Why Does the Divergence of aRab + bRgab Vanish at b = -1/2a?

solveforX
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I'm reading gravitation and having trouble with one of the exercises.

aRab+bRgab is the general tensor

the exercise asks to show that the divergence of this tensor vanishes if and only if b=-1/2a
how do I go about solving this problem?
 
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solveforX said:
I'm reading gravitation and having trouble with one of the exercises.

aRab+bRgab is the general tensor

the exercise asks to show that the divergence of this tensor vanishes if and only if b=-1/2a
how do I go about solving this problem?

Check this out: http://mathworld.wolfram.com/ContractedBianchiIdentities.html"
 
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