In the shell-model context we can interpret this as 4 \( s_{1/2} \) levels, with \( m = \pm 1/2 \), we can also think of these are simple four pairs, \( \pm k, k = 1,2,3,4 \). Later on we will assign single-particle energies, depending on the radial quantum number \( n \), that is, \( \epsilon_k = |k| \delta \) so that they are equally spaced.