  Let $R$ be a commutative ring and $\Gamma$ a commutative monoid of finite
type. We study algebraic properties of modules and derivations over the
associated ring $\mathcal F(\Gamma,R)$ of Dirichlet convolutions. If $\Gamma$
is cancellative and $G(\Gamma)$ is its associated Grothendieck group, we
construct a natural extension $\mathcal F^f(G(\Gamma),R)$ of $\mathcal
F(\Gamma,R)$ and we study its basic properties. Further properties are
discussed in the case $\Gamma=\mathbb N^*$ and $G(\Gamma)=\mathbb Q^*_+$. In
particular, we show that $\mathcal F^f(\mathbb Q^*_+,R)\cong
R[[x_1,x_2,\ldots,]][x_1^{-1},x_2^{-1},\ldots]$.
