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False statementConstruction: Literature-sourcedVerification: AI-adaptedprecheck passaudited 2026-09-04
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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 k, the ring R=k[ε]/(ε2), the abelian categories A=Vectkfd and C=R-Modfd, and the bifunctor B(A,C):=A∗⊗kHom⁡R(k,C) to Vectkfd.

[L1]

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).

[L2]

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

technique · direct
1.1L1givenconstructalgebra

The functor A↦A∗ is exact on finite-dimensional vector spaces, C↦Hom⁡R(k,C) is left exact, and tensoring over k is exact. Hence B is additive and left exact in each variable in the sense required by [L1]. Give k∈A its length-zero projective resolution. The first-variable right-derived object at (k,k) is then zero in every positive degree.

1.2givenconstructalgebra

The R-module R is injective: the coefficient-of-ε functional identifies R with Hom⁡k(R,k) as an R-module, and Hom⁡R(−,Hom⁡k(R,k))≅Hom⁡k(−,k) is exact. Thus 0→k→1↦εR→εR→εR→⋯ is an injective resolution of k: at every copy of R, both the image and kernel of multiplication by ε are the ideal (ε).

2.1L1L2step 1.1step 1.2algebra∎

Applying B(k,−)=Hom⁡R(k,−) to the deleted resolution in step 1.2 gives a cochain complex with one copy of k in every degree and zero differentials, since multiplication by ε annihilates Hom⁡R(k,R)≅(ε). Consequently the second-variable right-derived object in degree 1 is k, whereas the first-variable object from step 1.1 is 0. 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