14 FIG. 1: Basic cells of the WB family of hierarchical lattices, with a scaling factor b = 2. In each case, the empty circles ( µ and ν ) represent the external sites of the cell, whereas the black circles are internal sites to be decimated in the RG procedure. (a) The WB cell of fractal dimension D = (ln 5) / (ln 2) ≈ 2 . 32; (b) The WB cell of fractal dimension D = (ln 12) / (ln 2) ≈ 3 . 58; (c) The WB cell of fractal dimension D = (ln 28) / (ln 2) ≈ 4 . 81. For clearness, all the bonds of cell (c) are not shown explicitly; the external site µ is connected directly to sites 5 , 6 , 7 , and 8 (as illustrated by the bond connecting sites µ and 5, whereas site ν is connected directly to sites 1 , 2 , 3 , and 4 (as illustrated by the bond connecting sites ν and 3).