Kerodon

$\Newextarrow{\xRightarrow}{5,5}{0x21D2}$
$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$

Exercise 4.2.5.11. Define a category $\operatorname{Set}_{\ast }$ as follows:

  • The objects of $\operatorname{Set}_{\ast }$ are pairs $(X,x)$, where $X$ is a set and $x \in X$ is an element.

  • A morphism from $(X,x)$ to $(Y,y)$ in $\operatorname{Set}_{\ast }$ is a function $f: X \rightarrow Y$ satisfying $f(x) = y$.

We will refer to $\operatorname{Set}_{\ast }$ as the category of pointed sets. Let $F: \operatorname{Set}_{\ast } \rightarrow \operatorname{Set}$ denote the forgetful functor, given on objects by the construction $(X,x) \mapsto X$. Show that $F$ is an opfibration in groupoids, but not a fibration in groupoids.