Theorem. Löb induction for presheaves on a direct category

Let 𝒞︀ be a direct category and 𝑃 a presheaf on it. Every map

𝜑:⊳𝑃→𝑃

has a fixed point: a global element 𝗅ö𝖻𝜑:⊤→𝑃 with

𝗅ö𝖻𝜑=𝗅ö𝖻𝜑⋆𝗇𝖾𝗑𝗍⋆𝜑,

and this fixed point is unique.

The hypothesis says: the value of 𝑃 at any object is determined by its values over the strict past — 𝜑 turns a coherent family over the strict downset of 𝑥 into a value at 𝑥. The proof is recursion along the well-founded ≺: at each 𝑥, the section already constructed over the past assembles into an element of (⊳𝑃)(𝑥), and 𝜑 extends it to 𝑥.

lob-presheaf theorem entries/category/lob-presheaf.hel