  We solve exactly a generalized Hubbard ring with twisted boundary conditions.
The magnitude of the nearest-neighbor hopping depends on the occupations of the
sites involved and the term which modifies the number of doubly occupied sites
$t_{AB}=0$. Although $\eta$-pairing states with off-diagonal long-range order
are part of the degenerate ground state, the behavior of the energy as a
function of the twist rules out superconductivity in this limit. A small
$t_{AB}$ breaks the degeneracy and for moderate repulsive $U$ introduce
superconducting correlations which lead to ``anomalous'' flux quantization.
  PACS numbers: 74.20Mn, 71.27.+a, 71.30+h, 71.28.+d

Condensed Matter