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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116= ( Aε C Q ) ◦ ( cocan1 −1 Q ) ◦ (Qδ D ) ◦ (A µ Q D ) ◦ ( )Aρ D Q= ( Aε C Q ) ◦ ( cocan −11 Q ) ◦ (A µ Q P Q ) ◦ (AQδ D ) ◦ ( )Aρ D Q(127)= ( Aε C Q ) ◦ ( cocan −11 Q ) ◦ (A µ Q P Q ) ◦ (Aδ C Q) ◦ ( A C ρ Q)= ( Aε C Q ) ◦ ( cocan −11 Q ) ◦ (cocan 1 Q) ◦ ( A C ρ Q)= ( Aε C Q ) ◦ ( A C ρ Q) C ρ Q counital= AQso we obtaincocan −12 = ( Aε C Q ) ◦ ( cocan −11 Q ) ◦ (Qδ D ) .(b) ⇒ (a) Follows from Theorem 6.36. (a) ⇔ (c) follows by similar computations.□6.10. Coherds and distributive laws. The following result is a reformulation ofTheorem ong>2.ong>16 in [BV] in our categorical setting.Proposition 6.38. Let A and B be categories with equalizers and let χ : QP Q →Q be a regular coherd for X = (C, D, P, Q, δ C , δ D ) where the underlying functorsP : A → B, Q : B → A, C : A → A and D : B → B preserve coequalizers.Let A = (A, m A , u A ) and B = (B, m B , u B ) be the associated monads constructed inProposition 6.25 and in Proposition 6.26. Then1) There exists a mixed distributive law between the monad A and the comonadC, Λ : AC → CA such thatΛ ◦ (xC) = λ = (Cx) ◦ (C ρ Q P ) ◦ (χP ) ◦ (QP δ C ) .2) There exists an opposite mixed distributive law between the monad B and thecomonad D, Γ : DB → BD such thatΓ ◦ (Dy) = γ = (yD) ◦ ( P ρ D Q)◦ (P χ) ◦ (δD P Q) .Proof. 1) Consider the functorial morphism given byQP C QP δ C−→ QP QP −→ χPQP C ρ Q P−→ CQP −→ CxCAλ = (Cx) ◦ (C ρ Q P ) ◦ (χP ) ◦ (QP δ C )Recall that w l = (χP ) ◦ (QP δ C ) and w r = QP ε C : QP C → QP and let us provethatλ ◦ ( w l C ) ? = λ ◦ (w r C)that isLet us compute(Cx) ◦ (C ρ Q P ) ◦ (χP ) ◦ (QP δ C ) ◦ (χP C) ◦ (QP δ C C)?= (Cx) ◦ (C ρ Q P ) ◦ (χP ) ◦ (QP δ C ) ◦ ( QP ε C C ) .(Cx) ◦ (C ρ Q P ) ◦ (χP ) ◦ (QP δ C ) ◦ (χP C) ◦ (QP δ C C)χ= (Cx) ◦ (C ρ Q P ) ◦ (χP ) ◦ (χP QP ) ◦ (QP QP δ C ) ◦ (QP δ C C)δ C ,(111)= (Cx) ◦ (C ρ Q P ) ◦ (χP ) ◦ (QP χP ) ◦ (QP δ C QP ) ◦ (QP Cδ C )

117(112)= (Cx) ◦ (C ρ Q P ) ◦ (χP ) ◦ ( QP ε C QP ) ◦ (QP Cδ C )ε C = (Cx) ◦ (C ρ Q P ) ◦ (χP ) ◦ (QP δ C ) ◦ ( QP ε C C )Since (AC, xC) = Coequ Fun(w l C, w r C ) ,by the universal property of coequalizers,there exists a unique functorial morphism Λ : AC → CA such thatΛ ◦ (xC) = λ = (Cx) ◦ (C ρ Q P ) ◦ (χP ) ◦ (QP δ C ) .We want to prove that Λ is a mixed distributive law. We compute(Cm A ) ◦ (ΛA) ◦ (AΛ) ◦ (xxC) x = (Cm A ) ◦ (ΛA) ◦ (AΛ) ◦ (xAC) ◦ (QP xC)x= (Cm A ) ◦ (ΛA) ◦ (xCA) ◦ (QP Λ) ◦ (QP xC)defλ= (Cm A ) ◦ (CxA) ◦ (C ρ Q P A ) ◦ (χP A) ◦ (QP δ C A) ◦ (QP Cx)◦ ( QP C ρ Q P ) ◦ (QP χP ) ◦ (QP QP δ C )δ C= (Cm A ) ◦ (CxA) ◦ (C ρ Q P A ) ◦ (χP A) ◦ (QP QP x) ◦ (QP δ C QP )◦ ( QP C ρ Q P ) ◦ (QP χP ) ◦ (QP QP δ C )χ= (Cm A ) ◦ (CxA) ◦ (C ρ Q P A ) ◦ (QP x) ◦ (χP QP ) ◦ (QP δ C QP )◦ ( QP C ρ Q P ) ◦ (QP χP ) ◦ (QP QP δ C )C ρ Q= (CmA ) ◦ (CxA) ◦ (CQP x) ◦ (C ρ Q P QP ) ◦ (χP QP ) ◦ (QP δ C QP )◦ ( QP C ρ Q P ) ◦ (QP χP ) ◦ (QP QP δ C )x,(127)= (Cm A ) ◦ (Cxx) ◦ (C ρ Q P QP ) ◦ (χP QP ) ◦ (QP Qδ D P )◦ ( QP ρ D QP ) ◦ (QP χP ) ◦ (QP QP δ C )(102)= (Cx) ◦ (CχP ) ◦ (C ρ Q P QP ) ◦ (χP QP ) ◦ (QP Qδ D P )◦ ( QP ρ D QP ) ◦ (QP χP ) ◦ (QP QP δ C )χ= (Cx) ◦ (CχP ) ◦ (C ρ Q P QP ) ◦ (Qδ D P ) ◦ (χDP ) ◦ ( QP ρ D QP ) ◦ (QP χP )◦ (QP QP δ C )C ρ Q= (Cx) ◦ (CχP ) ◦ (CQδD P ) ◦ (C ρ Q DP ) ◦ (χDP ) ◦ ( QP ρ D QP ) ◦ (QP χP )◦ (QP QP δ C )(113)= (Cx) ◦ ( CQε D P ) ◦ (C ρ Q DP ) ◦ (χDP ) ◦ ( QP ρ D QP ) ◦ (QP χP )◦ (QP QP δ C )C ρ Q= (Cx) ◦( Cρ Q P ) ◦ ( Qε D P ) ◦ (χDP ) ◦ ( QP ρ D QP ) ◦ (QP χP ) ◦ (QP QP δ C )χ= (Cx) ◦ (C ρ Q P ) ◦ (χP ) ◦ ( QP Qε D P ) ◦ ( QP ρ D QP ) ◦ (QP χP ) ◦ (QP QP δ C )Qcomfun= (Cx) ◦ (C ρ Q P ) ◦ (χP ) ◦ (QP χP ) ◦ (QP QP δ C )(111)= (Cx) ◦ (C ρ Q P ) ◦ (χP ) ◦ (χP QP ) ◦ (QP QP δ C )

