Would you please give some references concerning the number of indecomposable modules over preprojective algebras of type $A_n$?

More precisely, I need references about the following claim: The number of indecomposable modules over the preprojective algebra of type $A_n$ is:

1) only one for $n=2$, 

2) four for $n=3$, 

3) $12$ for $n=4$, 

4) $40$ for $n=5$, and

5) an infinite number of indecomposable modules for $n\geq6$.