This question is entirely on tensorial notation in Wald's General Relativity. When specifying the properties of the Riemann tensor on pg39, he states:
$R_{[abc]}^{\quad \ \ \ d} = 0$
and
For the derivative operator $\nabla_a$ naturally associated with the metric, $\nabla_a g_{bc}=0$, we have $R_{abcd} = -R_{abdc}$.
and
The Bianchi identity holds: $\nabla_{[a}R_{bc]d}^{\quad \ \ e} = 0$
Questions:
What do the square brackets around "abc" mean?
Why does $R_{abc}^{\quad d}$ become $R_{abcd}$? What is the relation between the two?
What does having $R$ in the square brackets mean?
Thank you.