Upper and lower solutions for a superlinear singular boundary value problem

The singular boundary value problem u″ = −g(t, u), for t ε (0, 1), u(0) = u(1) = 0, is studied in this paper. The singularity may appear at u = 0, t = 0, and t = 1, the function g may be superlinear at u = ∞ and change sign. The existence of solutions is obtained via an upper and lower solution method.