If we take the function f(q,p) in eqn. (3.5.3) to be U−H(q,p), then clearly ⟨f⟩=0; formally, this would mean
∫[U−H(q,p)]e−βH(q,p)dω=0.
Derive from this equation expression (3.6.3) for the mean-square fluctuation in the energy of a system embedded in the canonical ensemble.