The Bloch-Wigner function $D(z)$, gives the volume of a ideal tetrahedron in hyperbolic space $\mathbb H3$, where z is the cross-ratio (z1,z2,z3,z4) parametrizing the tetrahedron in $\mathbb CP1$.
$D(z)= \tilde D(z1,z2,z3,z4)$
Relating to this, the five-term relation for the dilogarithm, could be interpreted, as the fact that the signed sum of some volume of tetrahedrons, is null :
$$\sum^4_{i=0} (-1)^i \tilde D(z_0, ...., \hat z_i, ... z_4) = 0$$
Here the $z_i$ are 5 points in $\mathbb CP1$, and the notation $\hat z_i$ means that we don't take the vertex $z_i$ in account. The above equation looks like some function of a boundary of some 5-simplex.
But what is this 5-simplex (which, I think, corresponds to a 4-volume), and in wich which space this simplex exists (hyperbolic space ?) ?
Reference (Zagier) : http://maths.dur.ac.uk/~dma0hg/dilog.pdf (Pages 10 - 11)
