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FALSE: derived functors in two variables are automatically balanced
Statement
Whenever a bifunctor can be derived in each variable, the two derived constructions are automatically balanced.
Facts & Assumptions
Given: The Axiom of Dependent Choice, a field , the ring , the abelian categories and , and the bifunctor to .
With supplied projective and injective data, one may derive an additive bifunctor in either variable and thereby obtain two candidate constructions (A bifunctor can be derived in either variable when the relevant resolution data are supplied).
A balanced derived bifunctor relative to the supplied data requires extra natural isomorphisms, natural in both variables and normalized by the degree-zero identifications (A balanced derived bifunctor).
Refutation
The functor is exact on finite-dimensional vector spaces, is left exact, and tensoring over is exact. Hence is additive and left exact in each variable in the sense required by [L1]. Give its length-zero projective resolution. The first-variable right-derived object at is then zero in every positive degree.
The -module is injective: the coefficient-of- functional identifies with as an -module, and is exact. Thus is an injective resolution of : at every copy of , both the image and kernel of multiplication by are the ideal .
Applying to the deleted resolution in step 1.2 gives a cochain complex with one copy of in every degree and zero differentials, since multiplication by annihilates . Consequently the second-variable right-derived object in degree is , whereas the first-variable object from step 1.1 is . They cannot be isomorphic, so the balance data required by [L2] do not exist and the displayed automatic-balance claim is false.
Depends on
Used by
Nothing in the library uses this result yet.
Dependency tree · two levels
8 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)