Variational model for strongly correlated fermions in a magnetic field

For a single band of correlated fermions at T = 0 in a magnetic field h we prove that: (a) if n < 1 and U > 2 W, the effective mass for spin-up (-down) particles decreases (increases) with h; (b) if n = 1 and U < 2 W, the effective mass is spin-independent, the magnetoresistance is positive, and h induces a first-order transition to the localized, magnetically saturated state.