I identify the class of even-dimensional conformal field theories that is m
ost similar to two-dimensional conformal field theory. In this class the fo
rmula, 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 (c
harge a) and the stress-tenser two-point function (charge c). More precisel
y, the tract anomaly in external gravity is quadratic in the Ricci tensor a
nd the Ricci scalar and contains a unique central charge. 1 check the predi
ction in detail in four, six and eight dimensions, and then in arbitrary ev
en dimension. (C) 2000 Published by Elsevier Science B.V. All rights reserv
ed.