Why the Casimir operator is proportional to the unit matrix ?

[Wonchu]

Hello,
now I'm reading Peskin Shroeder.
I have a question about the Casimir operator on page 500 in Chapter 15.

From the following eq,
$\ \ \ [T^b , T^a T^a ] = 0 \ \ \ \ \ \ \ (15.91)$
$T^2(=T^a T^a)$ is an invariant of the algebra.
Thus the author concludes that $T^2$ is proportional to the unit matrix.
Why is that ?
How to prove it ?

Please Anybody,help me !
Thanks in advance.

 

[kevinferreira]

That is called Schur's lemma. Look for it. If an element of the algebra commutes with every element it must be the proportional to the identity (as this is the unique element which has this property).

 

[Wonchu]

Now that you say that,
I recollect I also have heard about that lemma.
Now I can relate to.

Thanks a lot !