  I identify the class of even-dimensional conformal field theories that is
most similar to two-dimensional conformal field theory. In this class the
formula, elaborated recently, for the irreversibility of the
renormalization-group flow applies also to massive flows. This implies a
prediction for the ratio between the coefficient of the Euler density in the
trace anomaly (charge a) and the stress-tensor two-point function (charge c).
More precisely, the trace anomaly in external gravity is quadratic in the Ricci
tensor and the Ricci scalar and contains a unique central charge. I check the
prediction in detail in four, six and eight dimensions, and then in arbitrary
even dimension.
