  The general form of the stress-tensor three-point function in four dimensions
is obtained by solving the Ward identities for the diffeomorphism and Weyl
symmetries. Several properties of this correlator are discussed, such as the
renormalization and scheme independence and the analogies with the anomalous
chiral triangle. At the critical point, the coefficients a and c of the
four-dimensional trace anomaly are related to two finite, scheme-independent
amplitudes of the three-point function. Off-criticality, the imaginary parts of
these amplitudes satisfy sum rules which express the total
renormalization-group flow of a and c between pairs of critical points.
Although these sum rules are similar to that satisfied by the two-dimensional
central charge, the monotonicity of the flow, i.e. the four-dimensional
analogue of the c-theorem, remains to be proven.
