# Math Help - Matrix and endomorfism

1. ## Matrix and endomorfism

Hello

Prove that a matrix associated with an endomorfism $T$ has inverse if and only if $T$ is injective.

Thanks a lot¡¡

2. Originally Posted by osodud
Hello

Prove that a matrix associated with an endomorfism $T$ has inverse if and only if $T$ is injective.
Well, a matrix $T$ has an inverse if and only if...what? There are so many properties that finish off this sentence, but pick your favourite. Does this property tell you anything about the matrix begin injective? Surjective?
For instance, if you go for the obvious property, $T$ is invertible if and only if there exists a matrix $S$ such that $ST=I=TS$. Then, suppose $T$ is not injective. that means there exists some non-zero vector which is mapped to zero (that is to say, $ker(T) \neq \{0\}$). Call this vector $v$. Then $vI=v$ but $vTS = 0S=0$ an contradiction. Thus the matrix must be injective.