# Linear Algebra: Linear Transformation Problem

1. Oct 29, 2008

### Ertosthnes

1. The problem statement, all variables and given/known data
Let T$$\in$$L(V,V). Prove that T$$^{2}$$=0 iff T(V)$$\subset$$n(T).

2. Relevant equations
dim T(V) + dim n(T) = dim V comes to mind.

3. The attempt at a solution
Honestly, I don't know where to start. I have no idea what I'm doing. My book and my professor are both utterly useless and I'm frustrated at how badly I am failing at this.

2. Oct 29, 2008

### HallsofIvy

Staff Emeritus
Does your book have any definitions? For example does it give a definition of "n(T)"? You did not here, but I am going to assume that n(T) is the null space of T: the set of all vectors, v, such that T(v)= 0. One direction should be obvious. If T(v)= 0, then T2(v)= T(T(v))= T(0)= 0. Now the other way. Suppose that T2(v)= T((T(v))= 0. What does that tell you about the T(V)?

3. Oct 29, 2008

### Ertosthnes

Ah yes. n(T) is the null space of T.

Okay, suppose T$$^{2}$$(V) = T(T(V)) = 0:

Let another vector space W = T(V), so T(W) = 0. Then W $$\subset$$ n(T). Substituting we obtain T(V) $$\subset$$ n(T).

Does that work?

4. Oct 29, 2008

### HallsofIvy

Staff Emeritus
Strictly speaking, either T(W)= {0} or T(v)=0 for all v in W. It might be better to say "if v is in T(V) then v= T(u) for some u in V. Then T(v)= T(T(u))= T2(u).

That proves "If T2= 0, then T(V)$\subset$n(T). But you still need to prove "If T(V)$\subset$n(T) then If T2= 0". Of course, that's pretty easy.