A criterion for determining the stability of solutions of Hill's equat
ion containing even periodic potential is developed based on Hill's me
thod of solution. rn the special case of the Mathieu equation the theo
rem is applied to generate stability maps that compares well with thos
e obtained by existing methods. Finally, stability of Poynting oscilla
tions of a rigid disk attached to a neo-Hookean rubber rod is briefly
discussed.