Is there any properties with the curvature tensors in 3 dimensions?
(Maybe between the Ricci tensor and the Ricci scalar, they are proportional to each other? )
I heard about it in a lecture, but I can not remember the details. The 3 dimensional case is not discussed in many reference books...
There is a book which talks about the tools for practical computation,
"A relativist's toolkit:
the mathematics of black-hole mechanics "
I am now reading it, and I think it is very helpful~
In Jacobson's famous paper "Thermodynamics of Spacetime: The Einstein Equation of State" (gr-qc/9504004) Phys. Rev. Lett. 75, 1260–1263 (1995) , he wrote the Raychaudhuri equation as (Eq.(4) in his paper):
\frac{d\theta}{d\lambda}=-\frac{1}{2}\theta^{2}-\sigma^{2}-R_{ab}k^{a}k^{b}
However...