English

Roos axiom holds for quasi-coherent sheaves

Algebraic Geometry 2026-02-20 v4 Category Theory

Abstract

Let XX be either a quasi-compact semi-separated scheme, or a Noetherian scheme of finite Krull dimension. We show that the Grothendieck abelian category XQcohX{-}\mathsf{Qcoh} of quasi-coherent sheaves on XX satisfies the Roos axiom AB4\mathrm{AB}4^*-nn: the derived functors of infinite direct product have finite homological dimension in XQcohX{-}\mathsf{Qcoh}. In each of the two settings, two proofs of the main result are given: a more elementary one, based on the Cech coresolution, and a more conceptual one, demonstrating existence of a generator of finite projective dimension in XQcohX{-}\mathsf{Qcoh} in the semi-separated case and using the co-contra correspondence (with contraherent cosheaves) in the Noetherian case. The hereditary complete cotorsion pair (very flat quasi-coherent sheaves, contraadjusted quasi-coherent sheaves) in the abelian category XQcohX{-}\mathsf{Qcoh} for a quasi-compact semi-separated scheme XX is discussed.

Keywords

Cite

@article{arxiv.2407.13651,
  title  = {Roos axiom holds for quasi-coherent sheaves},
  author = {Leonid Positselski},
  journal= {arXiv preprint arXiv:2407.13651},
  year   = {2026}
}

Comments

LaTeX 2e, 27 pages; v.2: the first version v1 corresponds to Section 3 of the updated version v2; new Sections 1, 2, and 4 added; v.3: several misprints corrected, references updated; v.4: several misprints corrected