I'm currently self-studying Ravi Vakil's Rising Sea. I have been stuck on exercise 2.4.C, which ask one to prove that any compatible germs is the image of a section. The following definition etc are all taken from the notes. If I have understood it correctly we have the map $$I : \mathscr{F}(U) \rightarrow \prod_{p \in U} \mathscr{F}_p$$ given by $ s \mapsto (\bar{s})_p$, i.e mapping a section to its germ in the respective points. Now germs $(s_p)$ are defined to be compatible if there exists a covering $\{U_i\}$ of $U$ and sections $f_i \in U_i$ such that the germ of $f_i$ for all $p \in U_i$ is $s_p$. I think that one is supposed to glue together the sections given in the definition of compatible germs, though I haven't been able to show that the given sections' restrictions are equal on the overlaps/intersections. Many thanks for any help or hint.

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    $\begingroup$ Questions on this website should be as self-contained as possible. Please include the text of the exercise inside the question. $\endgroup$ – KReiser Mar 20 at 21:31
  • $\begingroup$ I have now added an image of the relevent definition and exercise from the book. Hope that this is sufficient :) $\endgroup$ – Najonathan Mar 21 at 15:50
  • $\begingroup$ Is this sufficient to open the question again or should I post a new question? $\endgroup$ – Najonathan Mar 23 at 10:30

Yeah, that's the approach you're supposed to take, I think.

Picking up where you left off, you want to show that, given two sections $f_i \in \mathscr F(U_i)$, $f_j \in \mathscr F(U_j)$, that they agree when restricted to $U_i \cap U_j$.

At any point $p \in U_i \cap U_j$, both $(f_i)\mid_{U_i \cap U_j}$ and $(f_j)\mid_{U_i \cap U_j}$ have the stalk $s_p$. But by 2 4.A, two sections being everywhere stalk-wise equal means that they are the same section.

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