  In a September 1976 PRL Eguchi and Freund considered two topological
invariants: the Pontryagin number $P \sim \int d^4x \sqrt{g}R^* R$ and the
Euler number $\chi \sim \int d^4x \sqrt{g}R^* R^*$ and posed the question: to
what anomalies do they contribute? They found that $P$ appears in the
integrated divergence of the axial fermion number current, thus providing a
novel topological interpretation of the anomaly found by Kimura in 1969 and
Delbourgo and Salam in 1972. However, they found no analogous role for $\chi$.
This provoked my interest and, drawing on my April 1976 paper with Deser and
Isham on gravitational Weyl anomalies, I was able to show that for Conformal
Field Theories the trace of the stress tensor depends on just two constants: \[
g^{\mu\nu}\langle T_{\mu\nu}\rangle=\frac{1}{(4\pi)^2}(cF-aG)\] where $F$ is
the square of the Weyl tensor and $\int d^4x\sqrt{g} G/(4\pi)^2$ is the Euler
number. For free CFTs with $N_s$massless fields of spin $s$ \[ 720c=6N_0 +
18N_{1/2} + 72 N_1~~~~ 720a=2N_0 + 11N_{1/2} + 124N_1 \]
