Where order-$k$ Voronoi diagrams partition according to the $k$ closest sites
(de Berg et al.
p. 169),
consider partitioning on the 2nd and 3rd closest —
call these [2,3]-Voronoi partitions.
Each cell is tagged with a set (unordered) of 2 sites $\{ p_i, p_j \}$.
Some pictures with Nsite = 5 random sites in the plane:
How does the number of cells in a [2,3]-Voronoi diagram vary with Nsite
and dimension $d$ in $\mathbb{R}^d$ ?
Added 18 Dec: some numbers from Monte Carlo
(despite "avoid experiment not backed up by a little theory"):
1-2-Voronoi 2d 3d 4d
----------------------
nsite 5: 7 9 9
nsite 10: 20 29 40
nsite 20: 44 79 139
2-3-Voronoi 2d 3d 4d
----------------------
nsite 5: 15 28 28
nsite 10: 57 129 246
nsite 20: 136 359 740