 Filling fraction of an independent cosine band

For a dispersion relation $E(k)=E_0+t\cos (ka)$, $t<0$, occupied states at zero temperature form the interval $|k|<k_F$ in the first Brillouin zone. Its occupied fraction is $f=2k_F/(2\pi /a)=k_{F}a/\pi$. A spin-degenerate band therefore holds $2f$ electrons per site. If the band is independent of the other two wavevector coordinates, its Fermi surface consists of two planes. For $t>0$ the occupied region is instead near the zone edges, bounded by $|k|=\pi (1-f)/a$.