Question

How can I solve the following exercise: Use perturbation theory to calculate the energy levels of a particle in a box with a cosine function botto. Let the box extend from x=0 to x=a and let the perturbing potential be H' = V0[1+cos(2*Pi*m*x/a)]. This potential oscilates between 2V0 and 0 (this means that H’(x=0) = 2V0 and H’(x=a/2) =. The number of oscilations is determined by m. Discuss restrictions on the possible values of k+1 and k-1 in the integrals H'kl
and I don't know what does mean k+1 and k-1. It refers to l=1?