Contents 1. Introduction 2 2. Preliminaries 4 2.1. Some results on ...

Contents 1. Introduction 2 2. Preliminaries 4 2.1. Some results on ... Contents 1. Introduction 2 2. Preliminaries 4 2.1. Some results on ...

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234Since we have(B • B (Q · A) , p B,Q·A ) = Coequ C (m B · 1 Q · 1 A , 1 B · λ Q·A )and we are assuming that the composition with 1-cells preserves coequalizer, we alsohave(P · (B • B (Q · A)) , 1 P · p B,Q·A ) = Coequ C (1 P · m B · 1 Q · 1 A , 1 P · 1 B · λ Q·A ) .Therefore, there exists a unique isomorphism h : (P · B)• B (Q · A) → P ·(B • B (Q · A))such that(248) h (p P ·B,Q·A ) = 1 P · p B,Q·A .Moreover, by Proposition 1ong>1.ong>6, P · (B • B (Q · A)) ≃ P · (Q · A) so that we get(P · B) • B (Q · A) ≃ P · (Q · A) = P · Q · A.Now, the left C-module structure λ (P ·B)•B (Q·A) of (P · B) • B (Q · A), by (236) isuniquely determined byλ (P ·B)•B (Q·A) (1 C · p P ·B,Q·A ) = p P ·B,Q·A (λ P ·B · 1 Q · 1 A ) .By (248) we getp P ·B,Q·A = h −1 (1 P · p B,Q·A )and thus we can rewrite the above relation(λ (P ·B)•B (Q·A) (1 C · p P ·B,Q·A ) = λ (P ·B)•B (Q·A) 1C · [h −1 (1 P · p B,Q·A ) ])andso that= λ (P ·B)•B (Q·A)(1C · h −1) (1 C · 1 P · p B,Q·A )p P ·B,Q·A (λ P ·B · 1 Q · 1 A ) = h −1 (1 P · p B,Q·A ) (λ P ·B · 1 Q · 1 A )defλ P ·B= h −1 (1 P · p B,Q·A ) (λ P · 1 B · 1 Q · 1 A )λ=Ph ( ) −1 λ P · 1 B•B (Q·A) (1C · 1 P · p B,Q·A )λ (P ·B)•B (Q·A)(1C · h −1) (1 C · 1 P · p B,Q·A ) = h −1 ( λ P · 1 B•B (Q·A))(1C · 1 P · p B,Q·A ) .Since 1 C · 1 P · p B,Q·A is epi, we get(λ (P ·B)•B (Q·A) 1C · h −1) = h ( )−1 λ P · 1 B•B (Q·A)and thusso that we get thatλ (P ·B)•B (Q·A) = h −1 ( λ P · 1 B•B (Q·A))(1C · h)λ (P ·B)•B (Q·A) ≃ λ P · 1 B•B (Q·A) ≃ λ P · 1 Q·A ≃ λ P ·Q·A .Similarly, the right A-module structure λ (P ·B)•B (Q·A) of (P · B) • B (Q · A), by (237)is uniquely determined byBy (248) we getρ (P ·B)•B (Q·A) (p P ·B,Q·A · 1 A ) = p P ·B,Q·A (1 P · 1 B · ρ Q·A ) .p P ·B,Q·A = h −1 (1 P · p B,Q·A )

