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16 views

Geometric equivalent of the degree zero divisor class group of an algebraic function field (in the singular case)

In an algebraic function field $F/k$ we have the degree zero divisor class group $\text{Cl}^0(F/k)$. Now since any such function field corresponds to the function field of a normal projective curve ...
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1answer
41 views
+100

Adeles for function fields

Usually the ring of adeles is defined for number fields: if $K$ is a number field the ring of adeles of $K$ is: $$\mathbb A_K:=\prod_{v}' K_v \;\;\;\;\;\;\;\;\;\;\;\;\;(\ast)$$ where $v$ ranges ...
6
votes
1answer
78 views

Quotients of Elliptic Curves

I am fairly inexperienced with elliptic curves so there might be aspects of my question that may need better wording but let me know if there are any issues: Question: Say I have an elliptic curve ...
2
votes
1answer
86 views

$A^\times/k^\times$ is a free $\mathbb{Z}$-module of rank of at most $r - 1$

Consider an algebraically closed field $k$, a finite extension $K$ of $k(T)$, the integral closure $A$ of $k[T]$ in $K$, the integral closure $A'$ of $k[1/T]$ in $K$, and the integral closure $A''$ of ...
2
votes
1answer
58 views

Characters on rings of residue classes modulo polynomials over finite fields

First recall the following orthogonality relation on $\mathbb{Z}/n\mathbb{Z}$. Fix $n \in \mathbb{Z}$, $n \neq 0$. For $r \in \mathbb{Q}$, let $e(r) := e^{2 \pi i r}$. Let $x \in \mathbb{Z}$. Then ...
1
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0answers
18 views

Finding Primitive Elements of Separable Function Field Extensions

Suppose you have a a curve $C$ defined by an equation in $x$ and $y$. There is a map from $C$ to $\mathbb{P}_1$ by projection onto $x$. This corresponds to a separable extension of function fields ...
1
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2answers
44 views

Showing automorphisms on $\mathbb{C}(x)$

Let $\mathbb{C}(x)$ denote the field of rational functions over $\mathbb{C}$, the field of complex numbers. Consider the six mappings $\phi : \mathbb{C}(x) → \mathbb{C}(x)$ defined by $\phi_{1}:f(x) ...
2
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0answers
28 views

Computing Galois groups of function fields in sage

I found documentation on how to compute galois groups for number fields in sage. Is it possible to do the same for function field extensions? I only need it in the simple case of $t - f(x)$ over $k(t)$...
1
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1answer
34 views

Algebraic function fields

I am trying to understand what an algebraic function field is, so i was looking for some examples. The example on Wiki says: Given a polynomial ring $k[X,Y]$. Consider the ideal generated by the ...
4
votes
1answer
50 views

$a(x)$, $b(x) \in \mathbb{C}(x)$ and $b(x)^2 = a(x)^3 + 1$ implies $a(x)$, $b(x)$ constant?

If $a(x)$, $b(x) \in \mathbb{C}(x)$ and $b(x)^2 = a(x)^3 + 1$, then does it necessarily follow that $a(x)$ and $b(x)$ are constant? Edit. To clarify, $\mathbb{C}(x)$ is the field of rational ...
1
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1answer
32 views

formal derivative algebraic [closed]

Let $q$ be a prime power, $\mathbb{F}_q$ the field with $q$ elements and $f \in \mathbb{C}_\infty$ be of the form $f = \prod_{i=1}^\infty f_i$ with $f_i \in \mathbb{F}_q(X)$ (here $\mathbb{C}_\infty$ ...
1
vote
1answer
75 views

Some elements of the function field of the Fermat curve

For $n>0$, consider the Fermat curve: $$C(n): \{X^n+Y^n=Z^n\}\subset\mathbb P^2(\mathbb C)$$ the function field of $\mathbb C(n)$ can be explicitly described in the following way. It is the set of ...
5
votes
1answer
55 views

Genus of extension $\mathbb{C}(T)(\sqrt{T^n + 1})$

Let $k = \mathbb{C}$ and $K$ is the extension $\mathbb{C}(T)(\sqrt{T^n + 1})$ of $\mathbb{C}(T)$ with $n \ge 2$ an even integer. I suspect that the genus of $K$ is $(n - 2)/2$, but all attempts at ...
1
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0answers
22 views

compute order x/z in Q

Let $x^3+y^2z+yz^2+\alpha^3z^3=0$ be a curve over $F_{16}$ where $\alpha$ is primitive element for $F_{16}$ and $\alpha^4+\alpha+1=0$. The point $Q=(0:1:0)$ is only infinity point this curve. I want ...
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0answers
26 views

