I am analyzing the sign of the eigenvalues of a singular Sturm-Liouville problem, with the function coefficient of the second order term equal to zero at the edges of the interval. The problem arises in a linear stability analysis of self similar solution of a diffusive-convective differential problem. A theorem states that for a regular SL problem the sign of the eigenvalues in some conditions is related to the sign of the coefficient functions in the interval. Is any theorem or lemma that can be used to justify for a singluar SL problem the same conclusions that can be drawn for a regular SL problem?