# Mixed anomaly

The anomaly usually appears as a Feynman diagram with a chiral fermion running in the loop (a polygon) with n−k external gravitons and k external gauge bosons attached to the loop where $n=1+D/2$ where $D$ is the spacetime dimension. Anomalies occur only in even spacetime dimensions. For example, the anomalies in the usual 4 spacetime dimensions arise from triangle Feynman diagrams.