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 Z G has projective cancellation, i.e. for which P ⊕ Z G Q ⊕ Z G implies P Q for projective Z G-modules P and Q. To do this, we prove a cancellation theorem based on a relative version of the Eichler condition and use it to establish the cancellation property for a large class of finite groups. 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 the cancellation property for certain groups.