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FALSE: any sequence of functors with long exact sequences is a delta functor
Statement
False. Any family of functors that sends every short exact sequence to a long exact sequence is automatically a delta functor.
Facts & Assumptions
Given: The homology delta functor on complexes and one short exact sequence of complexes whose connecting map is nonzero.
A delta functor requires naturality of the connecting maps, not only exactness of the long sequence (Homological delta functor, Cohomological delta functor).
Homology of complexes is a genuine homological delta functor (Homology of complexes satisfies the delta-functor naturality and exactness laws).
There exists a short exact sequence of complexes with a nonzero connecting map (A degreewise split sequence with nonzero connecting map).
Refutation
Start from the homology delta functor of [L2]. Keep all functors and all connecting maps unchanged except on one chosen short exact sequence with nonzero connector from [L3], where replace the connecting map by its negative. Each long exact sequence stays exact, because negating one map does not change its image or kernel.
By construction, the modified family still has long exact sequences, but on an isomorphism between the altered sequence and an unaltered copy the connecting square no longer commutes: one side uses and the other uses , which are different because . Thus [L1] fails, so the modified family is not a delta functor.
Depends on
Used by
Nothing in the library uses this result yet.
Dependency tree · two levels
11 results within two dependency steps of this one, each drawn at its shortest distance from it. An arrow runs from a result to what uses it, so the chart reads left to right and ends at this result, which carries a heavier outline. Every node is a link to that result. Click elsewhere on the chart to enlarge it.
Sources
- Charles A. Weibel, An Introduction to Homological Algebra, Chapter 2 `Derived Functors` (standard reference, not scraped)