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Phùng Hô Hai

Publications and source records attributed to Phùng Hô Hai.

At least 19 recordsLinked to original sources
arXiv2026

Tannakian duality and Gauss-Manin connections for a family of curves

Let $X/S$ be a smooth family of smooth projective varieties, where $S$ is a smooth affine curve over a field $k$ of characteristic $0.$ We relate the differential fundamental groupoid scheme of $X/k$ with the differential fundamental groupoid scheme of $S/k$ and the relative differential fundamental group of $X/S$ in a short exact sequence. This yields natural maps from the group cohomology of the geometric relative fundamental group to the Gauss-Manin connections. For families of curves of genus at least $1,$ we prove that these maps are isomorphisms. This gives an interpretation of the Gauss-Manin connection in terms of cohomology of the differential fundamental group. As a consequence we can shrink $X$ (as a family on $S$) to obtain a de Rham $K(π,1)$ surface.

math.AG↗
arXiv2025

Singular varieties and infinitesimal non-commutative Witt vectors

Given a projective variety $X$ over an algebraically closed field $k$, M. V. Nori introduced in 1976 a group scheme $π(X)$ which accounts for principal bundles $P\to X$ with finite structure, obtaining in this way an amplification the etale fundamental group. One drawback of this theory is that it is quite difficult to arrive at an explicit description of $π(X)$, whenever it does not vanish altogether. To wit, there are no known non-trivial examples in the literature where $π(X)$ is local, or local of some given height, etc. In this paper we obtain a description of $π(X)$ through amalgamated products of certain non-commutative local group schemes - we called them infinitesimal non-commutative Witt group schemes - in the case where $X$ is a non-normal variety obtained by pinching a simply connected one.

math.AG↗
arXiv2024

On the fundamental group schemes of certain quotient varieties

In \cite{armstrong}, M. Armstrong proved a beautiful result describing fundamental groups of quotient spaces. In this paper we prove an analogue of Armstrong's theorem in the setting of $F$-divided \cite{dS07} and essentially finite \cite{Nori76} fundamental group schemes.

math.AG↗
arXiv2024

Fiber criteria for flatness and homomorphisms of flat affine group schemes

A very useful result concerning flatness in Algebraic Geometry is EGA's ``fiber'' criterion. We propose similar fiber criteria to verify flatness of a module while avoiding ``finiteness'' assumptions. Motivated by a Tannakian viewpoint (where the category of representations comes to the front), we derive applications to the theory of affine and flat group schemes.

math.AC↗
arXiv2023

Algebraic theory of formal regular-singular connections with parameters

This paper is divided into two parts. The first is a review, through categorical lenses, of the classical theory of regular-singular differential systems over $C((x))$ and $\mathbb P^1_C\smallsetminus\{0,\infty\}$, where $C$ is algebraically closed and of characteristic zero. It aims at reading the existing classification results as an equivalence between regular-singular systems and representations of the group $\mathbb Z$. In the second part, we deal with regular-singular connections over $R((x))$ and $\mathbb P_R^1\smallsetminus\{0,\infty\}$, where $R=C[[t_1,\ldots,t_r]]/I$. The picture we offer shows that regular-singular connections are equivalent to representations of $\mathbb Z$, now over $R$.

math.AG↗
arXiv2023

Connections on trivial vector bundles over projective schemes

Over a smooth and proper complex scheme, the differential Galois group of an integrable connection may be obtained as the closure of the transcendental monodromy representation. In this paper, we employ a completely algebraic variation of this idea by restricting attention to connections on trivial vector bundles and replacing the fundamental group by a certain Lie algebra constructed from the regular forms. In more detail, we show that the differential Galois group is a certain ``closure'' of the aforementioned Lie algebra. This is then applied to construct connections on curves with prescribed differential Galois group.

