I'm a little bit suprised at the moment, so i'll ask here if I see this wrong:


Given a sheaf of algebras $R$ ( e.g. maximal order or Azumaya) on a smooth projective scheme $X$ with generic point $p$. Asumme $M$ and $N$ are two left $R$-modules, coherent and torsion free over $O\_X$, such that $M\_p$ and $N\_p$ are simple $R\_p$-modules. Now given a nontrivial R-morphism $f: M \rightarrow N$. I think that f is automatically injective in this case.


Injectivity can be check on the open subsets $U \subset X$, there we have $f\_U : M(U) \rightarrow N(U)$. We also have the injections $i\_{U,p} :M(U)\rightarrow M\_p$ and $j\_{U,p}: N(U)\rightarrow N\_p$ and the induced map $f\_p: M\_p \rightarrow N\_p$, with the property $j\_{U,p}\circ f\_U=f\_p\circ i\_{U,p} (1)$.


Assume $f\_p=0$, then we have $j\_{U,p}\circ f\_U=0$ by $(1)$ for every open subset $U$. But since $j\_{U,p}$ is injective, this would imply that $f\_U=0$ for every $U$ so $f=0$, which contradicts $f\neq 0$. So $f\_p: M\_p \rightarrow N\_p$ is a nontrivial map between simple modules, so it is an isomorphism, especially injective. This implies that $f\_U$ must be injective for every open subset $U \subset X$ because of $(1)$. So f is injective.


What do you think? Am I missing something?



Is every nontrivial morphism already injective in this case?
I think you are right. Another way to prove this is the following:
Let $K=\ker [f:M\to N]$ and $I={\rm im}[f:M\to N]$. 
Since $f$ is non-trivial, $I\neq 0$ and since it is torsion-free (as a subsheaf of $N$), $I\_p\neq 0$. Then $K\_p\subsetneq M\_p$, so $K\_p=0$, but then $K$ is a torsion-sheaf and hence $0$ since it is a subsheaf of the torsion-free sheaf $M$. 


