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The question is very simple and I couldn't find an answer.

What the notation $\gamma^{ [ \mu} \gamma^{\nu} \gamma^{\rho ]}$ and $\gamma^{ [ \mu} \gamma^{\nu} \gamma^{\rho} \gamma^{\sigma ]}$ means?

(From Peskin and Schroeder page 49)

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It means to antisymmetrize on the indices inside the brackets: Form all permutations, multiply each term by plus or minus one depending on whether the permutation is even or odd, and add the terms together. So the first expression represents 3!=6 terms and the second 4!=24 terms.

If you see curly braces instead of square brackets, it means to symmetrize. Add up all permutations, but with a plus sign.

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