1. Kittel p. 25 #1
2. Kittel p. 25 #2
3. Kittel p. 25 #3 I am not happy with Kittel's
designation "non-equilibrium state" for states with n not equal
to zero. Mostly this is because the equilibrium state in the presence
of the field does certainly have a non-zero expected value of n.
So I would replace his term by the alternate term "constrained state."
You may use the fact that W(n)=2N w(n) and use Eq. 6.8
for w(n).
4. Kittel p. 25 #4