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Let $K$ be a p-adic field, and let $\mathcal{O}$ be the ring of integers inside $K$ with residue field $k$. Let $\mathcal{X}$ be a smooth proper formal scheme over $\mathcal{O}$ (with topology given by $p$). In this situation, there is a beautiful/famous result goes by the saying "Newton above Hodge". The statement is that the Newton polygon of $H^i_{crys}(\mathcal{X}_0/W(k))[\frac{1}{p}]$ lies above or on the Hodge polygon of $H^i_{dR}(\mathcal{X}_{K})$.

The proof I'm aware of goes by proving Fontaine's Crystalline conjecture (due to Faltings, and many others, then most recently Colmez--Nizioł/Bhatt--Morrow--Scholze), which implies that the Crystalline cohomology $H^i_{crys}(\mathcal{X}_0/W(k))[\frac{1}{p}]$ as a filtered $\phi$ module is weakly admissible. (Obviously tons of details need to be said with this approach.)

This sounds like a somewhat complicated "proof". Does anyone know an alternative proof?

I have seen people stating the above result and refer to either Mazur's paper [Frobenius and the Hodge filtration (estimates)] (which is a special case) or the book by Berthelot--Ogus [Notes on Crystalline Cohomology, 8.36] (which shows the Newton polygon lies on or above the Hodge polygon $\textit{of the special fiber}$). I have also come across the paper by Ogus [Frobenius and the Hodge spectral sequence] (which generalizes the Berthelot--Ogus result above). These are certainly beautiful and related results, but it seems that one cannot quite deduce the statement in the first paragraph out of them.

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  • $\begingroup$ Other references are given on page 25 in Theorem 2.3 of Chambert-Loir's survey paper: webusers.imj-prg.fr/~antoine.chambert-loir/publications/papers/… $\endgroup$ Commented Jun 7, 2018 at 11:59
  • $\begingroup$ @AriyanJavanpeykar Thanks for your comment. I have come across that survey article, the statement of Theorem 2.3 is comparing Newton polygon with Hodge polygon of the special fibre. So does the references given there. This kind of comparison is neat, since it doesn't invoke a lift therefore is applicable to non-liftable varieties. On the other hand, assuming the presence of a lift, the comparison with the Hodge polygon of the generic fibre will be better in general. $\endgroup$
    – S. Li
    Commented Jun 7, 2018 at 14:26

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Frobenius and the Hodge Filtration of the generic fiber has an alternative proof of "Newton over Hodge".

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