Quantum anomalies from three-point on-shell bootstrap

We bootstrap quantum anomalies using on-shell techniques in the simplest setting: at three points. The necessary field-theoretic input includes the local or global classical symmetry, which the anomaly may break, and the symmetries of the effective action -- though not its explicit form. We use massless helicity spinors together with an on-shell representation of the discrete C, P, and T transformations. Our approach determines the Weyl, chiral (global and local), diffeomorphism, and Lorentz anomalies up to real constant prefactors. In particular, we recover gauge-anomaly cancelation conditions, as well as find that the Weyl anomaly should not involve Pontryagin densities.