117(112)= (Cx) ◦ (C ρ Q P ) ◦ (χP ) ◦ ( QP ε C QP ) ◦ (QP Cδ C )ε C = (Cx) ◦ (C ρ Q P ) ◦ (χP ) ◦ (QP δ C ) ◦ ( QP ε C C )Since (AC, xC) = Coequ Fun(w l C, w r C ) ,by the universal property of coequalizers,there exists a unique functorial morphism Λ : AC → CA such thatΛ ◦ (xC) = λ = (Cx) ◦ (C ρ Q P ) ◦ (χP ) ◦ (QP δ C ) .We want to prove that Λ is a mixed distributive law. We compute(Cm A ) ◦ (ΛA) ◦ (AΛ) ◦ (xxC) x = (Cm A ) ◦ (ΛA) ◦ (AΛ) ◦ (xAC) ◦ (QP xC)x= (Cm A ) ◦ (ΛA) ◦ (xCA) ◦ (QP Λ) ◦ (QP xC)defλ= (Cm A ) ◦ (CxA) ◦ (C ρ Q P A ) ◦ (χP A) ◦ (QP δ C A) ◦ (QP Cx)◦ ( QP C ρ Q P ) ◦ (QP χP ) ◦ (QP QP δ C )δ C= (Cm A ) ◦ (CxA) ◦ (C ρ Q P A ) ◦ (χP A) ◦ (QP QP x) ◦ (QP δ C QP )◦ ( QP C ρ Q P ) ◦ (QP χP ) ◦ (QP QP δ C )χ= (Cm A ) ◦ (CxA) ◦ (C ρ Q P A ) ◦ (QP x) ◦ (χP QP ) ◦ (QP δ C QP )◦ ( QP C ρ Q P ) ◦ (QP χP ) ◦ (QP QP δ C )C ρ Q= (CmA ) ◦ (CxA) ◦ (CQP x) ◦ (C ρ Q P QP ) ◦ (χP QP ) ◦ (QP δ C QP )◦ ( QP C ρ Q P ) ◦ (QP χP ) ◦ (QP QP δ C )x,(127)= (Cm A ) ◦ (Cxx) ◦ (C ρ Q P QP ) ◦ (χP QP ) ◦ (QP Qδ D P )◦ ( QP ρ D QP ) ◦ (QP χP ) ◦ (QP QP δ C )(102)= (Cx) ◦ (CχP ) ◦ (C ρ Q P QP ) ◦ (χP QP ) ◦ (QP Qδ D P )◦ ( QP ρ D QP ) ◦ (QP χP ) ◦ (QP QP δ C )χ= (Cx) ◦ (CχP ) ◦ (C ρ Q P QP ) ◦ (Qδ D P ) ◦ (χDP ) ◦ ( QP ρ D QP ) ◦ (QP χP )◦ (QP QP δ C )C ρ Q= (Cx) ◦ (CχP ) ◦ (CQδD P ) ◦ (C ρ Q DP ) ◦ (χDP ) ◦ ( QP ρ D QP ) ◦ (QP χP )◦ (QP QP δ C )(113)= (Cx) ◦ ( CQε D P ) ◦ (C ρ Q DP ) ◦ (χDP ) ◦ ( QP ρ D QP ) ◦ (QP χP )◦ (QP QP δ C )C ρ Q= (Cx) ◦( Cρ Q P ) ◦ ( Qε D P ) ◦ (χDP ) ◦ ( QP ρ D QP ) ◦ (QP χP ) ◦ (QP QP δ C )χ= (Cx) ◦ (C ρ Q P ) ◦ (χP ) ◦ ( QP Qε D P ) ◦ ( QP ρ D QP ) ◦ (QP χP ) ◦ (QP QP δ C )Qcomfun= (Cx) ◦ (C ρ Q P ) ◦ (χP ) ◦ (QP χP ) ◦ (QP QP δ C )(111)= (Cx) ◦ (C ρ Q P ) ◦ (χP ) ◦ (χP QP ) ◦ (QP QP δ C )

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