Suppose each a n is g -fold degenerate. Let the corresponding orthonormal eigenkets be | ψ ψ n r ⟩ ⟩ , r ∈ ∈ { 1 , 2 , … … , g } . Since [ A , B ] = 0 , we reason as above to find that B | ψ ψ n r ⟩ ⟩ is an eigenket of A corresponding to the degenerate eigenvalue a n . So, we can expand B | ψ ψ n r ⟩ ⟩ in the basis of the degenerate eigenkets of a n :