The cancellation property for projective modules over integral group rings

We obtain a partial classification of the finite groups $G$ for which the integral group ring $\mathbb{Z}G$ has projective cancellation, i.e. for which $P\oplus \mathbb{Z}G\cong Q\oplus \mathbb{Z}G$ implies $P\cong Q$ for projective $\mathbb{Z}G$-modules $P$ and $Q$. In particular, we determine when projective cancellation holds for a finite group with no exceptional binary polyhedral quotients. To do this, we prove a cancellation theorem based on a relative version of the Eichler condition. We then use a group theoretic argument to precisely determine the class of groups not covered by this result. The final classification is then obtained by applying results of Swan, Chen and Bley-Hofmann-Johnston which show failure of projective cancellation for certain groups.