# Thread: invertible operator

1. ## invertible operator

I'm stuck on this h/w problem...any help would be appreciated!!! thank u

Let V be a vector space of dimension n, and T: V --> V an invertible operator. Prove that if U is a T-invarient subspace of V, then U is also invariant under T^-1. Justify each step.

2. What ideas have you had so far?

3. Well I don't know too much about T-invarient subspaces, but I figured it means eigenvalues. So here is my prrof so far-

First suppose that U is an eigenvalue of T. Thus there exists a nonzero vector v exists in V such that: Tv=Uv
Appyling T^-1 to both sides of the equation above and we get v=UT^-1v, which is equivantly to the equation T^-1v=(1/U)v. Thus (1/U) is an eigenvalue of T^-1. Therefore proving that if U is a T-invarient subspace of V, then U is also invariant under T^-1.

What do u think??
Thank you.

4. No, I don't think you understand what's going on. A $T$-invariant subspace $U$ is a subspace of $V$ that remains the same under action from $T$. That is, $T(U) \subseteq U$. This relation says that $\forall\,u\in U,$ it is the case that $T(u)\in U.$

Can you write down, now, a mathematical statement that represents what you need to prove?

5. ok that helps thank you!! I will play around with the proof now.

6. Ok, let me know how it goes.