The integral form of APS boundary conditions in the Bag Model
  We propose an integral form of Atiah-Patodi-Singer spectral boundary conditions (SBC) and find explicitly the integral projector onto SBC for the 3-dimensional spherical cavity. After discussion of a simple example we argue that the relation between the projector and fermion propagator is universal and stays valid independently of the bag form and space dimension. 
Alexey A. Abrikosov jr., Andreas Wipf