The paper concerns the boundary value problem
It is assumed that the functions and are continuous and nonnegative and such that the conditions and hold. The linear operator , defined on the subspace of functions that belong to the Sobolev space and satisfy the boundary conditions is a Fredholm operator with index zero when the functions and satisfy a condition formulated in the preliminary part of the paper. Moreover, its kernel is two-dimensional.
Next, a fixed point theorem due to J. Mawhin [Topological degree methods in nonlinear boundary value problems. Regional Conference Series in Mathematics. No. 40. R.I.: The American Mathematical Society (1979; Zbl 0414.34025)] is recalled. The existence of at least one solution of the boundary value problem is proved when certain conditions are satisfied. They are formulated in the main result which proof is based on Mawhin’s fixed point theorem. An example is presented.