XX Heisenberg Spin Chain and an Example of Path Integral with   "Automorphic" Boundary Conditions
  New representation for the generating function of correlators of third components of spins in the XX Heisenberg spin chain is considered in the form given by the fermionic Gaussian path integrals. A part of the discrete anti-commuting integration variables is subjected to ``automorphic'' boundary conditions in respect of imaginary time. The situation when only a part of the integration variables is subjected to the unusual boundary conditions generalizes more conventional ones when ``automorphic'' boundary conditions appear for all sites in the lattice spin models. The results of the functional integration are expressed as determinants of the matrix operators. The generating function, as well as the partition function of the model, are calculated by means of zeta-regularization. Certain correlation functions at nonzero temperature are obtained explicitly. 
Cyril Malyshev