Hey guys!

I'm working through a proof from an old paper for my project and I'm really stumped with this notation. I want to understand the meaning of:

For , ,

Where the are Legendre polynomials degree .

Now I know that this is a kind of more generalised orthogonality relation and the legendre polynomials take scalar arguments (at least I think I know that...) in which case I think it would mean

For , ,

But then I dont understand the meaning of .

Ive read in an old text that used the notation "for where is the ordinary Lebesque measure", but I haven't been taught the Lebesque stuff and it looks real tricky...

So if anyone could shed a little light on whats going on, or point me to somewhere that does if its elementary (and im just being stupid) then id be so grateful!