  We study the Cu_{5}O_{4} cluster by exact diagonalization of a three-band
Hubbard model and show that bound electron or hole pairs are obtained at
appropriate fillings, and produce superconducting flux quantisation.
  The results extend earlier cluster studies and illustrate a canonical
transformation approach to pairing that we have developed recently for the full
plane. The quasiparticles that in the many-body problem behave like Cooper
pairs are W=0 pairs, that is, two-hole eigenstates of the Hubbard Hamiltonian
with vanishing on-site repulsion. The cluster allows W=0 pairs of d symmetry,
due to a spin fluctuation, and s symmetry, due to a charge fluctuation. Flux
quantisation is shown to be a manifestation of symmetry properties that hold
for clusters of arbitrary size.
