Some Characterizations of Relative Sequentially Cohen-Macaulay and Relative Cohen-Macaulay Modules

Abstract

Let $M$ be an $R$-module over a Noetherian ring $R$ and $\mathfrak{a}$ be an ideal of $R$ with $c={\rm cd}(\mathfrak{a},M)$. First, we prove that $M$ is finite $\mathfrak{a}$-relative Cohen-Macaulay if and only if ${\rm H}_i(\Lambda_{\mathfrak{a}}({\rm H}_{\mathfrak{a}}^c(M)))=0$ for all $i\neq c$ and ${\rm H}_c(\Lambda_{\mathfrak{a}}({\rm H}_{\mathfrak{a}}^c(M))) \cong \widehat{M}^{\mathfrak{a}}$. Next, over an $\mathfrak{a}$-relative Cohen-Macaulay local ring $(R,\mathfrak{m})$, we provide a characterization of $\mathfrak{a}$-relative sequentially Cohen-Macaulay modules $M$ in terms of $\mathfrak{a}$-relative Cohen-Macaulayness of the $R$-modules ${\rm Ext}^{d-i}_{R}(M,{\rm D}_{\mathfrak{a}})$ for all $i\geq 0$, where ${\rm D}_{\mathfrak{a}} = {\rm Hom}_R({\rm H}^d_{\mathfrak{a}}(R),{\rm E}(R/\mathfrak{m}))$ and $d={\rm cd}(\mathfrak{a},R)$. Finally, we provide another characterization of $\mathfrak{a}$-relative sequentially Cohen-Macaulay modules $M$ in terms of vanishing of the local homology modules ${\rm H}_j(\Lambda_{\mathfrak{a}}({\rm H}_{\mathfrak{a}}^i(M)))=0$ for all $0\leq i\leq c$ and for all $j\neq i$.