The 'n-equivalences' of boundary conditions of lattice models are introduced
and it is derived that the models with n-equivalent boundary conditions result
in the identical free energy. It is shown that the free energy of the
six-vertex model is classified through the density of left/down arrows on the
boundary. The free energy becomes identical to that obtained by Lieb and
Sutherland with the periodic boundary condition, if the density of the arrows
is equal to 1/2. The relation to the structure of the transfer matrix and a
relation to stochastic processes are noted.Comment: 6 pages with a figure, no change but the omitted figure is adde