ABSTRACT
The twisted Dirac operators involved in the APS theorem, as stated in the Introduction, are defined in terms of a spin structure on the manifold. This is now discussed, as is the more general notion of a Dirac operator associated to a Clifford module with connection. Needless to say these concepts are extended to the case of compact manifolds with boundary equipped with ò-metrics. 3.1. Euclidean Dirac operator.
