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Cofiniteness with respect to a Serre subcategory
Authors:A Hajikarimi
Institution:1. Mobarakeh Branch, Islamic Azad University, Isfahan, Iran
Abstract:Let Φ be a system of ideals in a commutative Noetherian ring R, and let ></img>                              </span> be a Serre subcategory of <em>R</em>-modules. We set <span class= $$ H_\Phi ^i ( \cdot , \cdot ) = \mathop {\lim }\limits_{\overrightarrow {\mathfrak{b} \in \Phi } } Ext_R^i (R/\mathfrak{b}| \otimes R \cdot , \cdot ). $$ . Suppose that a is an ideal of R, and M and N are two R-modules such that M is finitely generated and N ></img>                              </span>. It is shown that if the functor <span class= $ D_\Phi ( \cdot ) = \mathop {\lim }\limits_{\overrightarrow {\mathfrak{b} \in \Phi } } Hom_R (\mathfrak{b}, \cdot ) $ is exact, then, for any $ \mathfrak{b} \in \Phi ,Ext_R^j (R/\mathfrak{b},H_\Phi ^i (M,N)) $ ></img>                              </span> for all <em>i, j</em> ≥ 0. It is also proved that if there is a nonnegative integer <em>t</em> such that <span class= $ H_\mathfrak{a}^i (M,N) $ ></img>                              </span> for all <em>i</em> < <em>t</em>, then <span class= $ Hom_R (R/\mathfrak{a},H_\mathfrak{a}^t (M,N)) $ ></img>                              </span>, provided that <span class= ></img>                              </span> is contained in the class of weakly Laskerian<em>R</em>-modules. Finally, it is shown that if <em>L</em> is an <em>R</em>-module and <em>t</em> is the infimum of the integers <em>i</em> such that <span class= $ H_\mathfrak{a}^i (L) $ ></img>                              </span>, then <span class= $ Ext_R^j (R/\mathfrak{a},H_\mathfrak{a}^t (M,L)) $ ></img>                              </span> if and only if <span class= $ Ext_R^j (R/\mathfrak{a},Hom_R (M,H_\mathfrak{a}^t (L))) $ ></img>                              </span> for all <em>j</em> ≥ 0.</td>
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