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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.