On the Aharonov-Casher formula for different self-adjoint extensions of   the Pauli operator with singular magnetic field
  Two different self-adjoint Pauli extensions describing a spin-1/2 two-dimensional quantum system with singular magnetic field are studied. An Aharonov-Casher type formula is proved for the maximal Pauli extension and it is also checked that this extension can be approximated by operators corresponding to more regular magnetic fields. 
Mikael Persson