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Feng-Wen An

Publications and source records attributed to Feng-Wen An.

At least 19 recordsLinked to original sources
arXiv2019

Transcendental Galois Theory and Noether's Problem

In 1918, Noether published a paper where she studied such a problem, now called Noether's problem on rationality: Let $L=K\left( t_{1},t_{2},\cdots ,t_{n}\right) $ be a purely transcendental extension over a field $K$ and $G$ a finite subgroup acting transitively on $t_{1},t_{2},\cdots ,t_{n}$ in an evident manner. Is it true that the invariant subfield $L^{G}$ of $L$ under $% G$ is still purely transcendental over $K$? The problem has been open in general except for minor particular cases. In this paper we will attempt to understand a general theory for Noether's problem on rationality by transcendental Galois theory. Then new particular cases will be obtained. We will also give a generalization for the remarkable counter-example given by Swan in 1969.

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arXiv2010

On the étale fundamental groups of arithmetic schemes, revised

In this paper we will give a computation of the étale fundamental group of an integral arithmetic scheme. For such a scheme, we will prove that the étale fundamental group is naturally isomorphic to the Galois group of the maximal formally unramified extension over the function field. It consists of the main theorem of the paper. Here, formally unramified will be proved to be arithmetically unramified which is defined in an evident manner and coincides with that in algebraic number theory. Hence, formally unramified has an arithmetic sense. At the same time, such a computation coincides with the known result for a normal noetherian scheme.

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arXiv2010

Affine Structures on a Ringed Space and Schemes

In this paper we will first introduce the notion of affine structures on a ringed space and then obtain several properties. Affine structures on a ringed space, arising mainly from complex analytical spaces of algebraic schemes over number fields, behave like differential structures on a smooth manifold. As one does for differential manifolds, we will use pseudogroups of affine transformations to define affine atlases on a ringed space. An atlas on a space is said to be an affine structure if it is maximal. An affine structure is admissible if there is a sheaf on the underlying space such that they are coincide on all affine charts, which are in deed affine open sets of a scheme. In a rigour manner, a scheme is defined to be a ringed space with a specified affine structure if the affine structures are in action in some special cases such as analytical spaces of algebraic schemes. Particularly, by the whole of affine structures on a space, we will obtain respectively necessary and sufficient conditions that two spaces are homeomorphic and that two schemes are isomorphic, which are the two main theorems of the paper. It follows that the whole of affine structures on a space and a scheme, as local data, encode and reflect the global properties of the space and the scheme, respectively.

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arXiv2010

On the transcendental Galois extensions

In this paper the transcendental Galois extensions of a field will be introduced as counterparts to algebraic Galois ones. There exist several types of transcendental Galois extensions of a given field, from the weakest one to the strongest one, such as Galois, tame Galois, strong Galois, and absolute Galois. The four Galois extensions are distinct from each other in general but coincide with each other for cases of algebraic extensions. The transcendental Galois extensions arise from higher relative dimensional Galois covers of arithmetic schemes. In the paper we will obtain several properties of Galois extensions in virtue of conjugation and quasi-galois and will draw a comparison between the Galois extensions. Strong Galois is more accessible. It will be proved that a purely transcendental extension is strong Galois.

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arXiv2010

On the unramified extension of an arithmetic function field in several variables

In this paper we will give a scheme-theoretic discussion on the unramified extensions of an arithmetic function field in several variables. The notion of unramified discussed here is parallel to that in algebraic number theory and for the case of classical varieties, coincides with that in Lang's theory of unramified class fields of a function field in several variables. It is twofold for us to introduce the notion of unramified. One is for the computation of the étale fundamental group of an arithmetic scheme; the other is for an ideal-theoretic theory of unramified class fields over an arithmetic function field in several variables. Fortunately, in the paper we will also have operations on unramified extensions such as base changes, composites, subfields, transitivity, etc. It will be proved that a purely transcendental extension over the rational field has a trivial unramified extension. As an application, it will be seen that the affine scheme of a ring over the ring of integers in several variables has a trivial étale fundamental group.

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arXiv2009

Algebraic Anabelian Functors

In this paper we will prove that there exists a covariant functor, called algebraic anabelian functor, from the category of algebraic schemes over a given field to the category of outer homomorphism sets of groups. The algebraic anabelian functor, given in a canonical manner, is full and faithful. It reformulates the anabelian geometry over a field. As an application of the anabelian functor, we will also give a proof of the section conjecture of Grothendieck for the case of algebraic schemes.

