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Additive cocontinuous module functors admit right adjoints

Statement

Let A,B be unital rings. Every additive cocontinuous functor F:A-Mod→B-Mod admits a right adjoint B-Mod→A-Mod: if M is a (B,A)-bimodule with F≅TM, then F is left adjoint to Hom⁡B(M,−). In particular F is a left adjoint, so it preserves every colimit that exists. No commutativity and no choice are used.

Facts & Assumptions

Given: Unital rings A,B, an additive cocontinuous functor F:A-Mod→B-Mod, and G:=Hom⁡B(M,−) for M:=F(A).

[F1]

M=F(A) is a (B,A)-bimodule and F is naturally isomorphic to TM=M⊗A− (Eilenberg-Watts theorem for arbitrary unital rings, (S,R)-bimodules and commuting left and right scalar actions).

[F2]

TM is left adjoint to Hom⁡B(M,−): there are a unit η:1⇒GTM and a counit ε:TMG⇒1 satisfying the triangle identities (Tensor-Hom adjunction for bimodules over arbitrary unital rings).

[F3]

An adjunction is a unit and counit satisfying (εF)∘(Fη)=1F and (Gε)∘(ηG)=1G; componentwise, εFX∘F(ηX)=1FX and G(εY)∘ηGY=1GY (Adjunction by unit, counit, and the triangle identities).

[F4]

Left whiskering Hα has components H(αA) and right whiskering αK has components αKB (Whiskering and horizontal composition of natural transformations).

[F5]

A natural isomorphism σ has an inverse natural transformation σ−1 with σ−1∘σ=1 and σ∘σ−1=1 (Natural isomorphism); vertical composition is componentwise (Identity natural transformation and vertical composition).

[F6]

A left adjoint preserves every colimit that exists (Left adjoints preserve every colimit that exists).

Proof

technique · direct
1.1F1F2F5

By [F1] there is a natural isomorphism σ:TM⇒F with inverse σ−1:F⇒TM, and M is a (B,A)-bimodule. By [F2] there are a unit η:1⇒GTM and a counit ε:TMG⇒1 satisfying the triangle identities of [F3] for the adjunction TM⊣G.

2.1F3F4F5step 1.1

Transfer: put η′:=(Gσ)∘η:1⇒GF and ε′:=ε∘(σ−1G):FG⇒1, using the whiskerings of [F4]; these are natural transformations by [F4] and [F5]. They satisfy the triangle identities of [F3]: at X, using naturality of σ−1 at G(σX), naturality of σ at ηX and naturality of ε at σX, one computes εF(X)′∘F(ηX′)=εF(X)∘(σ−1)G(F(X))∘F(G(σX))∘F(ηX)=εF(X)∘TM(G(σX))∘TM(ηX)∘σX−1=σX∘εTM(X)∘TM(ηX)∘σX−1=σX∘σX−1=1F(X); and at Y one computes G(εY′)∘ηG(Y)′=G(εY)∘G((σ−1)G(Y))∘G(σG(Y))∘ηG(Y)=G(εY)∘ηG(Y)=1G(Y), where the middle step cancels the inverse components of [F5]. Hence F⊣G in the sense of [F3].

3.1F6step 2.1∎

By step 2.1 the functor F admits G=Hom⁡B(M,−) 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.

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