1. The problem statement, all variables and given/known data An electron has two spin states and a set of energy levels E1,E2,.... By the Fermi-dirac distrbution,the mean number of electrons in energy level Ek is https://en.wikipedia.org/wiki/Fermi–Dirac_statistics#Distribution_of_particles_over_energy Does it mean that,for an electron, the mean number is 2*F(E) ? But this sounds weird to me,because the mean number doubles just because of the degeneracy ? I am quite confused. *Additional If we take degeneracy into account,suppose there are N electrons(weakly-interacting).At some temperatures close to 0K,the N/2 states of lowest energy are occupied rather than N states? 2. Relevant equations 3. The attempt at a solution Described in 1.