'Symmetrisation Notation' - What is it?

[InsertName]

I'm reading Hobson, Efstathiou and Lasenby, where on page 490, some notation is used that I am not familiar with.

In the rather long equation (17.62), some terms contain things like

$\partial_{(\mu} h^{\rho}_{\nu)}$.

The author(s) then says "where we have made use of the symmetrisation notation discussed in Chapter 4."

I've tried looking in Chapter 4 but it's not at all clear what is going on, so I came here :)

 

[Bill_K]

The indices within the parentheses are understood to be totally symmetrized. So

∂_(μh^ρ_ν) = ½(∂_μh^ρ_ν + ∂_νh^ρ_μ)

 

[InsertName]

Thanks Bill_K!