The actual question is slightly more general than that in the title:

Let $p: U\to Y$ be a surjective étale morphism and $Y\to X$ be a finite morphism of schemes. Is there an étale cover $V\to X$ (surjective) such that the base change $p': V\times_X U\to V\times_X Y$ admits a section?

If necessary, one can also assume that $Y\to X$ is surjective.

I think this is implicitly used in Lecture Notes on Motivic Cohomology by Carlo Mazza, Vladimir Voevodsky, Charles Weibel, Lemma 6.16. I don't know how to go through. I can prove this when $p$ is also a finite morphism and $Y=X$ and $Y\to X$ is the identity but not in general.

It would be very helpful to me to have a reference or an answer to this question.

  • $\begingroup$ Consider the case $X=Y$. If $p:U\to Y$ is not surjective, then neither is $p'$, so it cannot have a section. Conversely, if $p$ is surjective, then you can take $V=U$ and then the diagonal morphism gives a section. $\endgroup$ Apr 9 '20 at 16:06
  • 2
    $\begingroup$ Sorry, I forgot to say $p$ is surjective...edited just now. But $V=U$ may be not étale over $X$... $\endgroup$
    – Lao-tzu
    Apr 9 '20 at 16:07

We can work locally on $X$ and even (by standard limit arguments) assume that $X=\mathrm{Spec}(R)$ where $R$ is local and strictly henselian. Then $Y=\coprod_{i=1}^{n}Y_i$ where each $Y_i$ is local and finite over $X$, in particular strictly henselian too. So $U\times_Y Y_i\to Y_i$ has a section since it is étale and surjective.

  • $\begingroup$ I was meant to have an étale cover $V\to X$ (surjective) of $X$ such that the base change $p': V\times_X U\to V\times_X Y$ admits a section. I think your argument gives an étale cover of $Y$. $\endgroup$
    – Lao-tzu
    Apr 9 '20 at 16:53
  • $\begingroup$ @Lao-tzu No, the argument gives an etale cover of $X$, which by the limit argument is equivalent to assuming $X$ the spectrum of a strictly henselian local ring. $\endgroup$
    – Will Sawin
    Apr 9 '20 at 17:29
  • $\begingroup$ @Will Sawin Sorry I don't quite know how to use a limit argument, could any of you give more detail on how this works? $\endgroup$
    – Lao-tzu
    Apr 9 '20 at 17:34
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    $\begingroup$ @Lao-tzu It suffices to show that for each point there is an etale neighborhood of that point where the base change admits a section, as then the disjoint union of these over all points (or, if $X$ is quasicompact, a finite disjoint union) will be a cover with the desired properties. Now Laurent's argument gives a section over the inverse limit of all etale neighborhoods of that point. We want to show the section is defined over one of them. $\endgroup$
    – Will Sawin
    Apr 9 '20 at 18:51
  • $\begingroup$ @Lao-tzu This follows because $p$, being etale, is of finite presentation, so the finitely many generators of the ring of functions on $p$ must go to functions defined on one of these etale neighborhoods (because, by definition, all functions on the limit are on one of the neighborhoods). $\endgroup$
    – Will Sawin
    Apr 9 '20 at 18:52

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