1. Jul 14, 2009

### kNYsJakE

1. The problem statement, all variables and given/known data
Let $$\mathrm{V}$$ be a vector space. Determine all linear transformations $$\mathrm{T}:V\rightarrow V$$ such that $$\mathrm{T}=\mathrm{T}^2$$.

2. Relevant equations
Hint was given and it was like this:
Note that $$x=\mathrm{T}(x)+(x-\mathrm{T}(x))$$ for every $$x$$ in $$V$$, and show that $$V=\{y:\mathrm{T}(y)=y\}\oplus\mathrm{N}(T)$$

3. The attempt at a solution
I tried to calculate $$T(x)$$ and $$T^2(x)$$ using $$x=T(x)+(x-T(x))$$ and put $$T$$ and $$T^2$$ equal. But, I do not think this is how to solve this problem... =(

2. Jul 14, 2009

### Office_Shredder

Staff Emeritus
Each x can be written as x = T(x) + (x-T(x)) so note that:

T(T(x)) = T(x) so T(x) is in the set {y:T(y)=y}

And T(x-T(x)) = T(x) - T2(x) = T(x) - T(x) = 0 so x-T(x) is in N(T).

So given a T, we can represent V as the direct sum of the kernel of T and the image of T. The opposite question is, given V as the direct sum of two subspaces, can we find a T such that one is the kernel and the other is the image?