  A Nilsson mean-field plus extended pairing interaction Hamiltonian with
many-pair interaction terms is proposed. Eigenvalues of the extended pairing
model are easy to obtain. Our investigation shows that one- and two-body
interactions continue to dominate the dynamics for relatively small values of
the pairing strength. As the strength of the pairing interaction grows,
however, the three- and higher many-body interaction terms grow in importance.
A numerical study of even-odd mass differences in the $^{154-171}$Yb isotopes
shows that the extended pairing model is applicable to well-deformed nuclei.
