# Prove null is invariant

1. Feb 21, 2008

### pte419

Suppose S, T $$\in$$ L(V) are such that ST=TS. Prove that null(T-$$\lambda$$I) is invariant under S for every $$\lambda$$ $$\in$$ F.

I can't find anything helpful in my book, I'm not sure where to start...

2. Feb 21, 2008

### Dick

null(T-lambda*I) is just the set of all vectors x such that (T-lambda*I)x=0. So Tx-lambda*x=0. Or Tx=lambda*x. To show this is invariant under S you just have to show that if x satisfies that property, then so does Sx.