  The periodic boundary conditions changed the plane square-lattice Ising model
to the torus-lattice system which restricts the spin-projection orientations.
Only two of the three important spin-projection orientations, parallel to the
x-axis or to the y-axis, are suited to the torus-lattice system. The
infinitesimal difference of the free-energies of the systems between the two
systems mentioned above makes their critical temperatures infinitely close to
each other, but their topological fundamental groups are distinct.
