In General Relativity, one uses the Riemann Tensor in its coordinate form $R_{abcd}$, and proves the Second Bianchi Identity-
$R_{abcd;e} + R_{abde;c} + R_{abec;d} = 0$
It is said that this identity amounts, in the Riemmanian geometry sense, to the saying that, loosely, "the boundary of the bounday is null". So my questions are
Question 1: What would be a more rigorous statement of "the boundary of the boundary is null" and how does the above identity is equivalent ot it?
Question 2: Is there an analog to this geometrical intuition in general relativity?
I assume both questions might be answered with a reference, which is even better.
Best Answer
From The "Foreword to Feynman Lectures on Gravitation" by John Preskill and Kip S. Thorne:
[cart 28] is Cartan's 1928 lectures in French about the geometry of Riemannian manifolds. Extended English version can be found in the Cartan's book "Geometry of Riemannian Spaces": http://www.amazon.com/Geometry-Riemannian-Spaces-Lie-Groups/dp/0915692341
[MTW 73] is Misner, Thorne, and Wheeler's classic book "Gravitation": http://www.amazon.com/Gravitation-Charles-W-Misner/dp/0716703440
[Whee 90] is Wheeler's book "A Journey into Gravity and Spacetime": http://www.amazon.com/Journey-Gravity-Spacetime-Scientific-American/dp/0716750163
"Feynman Lectures on Gravitation" can be found here: http://hixgrid.de/pg/file/read/3511/feynman-lectures-on-gravitation-frontiers-in-physics
The exposition in MTW is actually quite short: