The *G*_{s} may be regarded as tori. The 1-dimensional torus *G*_{1} may be obtained by identifying two end-points of the unit interval 0 ≤ *x*_{1} ≤ 1 and *G*_{2} by indentifying 2 opposite sides of the unit square 0 ≤ *x*_{1} ≤ 1, 0 ≤ *x*_{2} ≤ 1. In general, *G*_{s} is obtained by identifying the 2*s* opposite surfaces of the s-dimensional unit cube, i.e., the points
$$ \left( {{x_1},...,{x_{v - 1}},0,{x_{v + 1}},...,{x_s}} \right) $$
and
$$ \left( {{x_1},...,{x_{v - 1}},1,{x_{v + 1}},...,{x_s}} \right) $$
are identified, where 1 ≤ *v* ≤ *s*.