or H c = E c {\displaystyle \mathbf {Hc} =E\mathbf {c} } which is a 2 × {\displaystyle \times } 2 matrix Eigenvalues and eigenvectors problem. Because of the hermiticity of H {\displaystyle \mathbf {H} } the eigenvalues are real, or rather vice versa it is the requirement that the energies are real that implies the hermiticity of H {\displaystyle \mathbf {H} } . The eigenvectors represent the stationary states, i.e. those for whom the absolute magnitude of the squares of the probability amplitudes do not change with time.