Gamma-ray energy-network fit (${\chi }^2={\sum }_i[(E_{u_i}-E_{l_i}-E_{\gamma ,i})/{\sigma }_i{]}^2$)

For a proposed gamma-transition graph, fix the ground-state energy and fit the remaining level energies with a linear incidence matrix. Loops give consistency constraints and correlated excitation-energy errors. Different transition graphs can have the same minimized residuals if they rearrange the same energy-sum identities; a small chi-squared is not then a unique level assignment.