  A single l-shell model is investigated for a system of fermions of spin s and
an attractive s-wave, spin channel independent, interaction. The spectra and
eigenvectors are determined exactly for different l, s values and particle
numbers N. As a generalization of Cooper pairing it is shown that when
N=mu(2s+1), mu=1,2,...,2l+1, the ground state consists of clusters of (2s+1)
particles. The relevance of the results for more general situations including
the homogeneous system is briefly discussed.
