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    • CommentRowNumber1.
    • CommentAuthorUrs
    • CommentTimeJul 12th 2016

    I have added to the entry split idempotent the statement (here) that in a triangulated category in which the direct sum of two triangles is a triangle, then idempotents split.

    (Maybe that should rather go into the entry Cauchy complete category?)

    • CommentRowNumber2.
    • CommentAuthorUrs
    • CommentTimeJul 12th 2016
    • (edited Jul 12th 2016)

    I have used this to write out a full proof (here) of the “Adams universal coefficient theorem”, namely the statement that for EE a homotopy ring spectrum such that E (E)E_\bullet(E) is flat over π (E)\pi_\bullet(E) and XX a spectrum such that E (X)E_\bullet(X) is projective over π (E)\pi_\bullet(E), then

    [X,EY] Hom E (E) (E (X),E (Y)) [X, E \wedge Y]_\bullet \overset{\simeq}{\longrightarrow} Hom^\bullet_{E_\bullet(E)}( E_\bullet(X), E_\bullet(Y) )

    is an isomorphism, where on the right we have the graded hom of comodule homomorphisms over the dual EE-Steenrod algebra.

    The proof in Adams 74, p. 323 of this fact invokes the stronger statement that under these conditions and for ZZ any homotopy EE-module spectrum then also

    [X,Z] Hom π (E)(E (X),π (Z)) [X, Z]_\bullet \simeq Hom_{\pi_\bullet(E)}( E_\bullet(X), \pi_\bullet(Z) )

    is an isomorphism (where what is needed for the above is only the case that ZZ is a free module spectrum EYE \wedge Y ), but this stronger statement (which is really what Adams calles the UCT) holds only under much more restrictive conditions. The general proof of the above I am deducing from Schwede 12, chapter II, prop. 6.20, which however at the end is vague about how the conclusion via idempotent splitting in Ho(Spectra)Ho(Spectra) follows. (It looks like there may have been a copy-and-pasting mismatch in the compilation of that proof.) It’s precisely that result by Bökstedt-Neemn in #1 above which concludes that proof.

  1. The proof in Bokstedt-Neeman of idempotent completeness for triangulated categories appeals to the existence and exactness of arbitrary direct sums, not just binary direct sums, since it uses totalizations to construct the splittings. The proposition is false under the weaker hypothesis. Remark 3.2 in Schnurer’s “Homotopy categories and idempotent completeness, weight structures and weight complex functors” gives a counter-example.


    diff, v14, current

    • CommentRowNumber4.
    • CommentAuthorUrs
    • CommentTimeFeb 3rd 2019

    thanks for fixing the prop, but then we should also point to that counterexample, for good measure – have added the remark here

    diff, v15, current

    • CommentRowNumber5.
    • CommentAuthorKeith Harbaugh
    • CommentTimeJul 5th 2022

    Added reference to biproducts.

    diff, v16, current

    • CommentRowNumber6.
    • CommentAuthormaxsnew
    • CommentTimeAug 19th 2022

    convenient redirect

    diff, v17, current

  2. convenient redirect

    Milo Moses

    diff, v18, current

    • CommentRowNumber8.
    • CommentAuthorvarkor
    • CommentTimeNov 3rd 2023

    Added some references to split idempotents in Kleisli categories.

    diff, v20, current

    • CommentRowNumber9.
    • CommentAuthorvarkor
    • CommentTimeNov 15th 2023

    Added a reference to Enveloppe Karoubienne de catégories de Kleisli.

    diff, v21, current