Consider the boundary-value problem y'' + y = 0, y(0) = y(2), y'(0) = y'(2) Show that except for the case = 0, there

Consider the boundary-value problem

y'' + y = 0, y(0) = y(2π), y'(0) = y'(2π)

Show that except for the case λ = 0, there are two independent eigenfunctions corresponding to each eigenvalue.