Since n x and n y can be interchanged without changing the energy, each energy level has a degeneracy of at least two when n x and n y are different. Degenerate states are also obtained when the sum of squares of quantum numbers corresponding to different energy levels are the same. For example, the three states (n x = 7, n y = 1), (n x = 1, n y = 7) and (n x = n y = 5) all have E = 50 π π 2 ℏ ℏ 2 2 m L 2 and constitute a degenerate set.