Linear and nonlinear, second-order problems with Sturm-Liouville-type, multi-point boundary conditions

We consider the nonlinear equation−u′′=f(u)+h,on(−1,1),where f:R→R and h:[−1,1]→R are continuous, together with general Sturm-Liouville type, multi-point boundary conditions at ±1. We will obtain existence of solutions of this boundary value problem under certain `nonresonance' conditions, and also Rabinowitz-type global bifurcation results, which yield nodal solutions of the problem. These results rely on the spectral properties of the eigenvalue problem consisting of the equation−u′′=λu,on(−1,1),together with the multi-point boundary conditions. In a previous paper it was shown that, under certain `optimal' conditions, the basic spectral properties of this eigenvalue problem are similar to those of the standard Sturm-Liouville problem with single-point boundary conditions. In particular, for each integer k≥0 there exists a unique, simple eigenvalue λk, whose eigenfunctions have `oscillation count' equal to k, where the `oscillation count' was defined in terms of a complicated Pr\"ufer angle construction. Unfortunately, it seems to be difficult to apply the Pr\"ufer angle construction to the nonlinear problem. Accordingly, in this paper we use alternative, non-optimal, oscillation counting methods to obtain the required spectral properties of the linear problem, and these are then applied to the nonlinear problem to yield the results mentioned above.