module Cat.Instances.Shape.Involution where
The walking involution🔗
The walking involution is the category with a single
non-trivial morphism
satisfying
We can encode this by defining morphisms as booleans, and using xor as our composition
operation.
∙⤮∙ : Precategory lzero lzero ∙⤮∙ .Precategory.Ob = ⊤ ∙⤮∙ .Precategory.Hom _ _ = Bool ∙⤮∙ .Precategory.Hom-set _ _ = Bool-is-set ∙⤮∙ .Precategory.id = false ∙⤮∙ .Precategory._∘_ = xor ∙⤮∙ .Precategory.idr f = xor-falser f ∙⤮∙ .Precategory.idl f = refl ∙⤮∙ .Precategory.assoc f g h = xor-associative f g h open Cat.Reasoning ∙⤮∙
This is the smallest precategory with a non-trivial isomorphism.
Bool≃∙⤮∙-isos : Bool ≃ (tt ≅ tt) Bool≃∙⤮∙-isos = Iso→Equiv (Bool→Iso , iso Iso→Bool right-inv left-inv) where Bool→Iso : Bool → tt ≅ tt Bool→Iso true = make-iso true true refl refl Bool→Iso false = id-iso Iso→Bool : tt ≅ tt → Bool Iso→Bool i = i .to right-inv : is-right-inverse Iso→Bool Bool→Iso right-inv f = Bool-elim (λ b → b ≡ f .to → Bool→Iso b ≡ f) ext ext (f .to) refl left-inv : is-left-inverse Iso→Bool Bool→Iso left-inv true = refl left-inv false = refl
Skeletality and (non)-univalence🔗
As the walking involution only has one object, it is trivially skeletal.
∙⤮∙-skeletal : is-skeletal ∙⤮∙ ∙⤮∙-skeletal .to-path _ = refl ∙⤮∙-skeletal .to-path-over _ = is-prop→pathp (λ _ → squash) (inc id-iso) _
However, it is not univalent, since its only object has more internal automorphisms than it has loops. By definition, univalence for the walking involution would mean that there are two paths in the unit type: but this contradicts the unit type being a set.
∙⤮∙-not-univalent : ¬ is-category ∙⤮∙ ∙⤮∙-not-univalent is-cat = true≠false (Bool-is-prop true false) where Bool≃Ob-path : Bool ≃ (tt ≡ tt) Bool≃Ob-path = Bool≃∙⤮∙-isos ∙e identity-system-gives-path is-cat Bool-is-prop : is-prop Bool Bool-is-prop = Equiv→is-hlevel 1 Bool≃Ob-path (hlevel 1)