We give a simple formula for the volume and Chern-Simons invariant of a PSL(2, C )-representation of a tame 3-manifold. If the representation is the geometric representation of a hyperbolic 3-manifold, our formula gives the volume and Chern-Simons invariant directly from an ideal triangulation without the use of additional combinatorial topology. In particular, our formula computes the Chern-Simons invariant just as fast as the volume.
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