technical note

§ Yoneda Preserves Limits

created 2021-09-24 · last edited 2021-10-04
  • Let JJJ be small, CCC locally small.
  • Let F:J→CF: J \to CF:J→C be a diagram. Let y:C→[Cop,Set]y : C \to [C^{op}, Set]y:C→[Cop,Set] be the contravariant yoneda defined by y(c)≡Hom(−,c)y(c) \equiv Hom(-, c)y(c)≡Hom(−,c).
  • Consider y(lim⁡F):Cop→Sety(\lim F) : C^{op} \to Sety(limF):Cop→Set. Is this equal to lim⁡(y∘F:J→[Cop,Set):Cop→Set\lim (y \circ F : J \to [C^{op}, Set) : C^{op} \to Setlim(y∘F:J→[Cop,Set):Cop→Set?
  • We know that limits in functor categories are computed pointwise. So let's start with lim⁡(y∘F):Cop→Set\lim (y \circ F) : C^{op} \to Setlim(y∘F):Cop→Set. Let's Evaluate at some e∈Cope \in C^{op}e∈Cop.
  • That gives us (lim⁡(y∘F))(e)=lim⁡(eve∘y∘F:J→Set):Set(\lim (y \circ F))(e) = \lim (ev_e \circ y \circ F : J \to Set) : Set(lim(y∘F))(e)=lim(eve​∘y∘F:J→Set):Set.
  • Writing the above out, we get lim⁡(eve∘y∘F)=lim⁡(λj.(y(F(j))(e))\lim (ev_e \circ y \circ F) = \lim(\lambda j. (y(F(j))(e))lim(eve​∘y∘F)=lim(λj.(y(F(j))(e)).
  • Plugging in the definition of yyy, we ge lim⁡(λj.Hom(−,F(j))(e))\lim( \lambda j. Hom(-, F(j))(e))lim(λj.Hom(−,F(j))(e)).
  • Simplifying, we get lim⁡(λj.Hom(e,F(j))\lim (\lambda j. Hom(e, F(j))lim(λj.Hom(e,F(j)).
  • We know from a previous theorem that lim⁡Hom(e,F(−))==Hom(e,lim⁡F)\lim Hom(e, F(-)) = = Hom(e, \lim F)limHom(e,F(−))==Hom(e,limF)
  • Thus, we get lim⁡(λj.Hom(e,F(j))=Hom(e,lim⁡F)\lim(\lambda j. Hom(e, F(j)) = Hom(e, \lim F)lim(λj.Hom(e,F(j))=Hom(e,limF).
  • So we get (lim⁡(y∘F))(e)=Hom(e,lim⁡F)(\lim (y \circ F))(e) = Hom(e, \lim F)(lim(y∘F))(e)=Hom(e,limF).
  • In general, we get lim⁡(y∘F)=Hom(−,lim⁡F)\lim (y \circ F) = Hom(-, \lim F)lim(y∘F)=Hom(−,limF), which is the same as y∘lim⁡Fy \circ \lim Fy∘limF.
  • So we find that lim⁡(y∘F)=y∘lim⁡F\lim (y \circ F) = y \circ \lim Flim(y∘F)=y∘limF, thereby proving that yoneda preserves limits.
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