Occupation numbers in a shell-model approach

It is reminded that a shell-model approach for occupation numbers in nuclei imposes constraints on the number of particles that can be lifted out of the Fermi sea. These constraints do not depend on the details of the calculation, such as the applied effective interaction, the size of the shell-model space or the way of tackling the shell-model equations. In some recent papers these constraints are violated. It is shown that this points to inconsistencies in the normalization of the many-body wave function calculated within perturbation theory.