Support and adic finiteness for complexes

Abstract

Let $X$ be a chain complex over a commutative noetherian ring $R$, that is, an object in the derived category $\mathcal{D}(R)$. We investigate the small support and co-support of $X$, introduced by Foxby and Benson, Iyengar, and Krause. We show that the derived functors $M \otimes_R^{\mathbf{L}} -$ and $\mathbf{R}\operatorname{Hom}_R(M,-)$ can detect isomorphisms in $\mathcal{D}(R)$ between complexes with restrictions on their supports or co-supports. In particular, the derived local (co)homology functors $\mathbf{R}\Gamma_{\mathfrak{a}}(-)$ and $\mathbf{L}\Lambda_{\mathfrak{a}}(-)$ with respect to an ideal $\mathfrak{a}\subsetneq R$ have the same ability. Furthermore, we give reprove some results of Benson, Iyengar, and Krause in our setting, with more direct proofs. Also, we include some computations of co-supports, since this construction is still quite mysterious. Lastly, we investigate "$\mathfrak{a}$-adically finite" $R$-complexes, that is, the $X\in\mathcal{D}(R)$ that are $\mathfrak{a}$-cofinite \textit{\`a la} Hartshorne. For instance, we characterize these complexes in terms of a finiteness condition on $\mathbf{L}\Lambda_{\mathfrak{a}}(X)$.