Skip to content

Commit

Permalink
Fix example
Browse files Browse the repository at this point in the history
Argh... not sure what I was thinking before...
  • Loading branch information
aisejohan committed Jun 7, 2024
1 parent 208eca8 commit 9daea86
Showing 1 changed file with 13 additions and 11 deletions.
24 changes: 13 additions & 11 deletions sites.tex
Original file line number Diff line number Diff line change
Expand Up @@ -4950,17 +4950,19 @@ \section{Cocontinuous functors which have a right adjoint}
Lemma \ref{lemma-continuous-with-continuous-left-adjoint}
shows that if $v$ is continuous, then $u$ is cocontinous.
Conversely, if $u$ is cocontinuous, then we can't conclude that $v$
is continuous. Namely, consider a topological space $X$ and
the site $X_{Zar}$ of Example \ref{example-site-topological}.
On the other hand, given an open covering $X = W \cup V$
we can consider the site $X'_{Zar}$ defined in exactly the
same manner, except that we declare $U = \bigcup U_i$ to be
a covering if and only if for each $i$ either $U_i \subset W$
or $U_i \subset V$. Set $u = v = \text{id}$. Then
$v$ viewed as functor $X'_{Zar} \to X_{Zar}$ is continuous
(as any covering in $X_{Zar}$ can be refined by a covering in
$X'_{Zar}$) but $u$ viewed as a functor $X_{Zar} \to X'_{Zar}$
is not continuous (provided neither $W \not = X$ and $V \not = X$).
is continuous. We will give an example of this phenomenon using
the big \'etale and smooth sites of a scheme, but presumably there is an
elementary example as well. Namely, consider a scheme $S$ and the sites
$(\Sch/S)_\etale$ and $(\Sch/S)_{smooth}$. We may assume these
sites have the same underlying category, see
Topologies, Remark \ref{topologies-remark-choice-sites}.
Let $u = v = \text{id}$. Then $u$ as a functor
from $(\Sch/S)_\etale$ to $(\Sch/S)_{smooth}$ is cocontinuous
as every smooth covering of a scheme can be refined by an \'etale
covering, see More on Morphisms, Lemma
\ref{more-morphisms-lemma-etale-dominates-smooth}.
Conversely, the functor $v$ from $(\Sch/S)_{smooth}$ to $(\Sch/S)_\etale$
is not continuous as a smooth covering is not an \'etale covering in general.
\end{example}


Expand Down

0 comments on commit 9daea86

Please sign in to comment.