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The flat model structure on
Authors:James Gillespie
Affiliation:Department of Mathematics, 4000 University Drive, Penn State--McKeesport, McKeesport, Pennsylvania 15132-7698
Abstract:Given a cotorsion pair $(mathcal{A},mathcal{B})$ in an abelian category $mathcal{C}$ with enough $mathcal{A}$ objects and enough $mathcal{B}$ objects, we define two cotorsion pairs in the category $mathbf{Ch(mathcal{C})}$ of unbounded chain complexes. We see that these two cotorsion pairs are related in a nice way when $(mathcal{A},mathcal{B})$ is hereditary. We then show that both of these induced cotorsion pairs are complete when $(mathcal{A},mathcal{B})$ is the ``flat' cotorsion pair of $R$-modules. This proves the flat cover conjecture for (possibly unbounded) chain complexes and also gives us a new ``flat' model category structure on $mathbf{Ch}(R)$. In the last section we use the theory of model categories to show that we can define $operatorname{Ext}^n_R(M,N)$using a flat resolution of $M$ and a cotorsion coresolution of $N$.

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