  Let $F$ be a differential field of characteristic zero with algebraically
closed constant field $C$. Let $E$ be a Picard--Vessiot closure of $F$, $R
\subset E$ its Picard--Vessiot ring and $\Pi$ the differential Galois group of
$E$ over $F$. Let $V$ be a differential $F$ module, finite dimensional as an
$F$ vector space. Then $V$ is singly generated as a differential $F$ module if
and only if there is a $\Pi$ module injection $\text{Hom}_F^\text{diff}(V,R)
\to R$. If $C \neq F$ such an injection always exists.
