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The perfection of a ring $A$ of prime characteristic $p$ is the perfect ring $A_\rm{pf}=$ lim{$A\to A\to ...$} where all maps are Frobenius, so being a filtered colimit in Rings, the perfection functor commutes with finite products. But, in general, does perfection commutes with products? If not, why in a scheme $X$, the presheaf associated to $U\to \Gamma (U,\mathcal{O_\rm{X}})_\rm{pf}$ is a sheaf? This is stated in section 6 in Greenberg: Perfect closures of rings and schemes but I do not understand the proof if perfection does not commute with products.

I asked this question in math.stackexchange a few days ago, but there was no answer.

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    $\begingroup$ Perfection is the state of being perfect. Isn't this operation rather "perfectification"? $\endgroup$
    – YCor
    Commented Mar 16, 2020 at 18:57
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    $\begingroup$ I don't know enough english to have an opinion (I take the word from some papers, for instance by Bhatt, Scholze, ...). It is also called perfect closure or direct perfection in other papers. However I have at hand the Collins English Dictionary, and as first meaning of perfection says: 1. the act of perfecting or the state or quality of being perfect. $\endgroup$
    – A.G
    Commented Mar 17, 2020 at 10:24
  • $\begingroup$ Indeed I only knew the second meaning, thanks! $\endgroup$
    – YCor
    Commented Mar 17, 2020 at 12:13
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    $\begingroup$ The reason it is not "perfectification" is that "perfect" is both a noun and a verb (meaning "to make perfect", pronounced with the stress on the second syllable instead of the first), so there is no verb "perfectify". $\endgroup$ Commented Mar 18, 2020 at 2:07

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No, even restricting to domains: it doesn't even commute to infinite powers.

Indeed, let $A$ be a non-perfect domain. So $A_{\mathrm{pf}}$ is an overring of $A$, such that for every $x\in A_{\mathrm{pf}}$ there exists some minimal $n=N(x)\ge 0$ such that $x^{p^n}\in A$. Then $x\mapsto N(x)$ is surjective onto non-negative integers. Choose $x_n\in A_{\mathrm{pf}}$ with $N(x_n)=n$. Then the sequence $(x_n)$, viewed as element of $(A_{\mathrm{pf}})^\mathbf{N}$ has no $p$-power power in $A^\mathbf{N}$. Thus, in this case we see that $(A^\mathbf{N})_{\mathrm{pf}}$ is a proper subring of $(A_{\mathrm{pf}})^\mathbf{N}$.

Note: $A_{\mathrm{pf}}$ is the initial object in the category of unital commutative rings (of characteristic $p$), endowed with a homomorphism from $A$, and in which $x\mapsto x^p$ is bijective. In particular there's a canonical homomorphism $\big(\prod_i A_i\big)_{\mathrm{pf}}\to \prod_i ((A_i)_{\mathrm{pf}})$. In the above example, it fails to be surjective. I haven't checked carefully but it seems to always be injective.


Edit: it's not always injective, even for infinite powers. Denote $\phi=\phi_A:A\to A$, $x\mapsto x^p$ (for $A$ commutative ring of characteristic $p$). First observe that the kernel of $A\to A_{\mathrm{pf}}$ is $\bigcup_n\mathrm{Ker}(\phi_A^n)$ (indeed, one directly check that for $B=A/\bigcup_n\mathrm{Ker}(\phi_A^n)$, we have $\phi_B$ injective).

Let $A$ be such that $\phi:x\mapsto x^p$ has nontrivial kernel. Hence for every $n\ge 0$ there exists $x_n$ in $A$ such that $x_n\in\mathrm{Ker}(\phi^{n+1})\smallsetminus\mathrm{Ker}(\phi^{n})$. Then $(x_n)_{n\ge 0}$ is an element of $A^\mathbf{N}$ inducing a nonzero element in the kernel of $(A^\mathbf{N})_{\mathrm{pf}}\to (A_{\mathrm{pf}})^\mathbf{N}$.

Note however that if each $A_i$ is reduced (i.e., has injective $\phi_{A_i}$), then $\big(\prod_i A_i\big)_{\mathrm{pf}}\to \prod_i ((A_i)_{\mathrm{pf}})$ is injective.

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    $\begingroup$ I'll leave the second question to competent users. At this point, I guess it would better be asked separately. $\endgroup$
    – YCor
    Commented Mar 16, 2020 at 17:13
  • $\begingroup$ Thank you. I will check the injectivity (that would be sufficient for the second question), $\endgroup$
    – A.G
    Commented Mar 16, 2020 at 17:22
  • $\begingroup$ Thank you again (after your second edit) for your time. $\endgroup$
    – A.G
    Commented Mar 16, 2020 at 19:15

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