In the absence of degeneracy, if a measured value of energy of a quantum system is determined, the corresponding state of the system is assumed to be known, since only one eigenstate corresponds to each energy eigenvalue. However, if the Hamiltonian H ^ ^ has a degenerate eigenvalue E n of degree g n, the eigenstates associated with it form a vector subspace of dimension g n. In such a case, several final states can be possibly associated with the same result E n , all of which are linear combinations of the g n orthonormal eigenvectors | E n , i ⟩ ⟩ .