w''(z) + (a - 2 q cos(2 z)) w(z) = 0 for w(z) = c_1 Ce(a, q, z) + c_2 Se(a, q, z) w''(z) + (a - 2 q cos(2 z)) w(z) = 0 for w(z) = c_1 Se(a, q, z) + c_2 Ce(a, q, z)