I have the following equation: (k(x)*g(x)')'=p*g(x) where k(x) = k(x+T) -- k(x) is a known periodical function of period T, k(x) real, x real, T real. p = some constant that have to be determined. g(x) = an unknown function. Question: Is there a method that can tell from the beginning that all the values of p should be real? If p is always real I will try to solve the equation if p could also be complex the equation will not be of so much interest for me.