math.AG↗
arXiv2022

Regular-singular connections on relative complex schemes

Deligne's celebrated "Riemann--Hilbert correspondence" relates representations of the fundamental group of a smooth complex algebraic variety and regular-singular integrable connections. In this work, we show how to arrive at a similar statement in the case of a smooth scheme $X$ over the spectrum of a ring $R=\mathbb C[[t_1,\ldots, t_r]]/I$. On one side of the correspondence we have representations on $R$-modules of the fundamental group of the special fibre, and on the other we have certain integrable $R$-connections admitting logarithmic models. The correspondence is then applied to give explicit examples of differential Galois groups of $\mathbb C[[t]]$--connections.

math.AG↗
arXiv2020

On the structure of affine flat group schemes over discrete valuation rings, II

In the 1st part of this work [DHdS18], we studied affine group schemes over a discrete valuation ring (DVR) by means of Neron blowups. We also showed how to apply these findings to throw light on the group schemes coming from Tannakian categories of D-modules. In the present work, we follow up this theme. We show that a certain class of affine group schemes of "infinite type", Neron blowups of formal subgroups, are quite typical. We also explain how these group schemes appear naturally in Tannakian categories of D-modules. To conclude, we isolate a Tannakian property of affine group schemes, named prudence, which allows one to verify if the underlying ring of functions is a free module over the base ring. This is then successfully applied to obtain a general result on the structure of differential Galois groups over complete DVRs.

math.AG↗
arXiv2017

Tannakian duality over Dedekind rings and applications

We establish a duality between flat affine group schemes and rigid tensor categories equipped with a neutral fiber functor (called Tannakian lattice), both defined over a Dedekind ring. We use this duality and the known Tannakian duality due to Saavedra to study morphisms between flat affine group schemes. Next, we apply our new duality to the category of stratified sheaves on a smooth scheme over a Dedekind ring R to define the relative differential fundamental group scheme of the given scheme and compare the fibers of this group scheme with the fundamental group scheme of the fibers. When R is a complete DVR of equal characteristic we show that this category is Tannakian in the sense of Saavedra.

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arXiv2015

On an injectivity lemma in the proof of Tannakian duality

In this short work we give a very short and elementary proof of the injectivity lemma, which plays an important role in the Tannakian duality for Hopf algebras over a field. Based on this we provide some generalizations of this fact to the case of flat algebras over a noetherian domain.

math.RA↗
arXiv2009

Two small remarks on Nori fundamental group scheme

For $X$ a complete, reduced, geometrically connected scheme over a perfect field of characteristic $p>0$, we analyze the decomposition of Nori's fundamental group scheme into its local and étale parts and raise the question of the relation between the geometry and the splitting of the group scheme. We also describe in categorial terms the functor which corresponds to the inclusion of the maximal reduced subgroup scheme.

math.AG↗
arXiv2007

Packets in Grothendieck's Section Conjecture

Using the identification of sections of the Galois group of the ground field into the arithmetic fundamental group with neutral fiber functors of the category of finite connections, we define the "packets" in Grothendieck's section conjecture and show their properties predicted by him.

math.NT↗
arXiv2006

The fundamental groupoid scheme and applications

We define a linear structure on Grothendieck's arithmetic fundamental group $π_1(X, x)$ of a scheme $X$ defined over a field $k$ of characteristic 0. It allows us to link the existence of sections of the Galois group ${\rm Gal}(\bar k/k)$ to $π_1(X, x)$ with the existence of a neutral fiber functor on the category which linearizes it. When applied to Grothendieck section conjecture, it allows us to find a $k$-structure on the universal covering, and $k$-rational pro-points at finite and infinite distance which lift given $k$-rational points. Changes as compared to the first version: (some) typos corrected, english impoved (?), (2.6) better explained, Thm 3.2, 3) made more precise, correspondingly Cor. 4.6 made more precise as well. It equates conjugacy classes of sections with neutral fiber functors. Finally Thm 5.1 establishes a bijection between rational points and neutral fiber functors under the assumption that Grothendieck's conjecture is true.

math.AG↗
arXiv2006

On Nori's Fundamental Group Scheme

We determine the quotient category which is the representation category of the kernel of the homomorphism from Nori's fundamental group scheme to its étale and local parts. Pierre Deligne pointed out an error in the first version of this article. We profoundly thank him, in particular for sending us his enlightning example reproduced in Remark 2.4 2).

math.AG↗