  It was suggested by Rasolt and Te\v{s}anovi\'c that the Landau level
quantization in isotropic superconductors could enhance superconductivity in a
very strong magnetic field, above the upper critical field ($H_{c2}$). We
derive a generalized Harper equation for superconducting systems, and show that
a similar reentrant behavior appears in a lattice model, even though the
Landau--level--structure is destroyed by the periodic potential in that case.
Both the orbital and the Zeeman field--induced effects are taken into account.
