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DefinitionDefinition: Literature-sourcedProof: Not applicableaudited 2026-09-04
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A balanced derived bifunctor

Definition

Assume the Axiom of Dependent Choice. Let A,C,D be abelian categories, let B:Aop×C→D be additive in each variable, let P be supplied projective resolution data on a class DA in A, and let I be supplied injective resolution data on a class DC in C. Assume moreover that for each fixed A∈DA the covariant functor B(A,−):C→D is left exact, and that for each fixed C∈DC the functor B(−,C):Aop→D is left exact.

A balanced derived bifunctor relative to (P,I) on DAop×DC consists of the two candidate one-variable right-derived constructions from A bifunctor can be derived in either variable when the relevant resolution data are supplied together with, for every n≥0, a natural isomorphism RIn(B(A,−))(C)→∼RPopn(B(−,C))(A) natural in A∈DA and C∈DC. These isomorphisms must satisfy:

  1. in degree 0, when the two candidates are identified with B(A,C) by The zero-th right derived functor of a left exact functor recovers the functor on C and on Aop, the balance isomorphism becomes the identity of B(A,C);
  2. the isomorphisms are natural in both variables in the sense of Natural transformation and its components.

This is a definition relative to the displayed supplied data P and I; it does not impose an unquantified condition involving alternative data. The definition records extra comparison data, while the previous proposition only constructs the two candidates.

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