# LA - Identity Maps and Injectivity

1. Jul 17, 2011

### MushroomPirat

1. The problem statement, all variables and given/known data
Suppose that $W$ is finite dimensional and $T:V\rightarrow W$. Prove that $T$ is injective if and only if there exists $S:W\rightarrow V$ such that $ST$ is the identity map on $V$.

2. Relevant equations

3. The attempt at a solution

First, suppose that $T$ is injective and let $Tu=Tv$ for $u,v\in V$. Clearly, $Tu-Tv=0$ and thus $S(Tu-Tv)=0$. From this, we can see that $STu=STv$. However, since $T$ is injective, then $u=v$. Therefore, there exists an $S$ such that $ST$ is the identity map on $V$. In the other direction, suppose $ST$ is the identity map on $V$, and let $Tu=Tv$. From the previous argument, we can see that $STu=STv$, and thus $u=v$, so $T$ is injective.

I think the second part of my proof is right, going from identity map to injectivity, but I'm just not sure about my argument for the first half.

2. Jul 18, 2011

### micromass

Staff Emeritus
I can't see how you concluded that. The only thing you can conclude is that STu=STu. The thing you must show is that STu=u!!
In fact, you haven't even defined a suitable S!! You must first define a candidate S, and only then can you show that ST is the identity. So, how would you define such an S?

This is ok.

3. Jul 22, 2011

### MushroomPirat

Suppose T is injective and let $(v_1,...,v_n)$ be a basis of V. Because this basis is linearly independent and T is injective, then $(Tv_1,...,Tv_n)$ is also linearly independent (this was a result from a previous problem). Because T is injective, then dim V = dim rangeT $\leq$ dimW. Extend the previous list to a basis $(Tv_1,...,Tv_n,w_1,...,w_m)$ of W. Now, define $S: W\rightarrow V$ such that $STv_i=v_i$ for all i=1,...,n, and $Sw_j=0$ for all j=1,...,m. Thus, if u is an arbitrary element of V, written as $u=a_1v_1+...+a_nv_n$ then $STu=ST(a_1v_1+...+a_nv_n)=a_1v_1+...+a_nv_n$ so ST must be the identity map on V.