Wednesday, 14 September 2011

Is there an additive model of the stable homotopy category?

The answer is: no there isn't such a thing. Here is a rough argument (a full proof would deserve a little more care).



Using the main result of



S. Schwede, The stable homotopy category is rigid, Annals of Mathematics 166 (2007), 837-863



your question is equivalent to the following: does there exist a model category C, which is additive, and such that C is Quillen equivalent to the usual model category of spectra?



In particular, we might ask: does there exist an additive category C, endowed with a Quillen stable model category structure, such that the corresponding stable (infty,1)-category is equivalent to the stable (infty,1)-category of spectra?



Replacing C by its full subcategory of cofibrant objects, your question might be reformulated as: does there exist a category of cofibrant objects C (in the sense of
Ken Brown), with small sums (and such that weak equivalences are closed under small sums), and such that the corresponding (infty,1)-category (obtained by inverting weak equivalence of C in the sense of (infty,1)-categories) is equivalent to the stable (infty,1)-category of spectra? If the answer is no, then there will be no additive model category C such that Ho(C) is (equivalent to) the category of spectra (as a triangulated category).



So, assume there is an additive category of cofibrant objects C, with small sums, such that Ho(C) is (equivalent to) the category S of spectra (as a triangulated category). Let Cf be the full subcategory of C spanned by the objects which correspond to finite spectra in S. Then Ho(Cf)simeqSf, where, by abuse of notations, Ho(Cf) is the (infty,1)-category obtained from Cf by inverting weak equivalences, while Sf stands for the stable (infty,1)-category of finite spectra (essentially the Spanier-Whitehead category of finite CW-complexes). Given any (essentially) small additive category A denote by K(A) the "derived (infty,1)-category of A" (that is the (infty,1)-category obtained from the category of bounded complexes of A, by inverting the chain homotopy equivalences). Then, the canonical functor AtoK(A) (which sends an object X to itself, seen as a complex concentrated in degree 0), has the following universal property: given a stable (infty,1)-category T, any functor AtoT which sends split short exact sequences of A to distinguished triangles (aka homotopy cofiber sequences) in T extends uniquely into a finite colimit preserving functor K(A)toT. In particular, the functor CftoHo(Cf)simeqSf extends uniquely to a finite colimit preserving functor F:K(Cf)toSf. Let Ker(F) be the full (infty,1)-subcategory of K(Cf) spanned by objects which are sent to zero in Sf. Then the induced functor
K(Cf)/Ker(F)toSf


is an equivalence of (stable) (infty,1)-categories (to see this, you may use the universal property of Sf: given a stable (infty,1)-category T, a finite colimit preserving functor SftoT is the same as an object of T; see Corollary 10.16 in DAG I). This implies that, for any object X of Sf, if X/n denotes the cone of the map n:XtoX (multiplication by an integer n), then n.X/nsimeq0 (see Proposition 1 in Schwede's paper Algebraic versus topological triangulated categories). But such a property is known to fail whenever X is a finite spectrum for n=2 (see Proposition 2 in loc. cit.). Hence there isn't such a C...

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