Lemma 13.31.8. Let \mathcal{A} be an abelian category. Let
\ldots \to I_3^\bullet \to I_2^\bullet \to I_1^\bullet
be an inverse system of complexes. Assume
each I_ n^\bullet is K-injective,
each map I_{n + 1}^ m \to I_ n^ m is a split surjection,
the limits I^ m = \mathop{\mathrm{lim}}\nolimits I_ n^ m exist.
Then the complex I^\bullet is K-injective.
Proof.
We urge the reader to skip the proof of this lemma. Let M^\bullet be an acyclic complex. Let us abbreviate H_ n(a, b) = \mathop{\mathrm{Hom}}\nolimits _\mathcal {A}(M^ a, I_ n^ b). With this notation \mathop{\mathrm{Hom}}\nolimits _{K(\mathcal{A})}(M^\bullet , I^\bullet ) is the cohomology of the complex
\prod _ m \mathop{\mathrm{lim}}\nolimits \limits _ n H_ n(m, m - 2) \to \prod _ m \mathop{\mathrm{lim}}\nolimits \limits _ n H_ n(m, m - 1) \to \prod _ m \mathop{\mathrm{lim}}\nolimits \limits _ n H_ n(m, m) \to \prod _ m \mathop{\mathrm{lim}}\nolimits \limits _ n H_ n(m, m + 1)
in the third spot from the left. We may exchange the order of \prod and \mathop{\mathrm{lim}}\nolimits and each of the complexes
\prod _ m H_ n(m, m - 2) \to \prod _ m H_ n(m, m - 1) \to \prod _ m H_ n(m, m) \to \prod _ m H_ n(m, m + 1)
is exact by assumption (1). By assumption (2) the maps in the systems
\ldots \to \prod _ m H_3(m, m - 2) \to \prod _ m H_2(m, m - 2) \to \prod _ m H_1(m, m - 2)
are surjective. Thus the lemma follows from Homology, Lemma 12.31.4.
\square
Comments (3)
Comment #853 by Bhargav Bhatt on
Comment #4337 by Manuel Hoff on
Comment #4487 by Johan on
There are also: