  In a four-dimensional quantum field theory that flows between two fixed
points under the renormalization group, the change in the conformal anomaly
$\Delta a$ has been related to the average null energy. We extend this result
to derive a sum rule for the other anomaly coefficient, $\Delta c$, in terms of
the stress tensor three-point function. While the sum rule for $\Delta a$ is an
expectation value of the averaged null energy operator, and therefore positive,
the result for $\Delta c$ involves the off-diagonal matrix elements, so it does
not have a fixed sign.
