  Let $R = k[x_1, \ldots, x_n]$ be a polynomial ring over a field $k$ of
characteristic zero and $\cR$ be the formal power series ring $k[[x_1, \ldots,
x_n]]$. If $M$ is a $\D$-module over $R$, then $\cR \otimes_R M$ is naturally a
$\D$-module over $\cR$. Hartshorne and Polini asked whether the natural maps
$H^i_{\dR}(M)\to H^i_{\dR}(\cR \otimes_R M)$ (induced by $M\to \cR \otimes_R
M$) are isomorphisms whenever $M$ is graded and holonomic. We give a positive
answer to their question, as a corollary of the following stronger result. Let
$M$ be a finitely generated graded $\D$-module: for each integer $i$ such that
$\dim_kH^i_{\dR}(M)<\infty$, the natural map $H^i_{\dR}(M)\to H^i_{\dR}(\cR
\otimes_R M)$ (induced by $M\to \cR \otimes_R M$) is an isomorphism.
