Copyright © 2011 Alberto Cabada and José Ángel Cid. This is an open access article distributed under the
Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
We deal with the existence and multiplicity of solutions for the
periodic boundary value problem , where is a positive parameter. The function is allowed to be singular, and the related Green's function is nonnegative
and can vanish at some points.