Definition. Earlier on presheaves

Later takes a limit over smaller indices. Dually, the earlier modality takes a colimit over larger indices: an element of βŠ²π‘ƒ at π‘₯ is a 𝑃-element sitting at some object 𝑦 strictly above π‘₯, carried down along a chosen morphism π‘₯→𝑦.

Earlier is left adjoint to later:

⊲⊣⊳

Under this adjunction, 𝗇𝖾𝗑𝗍:π‘ƒβ†’βŠ³π‘ƒ corresponds to

𝗉𝗋𝖾𝗏:βŠ²π‘ƒβ†’π‘ƒ,

.

earlier-presheaf definition entries/category/earlier-presheaf.hel