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Existence and localization of solutions of second order elliptic problems using lower and upper solutions in the reversed order. (English) Zbl 0897.35030

For a second order elliptic problem an upper solution above a lower solution implies existence of a solution in between. The authors address the question what happens if semisolutions for the semilinear boundary value problem \({\mathcal L} u=f(x,u)\), \({\mathcal B} u=0 \) have reversed order. Introducing an admissible box \([a,b]\times [c,d] \subset L^p \times L^p\) with the functions \(a,b,c,d\) bounding \(\liminf_{s\to \pm\infty}\) and \(\limsup_{s \to\pm \infty} f(x,s)/s\), their main result is that, if such a box exists and if there is a reversely ordered pair of semisolutions, a solution \(u\) exists with \(u\) in between for at least one point. A box is admissible if for \((q_+,q_-) \in[a,b] \times [c,d]\) the Fučík type eigenvalue problem \({\mathcal L} u=q_+ u^+- q_-u^-\), \({\mathcal B} u=0\) only has fixed sign solutions and if for \(q_+\), \(q_-\) in this box with \(q_+\) \(q_- \geq\lambda_1\) there exists a nondegenerate homotopy to some \(q^0_+ =q^0_-\). The paper extends earlier results of J. P. Gossez and P. Omari in [Commun. Partial Differ. Equations 19, No. 7-8, 1163-1184 (1994; Zbl 0814.35019)].
Reviewer: G.H.Sweers (Delft)

MSC:

35J65 Nonlinear boundary value problems for linear elliptic equations

Citations:

Zbl 0814.35019
Full Text: DOI