# Notation in Srednicki's QFT

In the book Quantum Field Theory by Srednicki, equation 21 for the commutators of the generators of the Lorentz group is

$$[M^{\mu\nu},M^{\rho\sigma}]=i\hbar\Big(g^{\mu\rho}M^{\nu\sigma}-(\mu\leftrightarrow\nu)\Big)-(\rho\leftrightarrow\sigma).$$

Clearly the $\leftrightarrow$ symbol has something to do with the indices, however I'm not sure exactly what it means. Is this notation standard and if so what does it mean?

It means "repeat the terms I've written, but with these indices swapped". So in your case, $$i\hbar\Big(g^{\mu\rho}M^{\nu\sigma}-(\mu\leftrightarrow\nu)\Big)-(\rho\leftrightarrow\sigma)$$ is short for $$i\hbar\Big(g^{\mu\rho}M^{\nu\sigma}-g^{\nu\rho}M^{\mu\sigma}\Big)-i\hbar\Big(g^{\mu\sigma}M^{\nu\rho}-g^{\nu\sigma}M^{\mu\rho}\Big).$$