and thus we can rewrite the above relation([ρ (P ·B)•B (Q·A) (p P ·B,Q·A · 1 A ) = ρ (P ·B)•B (Q·A) h −1 (1 P · p B,Q·A ) ] )· 1 Aandso that= ρ (P ·B)•B (Q·A)(h−1 · 1 A)(1P · p B,Q·A · 1 A )p P ·B,Q·A (1 P · 1 B · ρ Q·A ) = h −1 (1 P · p B,Q·A ) (1 P · 1 B · ρ Q·A )(237)= h −1 ( 1 P · ρ B•B (Q·A))(1P · p B,Q·A · 1 A )ρ (P ·B)•B (Q·A)(h−1 · 1 A)(1P · p B,Q·A · 1 A ) = h −1 ( 1 P · ρ B•B (Q·A))(1P · p B,Q·A · 1 A ) .Since 1 P · p B,Q·A · 1 A is epi, we get( )ρ (P ·B)•B (Q·A) h−1 · 1 ( )A = h−11 P · ρ B•B (Q·A)and thusso that we get thatρ (P ·B)•B (Q·A) = h −1 ( 1 P · ρ B•B (Q·A))(h · 1A )ρ (P ·B)•B (Q·A) ≃ 1 P · ρ B•B (Q·A) ≃ 1 P · ρ Q·A = ρ P ·Q·A .1ong>2.ong> Entwined modules and comodulesLet (X, 1 X ) , (Y, B) be monads in C and let us compute the categoryMnd (C) ((X, 1 X ) , (Y, B)) . Note that C (X, B) : C (X, Y ) → C (X, Y ) is a monadover the category C (X, Y ) . In fact, we set multiplication and unit of the monadto be C (X, m B ) = m B (−) : C (X, B · B) → C (X, B) and C (X, u B ) = u B (−) :C (X, 1 Y ) → C (X, B). In fact we haveandC (X, m B ) C (X, m B · 1 B ) = m B (m B · 1 B ) = m B (1 B · m B )= C (X, m B ) C (X, 1 B · m B )C (X, m B ) C (X, u B · 1 B ) = m B (u B · 1 B ) = 1 B = m B (1 B · u B )= C (X, m B ) C (X, 1 B · u B )The objects of such category are the monad functors (Q, φ) from (X, 1 X ) to (Y, B) ,i.e. the 1-cells Q : X → Y together with the 2-cells φ : B · Q = C (X, B) Q → Qsatisfying the following conditionsφ (1 B · φ) = φ (m B · 1 Q )φ (u B · 1 Q ) = 1 Qwhich says that φ gives a structure of left C (X, B)-module to the 1-cell Q : X → Y .Therefore, we can conclude thatMnd (C) ((X, 1 X ) , (Y, B)) = C(X,B) C (X, Y ).Now, following [St, pg. 158], we define the bicategory of comonads as follows:Cmd (C) = Mnd (C ∗ ) ∗where (−) ∗denotes the bicategory obtained by reversing235□

234Since we have(B • B (Q · A) , p B,Q·A ) = Coequ C (m B · 1 Q · 1 A , 1 B · λ Q·A )and we are assuming that the compositi<strong>on</strong> with 1-cells preserves coequalizer, we alsohave(P · (B • B (Q · A)) , 1 P · p B,Q·A ) = Coequ C (1 P · m B · 1 Q · 1 A , 1 P · 1 B · λ Q·A ) .Therefore, there exists a unique isomorphism h : (P · B)• B (Q · A) → P ·(B • B (Q · A))such that(248) h (p P ·B,Q·A ) = 1 P · p B,Q·A .Moreover, by Propositi<strong>on</strong> 1<str<strong>on</strong>g>1.</str<strong>on</strong>g>6, P · (B • B (Q · A)) ≃ P · (Q · A) so that we get(P · B) • B (Q · A) ≃ P · (Q · A) = P · Q · A.Now, the left C-module structure λ (P ·B)•B (Q·A) of (P · B) • B (Q · A), by (236) isuniquely determined byλ (P ·B)•B (Q·A) (1 C · p P ·B,Q·A ) = p P ·B,Q·A (λ P ·B · 1 Q · 1 A ) .By (248) we getp P ·B,Q·A = h −1 (1 P · p B,Q·A )and thus we can rewrite the above relati<strong>on</strong>(λ (P ·B)•B (Q·A) (1 C · p P ·B,Q·A ) = λ (P ·B)•B (Q·A) 1C · [h −1 (1 P · p B,Q·A ) ])andso that= λ (P ·B)•B (Q·A)(1C · h −1) (1 C · 1 P · p B,Q·A )p P ·B,Q·A (λ P ·B · 1 Q · 1 A ) = h −1 (1 P · p B,Q·A ) (λ P ·B · 1 Q · 1 A )defλ P ·B= h −1 (1 P · p B,Q·A ) (λ P · 1 B · 1 Q · 1 A )λ=Ph ( ) −1 λ P · 1 B•B (Q·A) (1C · 1 P · p B,Q·A )λ (P ·B)•B (Q·A)(1C · h −1) (1 C · 1 P · p B,Q·A ) = h −1 ( λ P · 1 B•B (Q·A))(1C · 1 P · p B,Q·A ) .Since 1 C · 1 P · p B,Q·A is epi, we get(λ (P ·B)•B (Q·A) 1C · h −1) = h ( )−1 λ P · 1 B•B (Q·A)and thusso that we get thatλ (P ·B)•B (Q·A) = h −1 ( λ P · 1 B•B (Q·A))(1C · h)λ (P ·B)•B (Q·A) ≃ λ P · 1 B•B (Q·A) ≃ λ P · 1 Q·A ≃ λ P ·Q·A .Similarly, the right A-module structure λ (P ·B)•B (Q·A) of (P · B) • B (Q · A), by (237)is uniquely determined byBy (248) we getρ (P ·B)•B (Q·A) (p P ·B,Q·A · 1 A ) = p P ·B,Q·A (1 P · 1 B · ρ Q·A ) .p P ·B,Q·A = h −1 (1 P · p B,Q·A )

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