Categories Definition. class Ob(C), Mor(C) ༩⿺〾ぁ〛⿶〝『ɻ〳〔ɺҎԼ〣݅ ຬ〔《ぁ〛⿶〝『ɻ 1. શ〛〣 f ∈ Mor(C) 〠ର「ɺdom(f ), cod(f ) ∈ Ob(C) ༩⿺ 〾ぁ〛⿶ɻ (f ∈ Mor(C), dom(f ) = a, cod(f ) = b 〣࣌ɺ f : a → b 〝ॻ。ɻ) 2. શ〛〣 c ∈ Ob(C) 〠ର「ɺidc : c → c 3. શ〛〣 f : a → b, g : b → c 〠ର「ɺg ◦ f : a → c 4. f : a → b 〠ର「ɺf ◦ ida = idb ◦ f = f 5. f : a → b, g : b → c, h: c → d 〠ର「 h ◦ (g ◦ f ) = (h ◦ g) ◦ f 〈〣〝 ɺC 〤 category 〜⿴ɺ〝ݴ⿸ɻ 〳〔ɺOb(C) 〣ݩぇ objectɺMor(C) 〣ݩぇ morphism 〝ݺ〫ɻ《 〾〠ɺobject a, b 〠ର「 Hom(a, b) = {f : a → b | f ∈ Mor(C)} 〝『ɻ
Functors Definition. F C 〾 D 〭〣 functor 〜⿴〝〤ɺ 1. c ∈ C 〠ର「 Fc ∈ D ༩⿺〾ぁ〛⿶ 2. f : a → b 〠ର「〛 Ff : Fa → Fb ༩⿺〾ぁ〛⿶ 3. F(ida) = idFa 4. F(g ◦ f ) = Fg ◦ Ff ぇຬ〔『〝 ぇݴ⿸ɻ〳〔ɺ〈〣࣌ F : C → D 〝ॻ。ɻ 〳〔ɺcategory C 〠ର「 IdC : C → C ぇɺId(c) = c, Id(f ) = f 〝 〟〽⿸〠ఆ〶ɻ 《〾〠ɺF : C → D, G : D → E 〠ର「ɺG ◦ F : C → E ぇ G ◦ F(c) = G(Fc), G ◦ F(f ) = G(Ff ) 〝〟〽⿸〠ఆ〶ɻ
Natural transformations Definition. F, G : C → D 〠ର「ɺθ: F ⇒ G ぇɺfunctor θ: C × I → D 〜 ⿴〘〛࣍〣ਤࣜぇՄ〠『〷〣〣〈〝〝『ɻ C C C × I D 0 1 F G θ(idc, !) ぇ θc 〝ॻ。ɻ
Hom functor and Yoneda emedding Definition. Category C 〝〒〣 object c ∈ C 〠ର「ɺFunctor Hom(−, c): Cop → Set ぇɺ࣍〣〽⿸〠ఆٛ『ɻ 1. object d ∈ C 〠ର「ɺHom(d, c) ∈ Set ぇׂ〿〛ɻ 2. f : d → d′ 〠ର「ɺHom(f , c): Hom(d′, c) → Hom(d, c) ぇׂ 〿〛ɻ〔〕「 Hom(f , c) 〤 g : d′ → c ぇ g ◦ f 〠ׂ〿 〛ɻ 〳〔ɺFunctor y : C → SetCop ぇɺc ∈ C 〠 y(c) = Hom(−, c) ぇɺ f : c → c′ 〠ର「 y(f ): Hom(−, c) ⇒ Hom(−, d) ぇׂ〿⿴〛〷 〣〝『ɻ 〔〕「ɺy(f ) 〤 y(f )d : Hom(d, c) → Hom(d, c′), Hom(d, c) ∋ g → f ◦ g ∈ Hom(d, c′) 〠〽〘〛ఆ〳〷〣〝『ɻ
Universality of comma categories F : C → U, G : D → U 〠ର「ɺ pr0 : F ↓ G → C, pr1 : F ↓ G → D ぇࣗવ〟ํ๏〜ఆٛ『ɻ〈〣 〝 ɺnatural transformation η: F ◦ pr0 ⇒ G ◦ pr1 ぇɺ η⟨c,d,f ⟩ : Fc → Gd f 〝〟〽⿸〠ఆٛ〜 ɻ〈〣〝 ɺҙ〣 category X 〝 functor K : X → C, K′ : X → Dɺnatural transformation θ: F ◦ K ⇒ G ◦ K′ 〠〙⿶〛ɺH : X → F ↓ G Ұ ҙ〠ଘࡏ「〛࣍〣ਤࣜ〿ཱ〙ɻ X F ↓ G D C U H G F η
Homotopy simplicial set X, Y 〝 morphisms f , g : X → Y ⿴〝 ɺ X, Y homotopic 〜⿴〝〤ɺ࣍〣ਤࣜぇՄ〠『 h: X × ∆1 → Y ଘࡏ『〈〝ぇݴ⿸ɻ X X X × ∆1 Y d1 d0 f g h 〳〔ɺ〈〣〝 h 〤 f 〾 g 〭〣 Homotopy 〜⿴〝ݴ⿸ɻ