• Let $\mathcal{D}$ be the $n$th Weyl algebra $\mathcal{D} :=k[x_1,...,x_n,\partial_1,...,\partial_n]$, where $\partial_ix_i-x_i\partial_i=1$.
• Let $\widetilde{\mathcal{D}}$ be its Rees algebra, which is $\mathcal{D} :=k[t, x_1,...,x_n,\partial_1,...,\partial_n]$, where $\partial_ix_i-x_i\partial_i=t$, and $t$ is central.