Figure 2

Two kinds of the finite area element of an upper-right piece. Two kinds of finite area element vectors $\overrightarrow{{\mathrm{\Delta }}_j^2{\sigma }_{j,k}}$ and $\overrightarrow{{\mathrm{\Delta }}_k^2{\sigma }_{j,k}}$ in (3.1) shown in Figure 1 are introduced as </p><p></p><p> for $j=1,2,\dots ,k$ and $k=1,2,\dots ,n$.