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Additive cocontinuous module functors admit right adjoints
Statement
Let be unital rings. Every additive cocontinuous functor admits a right adjoint : if is a -bimodule with , then is left adjoint to . In particular is a left adjoint, so it preserves every colimit that exists. No commutativity and no choice are used.
Facts & Assumptions
Given: Unital rings , an additive cocontinuous functor , and for .
is a -bimodule and is naturally isomorphic to (Eilenberg-Watts theorem for arbitrary unital rings, -bimodules and commuting left and right scalar actions).
is left adjoint to : there are a unit and a counit satisfying the triangle identities (Tensor-Hom adjunction for bimodules over arbitrary unital rings).
An adjunction is a unit and counit satisfying and ; componentwise, and (Adjunction by unit, counit, and the triangle identities).
Left whiskering has components and right whiskering has components (Whiskering and horizontal composition of natural transformations).
A natural isomorphism has an inverse natural transformation with and (Natural isomorphism); vertical composition is componentwise (Identity natural transformation and vertical composition).
A left adjoint preserves every colimit that exists (Left adjoints preserve every colimit that exists).
Proof
By [F1] there is a natural isomorphism with inverse , and is a -bimodule. By [F2] there are a unit and a counit satisfying the triangle identities of [F3] for the adjunction .
Transfer: put and , using the whiskerings of [F4]; these are natural transformations by [F4] and [F5]. They satisfy the triangle identities of [F3]: at , using naturality of at , naturality of at and naturality of at , one computes ; and at one computes , where the middle step cancels the inverse components of [F5]. Hence in the sense of [F3].
By step 2.1 the functor admits as right adjoint, so it is a left adjoint and [F6] applies; in particular it preserves every colimit that exists, consistently with its assumed cocontinuity. The transfer used only the displayed units, counits and inverse components, so no commutativity and no choice are involved.
Depends on
- Eilenberg-Watts theorem for arbitrary unital rings
- Tensor-Hom adjunction for bimodules over arbitrary unital rings
- Adjunction by unit, counit, and the triangle identities
- Whiskering and horizontal composition of natural transformations
- Natural isomorphism
- Identity natural transformation and vertical composition
- $(S,R)$-bimodules and commuting left and right scalar actions
- Left adjoints preserve every colimit that exists
Used by
Nothing in the library uses this result yet.
Dependency tree · two levels
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Sources
- M. Kamensky, Non-Commutative Algebra (BGU course notes, Spring 2017), §5.1, Theorem 5.1.43, Proposition 5.1.40, Lemma 5.1.46, Corollaries 5.1.48-5.1.49 (standard reference, not scraped)
- A. Nyman and S. P. Smith, A Generalization of Watts's Theorem: Right Exact Functors on Module Categories, arXiv:0806.0832, Theorem 1.1-1.2, Propositions 3.2-3.3, Lemma 3.4 (standard reference, not scraped)