On a Projective Space Invariant of a Co-torsion Module of Rank Two over a Dedekind Domain

Abstract: For a Dedekind domain $\mathcal{O}$ and a rank two co-torsion module $M\subseteq \mathcal{O}^2$ with invariant factor ideals $\mathcal{L}\supseteq \mathcal{K}$ in $\mathcal{O}$, that is, $\frac{\mathcal{O}^2}{M}\cong \frac{\mathcal{O}}{\mathcal{L}}\oplus \frac{\mathcal{O}}{\mathcal{K}}$ we associate a new projective space invariant element in $\mathbb{PF}^1_{\mathcal{I}}$ where $\mathcal{I}$ is given by the ideal factorization $\mathcal{K} = \mathcal{L}\mathcal{I}$ in $\mathcal{O}$. This invariant element along with the invariant factor ideals determine the module $M$ completely as a subset of $\mathcal{O}^2$. As a consequence, projective spaces associated to ideals in $\mathcal{O}$ can be used to enumerate such modules. We compute the zeta function associated to such modules in terms of the zeta function of the one dimensional projective spaces for the ring $\mathcal{O}K$ of integers in a number field $K/\mathbb{Q}$ and relate them to Dedekind zeta function. Using the projective spaces as parameter spaces, we re-interpret the Chinese remainder reduction isomorphism $\mathbb{PF}^1{\mathcal{I}} \rightarrow \underset{i=1}{\overset{l}{\prod}} \mathbb{PF}^1_{\mathcal{I}_i}$ associated to a factorization of an ideal $\mathcal{I}=\underset{i=1}{\overset{l}{\prod}} \mathcal{I}_i$ into mutually co-maximal ideals $\mathcal{I}_i,1\leq i\leq l$ in terms of the intersection of associated modules arising from the projective space elements.