Let = + , so that f() appears as f( + ). Show that f(

Let ε = μ + δ, so that f(ε) appears as f(μ + δ). Show that
f(μ + δ) = 1 – f(μ + δ).
Thus the probability that an orbital δ above the Fermi level is occupied is equal to the probability an orbital δ below the Fermi level is vacant. A vacant orbital is sometimes known as a hole.