Mean energy of a two-dimensional Fermi gas
= Mean energy of a two-dimensional Fermi gas
{title2=$U/N=E_F/2$}
A quadratic-dispersion <Fermi gas> in two dimensions has constant <density of states> $g$. At zero <temperature>, $N=gE_F$ and $U=gE_F^2/2$. Hence the mean <energy> is half the <Fermi energy>, independent of a common spin-degeneracy factor.