  Odd-even staggering of binding energies is studied in finite fermion systems
with pairing correlations. We discuss contributions of the pairing and
mean-field to the staggering, and we construct the binding-energy filters which
measure the magnitude of pairing correlations and the effective single-particle
spacings in a given system The analysis is based on studying several
exactly-solvable many-body Hamiltonians as well as on the analytical formulas
that can be applied in the weak and strong pairing limits.
