(0)u− z∥+ ∥r′(0)v+ − s′(0)v− − w∥ as δ → 0+. Therefore, o(δ) := R(u(δ)− u, v(δ)− v) satisfies o(δ)/δ → 0 as δ → 0+ and Iλµ(u(δ), v(δ)) = Iλµ(u, v) + I ′λµ(u(δ), v(δ))(u(δ)− u, v(δ)− v) + o(δ) (2.16) setting Tδ(φ, ϕ) := I ′λµ(u(δ), v(δ))(φ, ϕ), by (2.13) and (2.16) we have 1 n ∥u(δ)− u, v(δ)− v∥ ≥ Iλµ(u, v)− Iλµ(u(δ), v(δ)) = Tδ(u− u(δ), v − v(δ)) + o(δ) = = (1− t(δ))Tδ(u− δz, 0) + Tδ(u, 0)− Tδ(u− δz, 0) + (1− r(δ))Tδ(0, [vn − δw]+) + Tδ(0, v +)− Tδ(0, [v − δw]+) − (1− s(δ))Tδ(0, [v − δw]−)− Tδ(0, v −) + Tδ(0, [v − δw]−) + o(δ) = (1− t(δ))Tδ(u− δz, 0) + (1− r(δ))Tδ(0, [v − δw]+) − (1− s(δ))Tδ(0, [v − δw]−) + δTδ(z, w) + o(δ). As a consequence, 1 n ∥u(δ)− u δ , v(δ)− v δ ∥ ≥ (1− t(δ) δ ) Tδ(u− δz, 0) + (1− r(δ) δ ) Tδ(0, [v − δw]+) − (1− s(δ) δ ) Tδ(0, [v − δw]−) + Tδ(z, w) + o(δ) δ . Given that limδ→0+ t(δ) = limδ→0+ r(δ) = limδ→0+ s(δ) = 1, taking the limit as δ → 0+ , the above inequality gives us 1 n ∥t′(0)u− z, r′(0)v+ − s′(0)v− − w∥ ≥ −t′(0)T0(u, 0)− r′(0)T0(0, v +) + s′(0)T0(0, v −) + T0(z, w) = −t′(0)T0(u, 0)− r′(0)T0(0, v +) + s′(0)T0(0, v −) + T0(z, w). (2.17) Since (u, v) ∈ Mλµ, it follows that T0(u, 0) = T0(0, v +) = T0(0, v −) = 0; therefore, from (2.17) and (2.15) we conclude that |t′(0)|+ |r′(0)|+ |s′(0)| n (∥u, v∥+ ∥z, w∥) ≥ T0(z, w) = I ′λµ(u, v)(z, w) = ∥I ′λµ(u, v)∥. (2.18) By Lemma 2.1 there exists C2 > 0 (that does not depend on the index n) such that |t′(0)| + |r′(0)| + |s′(0)| ≤ 3C2. From (2.18), (2.14) and (2.15) we obtain ∥I ′λµ(un, vn)∥ ≤ 3(2M1 +1)C2/n. The existence of the constant M0 > 0 is guaran- teed by Lemma 2.1. This completes the proof. □ EJDE-2024/32 SOLUTIONS TO SEMI-NODAL SOLUTIONS 7 Proof of Theorem 1.1. Firstly, we deal with the case N ∈ {3, 4, 5}. Let (un, vn) the sequence obtained in Proposition 2.2, by (2.12), ∥un∥, ∥v±n ∥ ∈ [M0,M1], M0 > 0 and λ, µ < λ1(Ω), we can deduce that cλµ > 0. It follows from the boundedness of un, vn in H1 0 (Ω) that there exists u0, v0 ∈ H1 0 (Ω) such that, up to a subsequence, un ⇀ u0 and vn ⇀ v0 weakly in H1 0 (Ω). Since I ′λµ(un, vn) → 0 and (v±n − v±0 ) is bounded in H1 0 (Ω), it follows that ∥v±n ∥2µ − ∫ Ω ( ∇vn∇v±0 − µvnv ± 0 ) − Γn = I ′λµ(un, vn)(0, v ± n − v±0 ) → 0, (2.19) where Γn := q p+ q ∫ Ω |un|p||v±n |q−2v±n (v ± n − v±0 )dx. Then Hölder’s inequality gives us∣∣ ∫ Ω |un|p||v±n |q−2v±n (v ± n − v±0 )dx ∣∣ ≤ |un|pp+q|v±n | q−1 p+q|v±n − v±0 |p+q. (2.20) Since 2 < p + q < 2∗, it follows that H1 0 (Ω) ↪→ Lp+q(Ω) is a compact embedding; therefore vn ⇀ v0 weakly in H1 0 (Ω) imply that |v±n − v±0 |p+q → 0, which combined with the boundedness of un, vn in H1 0 (Ω) and (2.20) gives us Γn → 0. As∫ Ω (∇vn∇v±0 − µvnv ± 0 )dx → ∥v±0 ∥2µ from (2.19) we obtain ∥v±n ∥2µ → ∥v±0 ∥2µ. Since v±n → v±0 strongly in L2(Ω) it follows that ∥v±n ∥2 → ∥v±0 ∥2 and therefore v±n → v±0 strongly in H1 0 (Ω). In a completely analogous manner, we can conclude that un → u0 strongly inH1 0 (Ω). It follows from Proposition 2.2 that Iλµ(u0, v0) = cλµ, I ′ λµ(u0, v0) = 0 and ∥u0∥ > 0 and ∥v±0 ∥ > 0. Notice also that we can replaced un by |un| and still have Iλµ(|un|, vn) → cλµ. Without loss of generality we assume that un ≥ 0 which implies that u0 ≥ 0. Since I ′λµ(u0, v0) = 0 we deduce that u0, v0 are the weak solutions of the system −∆u0 = λu0 + p p+ q |u0|p−2u0|v0|q, in Ω, −∆v0 = µv0 + q p+ q |u0|p|v0|q−2v0, in Ω, u0 = v0 = 0, on ∂Ω, u0 ≥ 0, v±0 ̸≡ 0 in Ω. 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Anal. 190 (2008), no. 1, 83-106. EJDE-2024/32 SOLUTIONS TO SEMI-NODAL SOLUTIONS 9 João Pablo Pinheiro da Silva Departamento de Matemática, Universidade Federal do Pará, Belem, Brazil Email address: jpabloufpa@gmail.com Edcarlos Domingos da Silva Departamento de Matemática, Universidade de Federal de Goiás, Goiánia, GO, 74690- 900, Brazil Email address: edcarlos@ufg.br 1. Introduction 2. Main result Acknowledgements References