\prod _{i=0}^{k}\left( \frac{\exp {\left( -\frac{\varepsilon _i\left| {q\left( I\right) }_i-s_i\right| }{{\nabla q}_i}\right) }}{\exp {\left( -\frac{\varepsilon _i\left| {q\left( I'\right) }_i-z_i\right| }{{\nabla q}