Whiskering #
Given a functor F : C ⥤ D and functors G H : D ⥤ E and a natural transformation α : G ⟶ H,
we can construct a new natural transformation F ⋙ G ⟶ F ⋙ H,
called whiskerLeft F α. This is the same as the horizontal composition of 𝟙 F with α.
This operation is functorial in F, and we package this as whiskeringLeft. Here
(whiskeringLeft.obj F).obj G is F ⋙ G, and
(whiskeringLeft.obj F).map α is whiskerLeft F α.
(That is, we might have alternatively named this as the "left composition functor".)
We also provide analogues for composition on the right, and for these operations on isomorphisms.
We show the associator and unitor natural isomorphisms satisfy the triangle and pentagon identities.
If α : G ⟶ H then whiskerLeft F α : F ⋙ G ⟶ F ⋙ H has components α.app (F.obj X).
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If α : G ⟶ H then whiskerRight α F : G ⋙ F ⟶ H ⋙ F has components F.map (α.app X).
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Left-composition gives a functor (C ⥤ D) ⥤ ((D ⥤ E) ⥤ (C ⥤ E)).
(whiskeringLeft.obj F).obj G is F ⋙ G, and
(whiskeringLeft.obj F).map α is whiskerLeft F α.
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Right-composition gives a functor (D ⥤ E) ⥤ ((C ⥤ D) ⥤ (C ⥤ E)).
(whiskeringRight.obj H).obj F is F ⋙ H, and
(whiskeringRight.obj H).map α is whiskerRight α H.
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If F : D ⥤ E is fully faithful, then so is
(whiskeringRight C D E).obj F : (C ⥤ D) ⥤ C ⥤ E.
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The isomorphism between left-whiskering on the identity functor and the identity of the functor between the resulting functor categories.
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The isomorphism between left-whiskering on the composition of functors and the composition of two left-whiskering applications.
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The isomorphism between right-whiskering on the identity functor and the identity of the functor between the resulting functor categories.
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The isomorphism between right-whiskering on the composition of functors and the composition of two right-whiskering applications.
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If α : G ≅ H is a natural isomorphism then
isoWhiskerLeft F α : (F ⋙ G) ≅ (F ⋙ H) has components α.app (F.obj X).
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If α : G ≅ H then
isoWhiskerRight α F : (G ⋙ F) ≅ (H ⋙ F) has components F.map_iso (α.app X).
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The obvious functor (C₁ ⥤ D₁) ⥤ (C₂ ⥤ D₂) ⥤ (D₁ ⥤ D₂ ⥤ E) ⥤ (C₁ ⥤ C₂ ⥤ E).
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Auxiliary definition for whiskeringLeft₃.
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Auxiliary definition for whiskeringLeft₃.
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Auxiliary definition for whiskeringLeft₃.
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Auxiliary definition for whiskeringLeft₃.
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Auxiliary definition for whiskeringLeft₃.
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Auxiliary definition for whiskeringLeft₃.
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The obvious functor
(C₁ ⥤ D₁) ⥤ (C₂ ⥤ D₂) ⥤ (C₃ ⥤ D₃) ⥤ (D₁ ⥤ D₂ ⥤ D₃ ⥤ E) ⥤ (C₁ ⥤ C₂ ⥤ C₃ ⥤ E).
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The "postcomposition" with a functor E ⥤ E' gives a functor
(E ⥤ E') ⥤ (C₁ ⥤ C₂ ⥤ E) ⥤ C₁ ⥤ C₂ ⥤ E'.
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The "postcomposition" with a functor E ⥤ E' gives a functor
(E ⥤ E') ⥤ (C₁ ⥤ C₂ ⥤ C₃ ⥤ E) ⥤ C₁ ⥤ C₂ ⥤ C₃ ⥤ E'.