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arXiv2009

On the arithmetic fundamental groups

In this paper we will define a qc fundamental group for an arithmetic scheme by quasi-galois closed covers. Then we will give a computation for such a group and will prove that the etale fundamental group of an arithmetic scheme is a normal subgroup in our qc fundamental group, which make up the main theorem of the paper. Hence, our group gives us a prior estimate of the etale fundamental group. The quotient group reflects the topological properties of the scheme.

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arXiv2009

On the algebraic fundamental groups

Passing from arithmetic schemes to algebraic schemes, in a similar manner we will have the computation of the étale fundamental group of an algebraic scheme and then will define and discuss the qc fundamental group of an algebraic scheme in this paper. The qc fundamental group will also give a prior estimate of the étale fundamental group.

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arXiv2009

Notes on the quasi-galois closed schemes

Let $f:X\to Y$ be a surjective morphism of integral schemes. Then $X$ is said to be quasi-galois closed over $Y$ by $f$ if $X$ has a unique conjugate over $Y$ in an algebraically closed field. Such a notion has been applied to the computation of étale fundamental groups. In this paper we will use affine coverings with values in a fixed field to discuss quasi-galois closed and then give a sufficient and essential condition for quasi-galois closed. Here, we will avoid using affine structures on a scheme since their definition looks copious and fussy.

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arXiv2009

On the section conjecture of Grothendieck

For a given arithmetic scheme, in this paper we will introduce and discuss the monodromy action on a universal cover of the étale fundamental group and the monodromy action on an \emph{sp}-completion constructed by the graph functor, respectively; then by these results we will give a proof of the section conjecture of Grothendieck for arithmetic schemes.

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arXiv2009

Notes on the section conjecture of Grothendieck

In this short note, we will give the key point of the section conjecture of Grothendieck, that is reformulated by monodromy actions. Here, we will also give the result of the section conjecture for algebraic schemes over a number field.

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arXiv2009

Automorphism Groups of Quasi-galois Closed Arithmetic Schemes

Assume that $X$ and $Y$ are arithmetic schemes, i.e., integral schemes of finite types over $Spec(\mathbb{Z})$. Then $X$ is said to be quasi-galois closed over $Y$ if $X$ has a unique conjugate over $Y$ in some certain algebraically closed field, where the conjugate of $X$ over $Y$ is defined in an evident manner. Now suppose that $ϕ:X\to Y$ is a surjective morphism of finite type such that $X$ is quasi-galois closed over $Y$. In this paper the main theorem says that the function field $ k(X)$ is canonically a Galois extension of $k(Y)$ and the automorphism group ${Aut}(X/Y)$ is isomorphic to the Galois group $Gal(k(X)/k(Y))$; in particular, $ϕ$ must be affine. Moreover, let $\dim X=\dim Y$. Then $X$ is a pseudo-galois cover of $Y$ in the sense of Suslin-Voevodsky.

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arXiv2009

The Combinatorial Norm of a Morphism of Schemes

In this paper we will prove that there exists a covariant functor from the category of schemes to the category of graphs. This functor provides a combination between algebraic varieties and combinatorial graphs so that the invariants defined on graphs can be introduced to algebraic varieties in a natural manner. By the functor, we will define the combinatorial norm of a morphism of schemes. Then we will obtain some properties of morphisms of norm not great than one. The topics discussed here can be applied to study the discrete Morse theory on arithmetic schemes and Kontsevich's theory of graph homology.

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arXiv2007

The Conjugates of Algebraic Schemes

Fixed an algebraic scheme $Y$. We suggest a definition for the conjugate of an algebraic scheme $X$ over $Y$ in an evident manner; then $X$ is said to be Galois closed over $Y$ if $X$ has a unique conjugate over $Y$. Now let $X$ and $Y$ both be integral and let $X$ be Galois closed over $Y$ by a surjective morphism $ϕ$ of finite type. Then $ϕ^{\sharp}(k(Y))$ is a subfield of $k(X)$ by $ϕ$. The main theorem of this paper says that $k(X) /ϕ^{\sharp}(k(Y)) $ is a Galois extension and the Galois group $Gal(k(X)/ϕ^{\sharp}(k(Y))) $ is isomorphic to the group of $k-$automorphisms of $X$ over $Y$.

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