文摘
For each Poincaré duality group 螕 there exists a class, which we call the tangential Thom class of 螕, in the group cohomology of 螕×螕 with a right choice of the coefficient module. The class has the crucial properties, even if stated in a purely algebraic language, which correspond to those of Thom class of the tangent bundle of a closed manifold. In particular the Thom isomorphism has been proved to exist by observing that certain two sequences of homological functors, one being the homology of 螕 and the other that of 螕×螕, being regarded as functors defined on the category of Z螕-modules are homological and effaceable.