Definition 6.30.1. Let $X$ be a topological space. Let $\mathcal{B}$ be a basis for the topology on $X$.

A presheaf $\mathcal{F}$ of sets on $\mathcal{B}$ is a rule which assigns to each $U \in \mathcal{B}$ a set $\mathcal{F}(U)$ and to each inclusion $V \subset U$ of elements of $\mathcal{B}$ a map $\rho ^ U_ V : \mathcal{F}(U) \to \mathcal{F}(V)$ such that $\rho ^ U_ U = \text{id}_{\mathcal{F}(U)}$ for all $U \in \mathcal{B}$ whenever $W \subset V \subset U$ in $\mathcal{B}$ we have $\rho ^ U_ W = \rho ^ V_ W \circ \rho ^ U_ V$.

A morphism $\varphi : \mathcal{F} \to \mathcal{G}$ of presheaves of sets on $\mathcal{B}$ is a rule which assigns to each element $U \in \mathcal{B}$ a map of sets $\varphi : \mathcal{F}(U) \to \mathcal{G}(U)$ compatible with restriction maps.

Your definition of presheaf seems non-standard to me. Do you not want to also demand that the restriction from to is the identity map? [this is not implied by what you have at the time of writing; for example if is your favourite set with two elements and is your favourite element of this set, then we can define for all and we can let be the constant map sending everything to for all and ]

Dear Kevin, OK I finally got to your comment! Of course, this is an omission and I didn't really intend to have a nonstandard definition of a presheaf! Fixed here.

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## Comments (2)

Comment #3218 by Kevin Buzzard on

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