Friday, 2 January 2009

homological algebra - Colimit of intersections

Let Bpi be a family of sets, where pinmathbbN and iinI, I being a directed set, and such that, for every i, we have a descending chain of inclusions



dotssupsetBp1isupsetBpisupsetBp+1isupsetdots



Question: is the following implication true?



bigcappBpi=emptyset,textforalliquadLongrightarrowquadbigcappvarinjlimiBpi=emptyset.



Since bigcap is a limit, this seems a problem of an interchange of limits and filtered colimits and indeed there is a universal map



varphi:varinjlimibigcappBpilongrightarrowbigcappvarinjlimiBpi



If varphi was a bijection, then my implication would be true with no doubts,
but, since the intersection is not finite, I can not say that varphi is a bijection. Nevertheless, could my implication still be true, without varphi being a bijection?



The reason behind my question is the following: let (Ai,Fi) be a directed family of filtered sets (or abelian groups, or modules; in fact, in my problem they are cochain complexes). Since filtered colimits (direct limits) are exact, you can define a filtration on the colimit like this:



FpvarinjlimiAi=varinjlimiFpiAi.



Now assume all the filtrations Fi are Hausdorff; that is, bigcappFpiAi=0 for all i. Is it then necessarily true that the filtration F on varinjlimiAi is Hausdorff too?



This question is a sequel to my previous question Convergence of right half-plane spectral sequence bounded on the right . Despite Tilman's counterexemple to my guess there, I think I've managed almost to prove it because my spectral sequences are right half-plane and this is the final detail I need.

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