A new approach to instanton calculations in the O(3) nonlinear sigma   model
  We construct all instantons in the \nlsig\ on a cylindrical space-time, known not to exist on a finite time interval. The scale parameter, $\rho$, is related to the boundary condition in time. This may cure the $\rho\rightarrow0$ divergent instanton gas, through a proper inclusion of in and out states in the path integral. 
Jeroen Snippe and Pierre van Baal