Fig. 19.

Example of bisection for $\widehat{d}=3$. The initial element of level 0 is bisected in the x-direction ($\mathrm{dir}(0)=1$) to obtain two elements of level 1. These are then bisected in the y-direction ($\mathrm{dir}(1)=2$) to obtain four elements of level 2, which are bisected in the z-direction ($\mathrm{dir}(2)=3$) to get eight elements of level 3