Asian Journal of Mathematics

Volume 17 (2013)

Number 3

A combinatorial invariant for spherical CR structures

Pages: 391 – 422



Elisha Falbel (Institut de Mathématiques, Université Pierre et Marie Curie, Paris, France)

Qingxue Wang (School of Mathematical Sciences, Fudan University, Shanghai, China)


We study a cross-ratio of four generic points of $S^3$ which comes from spherical CR geometry. We construct a homomorphism from a certain group generated by generic configurations of four points in $S^3$ to the pre-Bloch group $\mathcal{P}(\mathbb{C})$. If $M$ is a 3-dimensional spherical CR manifold with a CR triangulation, by our homomorphism, we get a $\mathcal{P}(\mathbb{C})$-valued invariant for $M$. We show that when applying to it the Bloch-Wigner function, it is zero. Under some conditions on $M$, we show the invariant lies in the Bloch group $\mathcal{B}(k)$, where $k$ is the field generated by the cross-ratio. For a CR triangulation of the Whitehead link complement, we show its invariant is a torsion in $\mathcal{B}(k)$ and for a triangulation of the complement of the 52-knot we show that the invariant is not trivial and not a torsion element.


Bloch group, cross-ratio, spherical CR structure

2010 Mathematics Subject Classification

19M05, 57M27, 57M50

Published 16 October 2013