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In Section 7 of Katz' paper:

https://web.math.princeton.edu/~nmk/old/travdwork.pdf

He asserts "Crystalline cohomology tells us that for each integer $i \geq 0$, the de Rham cohomology $H^i=Rf_*(\Omega_X/S_\infty)$ with its Gauss-Manin connection $\nabla$ is the underlying differential equation of an $F$-crystal..."

He cites a huge series of short notes by Berthelot and Berthelot's thesis but I presume by now there is a better reference. Is there a single reference where I can find a proof of this statement?

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  • $\begingroup$ I don't have access, but I imagine Berthelot's book would be the goto: jstor.org/stable/j.ctt130hk6f $\endgroup$ Commented Jan 15 at 13:52
  • $\begingroup$ @Carlo Beenakker Do you have a Theorem number? $\endgroup$ Commented Jan 15 at 17:08
  • $\begingroup$ no, as I said, I do not have access to the book. $\endgroup$ Commented Jan 15 at 21:03

1 Answer 1

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I shall try to stick to the notation in Katz's paper. Let $k$ be a perfect field of characteristic $p>0$. Let $S_{\infty}$ be a $p$-adically complete and separated smooth formal $W(k)$-scheme and $f:X\rightarrow S_{\infty}$ a proper smooth formal $S_{\infty}$-scheme such that de Rham cohomology modules $H^{i}_{\mathrm{dR}}(X/S_{\infty}):=\mathbb{R}^{i}f_{\ast}\Omega^{\bullet}_{X/S_{\infty}}$ are locally free. Let $\nabla$ be the Gauss-Manin connection on $H^{i}_{\mathrm{dR}}(X/S_{\infty})$. Now, let $f_{1}:X_{1}\rightarrow S_{1}$ be the reduction modulo $p$ of $f:X\rightarrow S_{\infty}$. Let $\varphi_{1}:S_{1}\rightarrow S_{1}$ be the absolute Frobenius and let $\varphi:S_{\infty}\rightarrow S_{\infty}$ be a lifting of $\varphi_{1}$. Write $X'\times_{S_{\infty}}S_{\infty}$ and $X'_{1}\times_{S_{1}}S_{1}$ for the base change along $\varphi$ and $\varphi_{1}$, respectively. Let $F:X_{1}\rightarrow X_{1}'$ be the relative Frobenius. Then by functoriality we get an induced morphism (horizontal for $\nabla$) \begin{equation*} F:H^{i}_{\mathrm{cris}}(X'_{1}/S_{\infty})\rightarrow H^{i}_{\mathrm{cris}}(X_{1}/S_{\infty})\,. \end{equation*} But \begin{equation*} H^{i}_{\mathrm{cris}}(X'_{1}/S_{\infty})\cong H^{i}_{\mathrm{dR}}(X'/S_{\infty})\cong\varphi^{\ast}H^{i}_{\mathrm{dR}}(X/S_{\infty}) \end{equation*} where I have used the main property of crystalline cohomology (that it computes the de Rham cohomology of a lift), base change and locally freeness. Composing with $F$ gives me a horizontal map \begin{equation*} F(\varphi):\varphi^{\ast}H^{i}_{\mathrm{dR}}(X/S_{\infty})\rightarrow H^{i}_{\mathrm{dR}}(X/S_{\infty}) \end{equation*} depending on the choice of Frobenius lift $\varphi$. If $\psi$ is another lift of $\varphi_{1}$ then the connection $\nabla$ gives an isomorphism (``parallel transport'') \begin{equation*} \chi(\varphi,\psi):\psi^{\ast}H^{i}_{\mathrm{dR}}(X/S_{\infty})\cong H_{\mathrm{cris}}^{i}(X'_{i}/S_{\infty})\cong \varphi^{\ast} H^{i}_{\mathrm{dR}}(X/S_{\infty})\,, \end{equation*} and $F(\varphi)\circ\epsilon(\varphi,\psi)=F(\psi)$. Overall we get that $(H^{i}_{\mathrm{dR}}(X/S_{\infty}),\nabla, F)$ is an $F$-crystal as in (1.3) of Katz's paper. Note that $\nabla$ satisfies Griffiths transversality with respect to the Hodge filtration, so $(H^{i}_{\mathrm{dR}}(X/S_{\infty}),\nabla, \mathrm{Fil}^{\bullet}, F)$ is a filtered $F$-crystal. If the Hodge cohomology modules $H^{i}(X,\Omega_{X/S_{\infty}}^{j})$ are locally-free then $(H^{i}_{\mathrm{dR}}(X/S_{\infty}),\nabla, \mathrm{Fil}^{\bullet}, F)$ is a Hodge $F$-crystal as in 5.0 of Katz's paper.

I don't know a single reference for all of these things. Surely most of it is in P. Berthelot, Cohomologie cristalline des sch'{e}mas de caract'{e}ristique $p>0$, Lecture Notes in Math., Vol. 407. or the textbook that Carlo Beenakker suggested in the comments: P. Berthelot, A. Ogus, Notes on crystalline cohomology, Princeton University Press, Princeton, NJ, University of Tokyo Press, Tokyo, 1978. There is also Remark (2.9) in P. Berthelot, A. Ogus, $F$-isocrystals and de Rham cohomology. I, Invent. Math. 72 (1983), no. 2, 159–199. which might help you. For the Griffiths transversality bit see section 1.4 of N. Katz, Algebraic solutions of differential equations ($p$-curvature and the Hodge filtration), Invent. Math. 18 (1972), 1–118. for a more general statement.

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  • $\begingroup$ Thanks! This is helpful. Where can I find the statement that 𝐹 is horizontal for ∇? Would you mind saying a few words about that? Also in a related point, how are we defining the Gauss-Manin connection on crystalline cohomology? (I'm sorry if that's a very ignorant question.) $\endgroup$ Commented Jan 18 at 0:50
  • $\begingroup$ That $F$ is horizontal is just functoriality. For the Gauss-Manin connection you can look at Katz-Oda, On the differentiation of de Rham cohomology classes with respect to parameters., J. Math. Kyoto Univ. 8 (1968), 199–213. To see that the connection coming from the crystal is the same as the Gauss-Manin connection, see Chapitre V.3.6 in the Berthelot reference in my answer. $\endgroup$ Commented Jan 18 at 21:50

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