  We test the response to a threading magnetic field of rings of 5-site
$C_{4v}$-symmetric repulsive Hubbard clusters connected by weak intercell
links; each 5-site unit has the topology of a CuO$_{4}$ cluster and a repulsive
interaction is included on every site. In a numerical study of the three-unit
ring with 8 particles, we take advantage of a novel exact-diagonalization
technique which can be generally applied to many-fermion problems. For O-O
hopping we find Superconducting Flux Quantization (SFQ), but for purely Cu-Cu
links bound pair propagation is hindered by symmetry. The results agree with
W=0 pairing theory.
