  We prove an upper bound on the ground state energy of the dilute
spin-polarized Fermi gas capturing the leading correction to the kinetic energy
resulting from repulsive interactions. One of the main ingredients in the proof
is a rigorous implementation of the fermionic cluster expansion of Gaudin,
Gillespie and Ripka (Nucl. Phys. A, 176.2 (1971), pp. 237-260).