Analogy between group theoretic number theory proofs and function fields

We know that it is possible to prove some number theoretic congruences by using group theoretic tools like Burnside's Lemma for orbit counting. Many of these congruences also hold for $F[x]$ where $F$ ...
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0answers
26 views

Analogue of orbit counting formula in $F[x]$

We know that if $G$ is a finite group acting on a finite set $\Omega$, then $$\sum_{g\in G}\chi(g)\equiv 0\pmod{|G|}$$ where $\chi$ is the permutation character. I wonder if is there an analogue of ...
3
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0answers
42 views

Number of square-free polynomials over a finite field - a combinatorial interpretation?

One can show using zeta functions that the number of (monic)square-free polynomials of degree $n$ over a finite field $\Bbb F_q$ is $q^n-q^{n-1}$. For instance, this is done here. The answer is ...
3
votes
0answers
114 views

Is the function field of a variety a function field?

Let $X$ be an integral Noetherian scheme of dimension $n$ over a field $k$ (arbitrary field). The function field of $X$ is defined as $K(X):=\mathcal O_{X,\eta}$ where $\eta$ is the generic point of ...
0
votes
1answer
40 views

Function field on a regular scheme of dimension $1$

Let $(X,\mathcal O_X)$ be a locally noetherian scheme of dimension $1$ and suppose that $X$ is regular, that is: $\mathcal O_{X,x}$ is a regular local ring. We have no other hypothesis on $X$. What ...
2
votes
1answer
49 views

There are no archimedean function fields

Definition: a field $L\supseteq K$ is called a function field over $K$ if the extension $L|K$ is finitely generated, regular and of transcendence degree $1$. In the book "Topics in the theory of ...
1
vote
1answer
45 views

Expected genus of a function field over a finite field

Let $K=\mathbb F_q(t)$ be the field of rational functions over $\mathbb F_q$ and $f(T)\in K[T]$ be an irreducible polynomial. Let $F=K[y]/f(y)$ and $E$ be the Galois closure of F/K. What should I ...
1
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0answers
33 views

extension of a valuation on a function field

Let $K$ be a field, and $K(x)$ the field of rational functions over $K$. Consider the degree valuation $v$ on $K(x)$, That is $v\left(\frac{f(x)}{g(x)}\right)=\deg(g)-\deg(f)$. So for every $f(x)\in ...
1
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0answers
41 views

The sum of all primitive $n$-th roots of unity in the algebraic closure of $F(x)$

If $n$ is any natural number, then $\mu(n)$ is the sum of all primitive $n$-th roots of unity in $\mathbb{C}$ where $\mu$ is the Möbius function defined as $\mu(n)=0$ if $n$ is not square free and $\...
2
votes
0answers
30 views

Generalisation of Fermat's Little Theorem in Function Fields

There is a well-known generalisation of Fermat's Little Theorem as for any $a\in\mathbb{Z}$ and $n\in\mathbb{N}$ we have $$\sum_{d\mid n} \mu(n/d) a^d\equiv 0\pmod n.$$ where $\mu$ is the Möbius ...
1
vote
2answers
44 views

Finite extensions of $\mathbb F_p(t)$ [closed]

Let $\mathbb F_p(t)$ the field of rational functions with coefficients in $\mathbb F_p$. Is it true or not that every finite extension $K$ of $\mathbb F_p(t)$ is $K\cong\mathbb F_{p^m}(t)$ for some $...
2
votes
1answer
66 views

Is $\mathbb{C}(x,y)$ a rational function field?

Let $\mathbb{C}(x,y)$ be a degree $2$ extension of $\mathbb{C}(x)$ where $y$ is a root of $p(Z)=Z^2 + (x^2+1)$. Is it true that $\mathbb{C}(x,y)$ is not a rational function field? In other words, ...
0
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0answers
23 views

A remark about Eisenstein Criterion in Stichtenoth's Algebraic Function Fields and Codes

I am reading Stichtenoth's Algebraic Function Fields and Codes and am confused about a remark in Chapter 3. He mentions that Proposition 3.1.15, which proves irreducibility of a certain kind of ...
3
votes
1answer
90 views

Endomorphism ring of Drinfeld modules.

Let $\mathcal{X}$ be a smooth geometrically irreducible projective curve over $\mathbb{F}_q$. Fix a closed point $\infty\in \mathcal{X}(\bar{\mathbb{F}_q})$. Let $K$ be the function field of $\mathcal{...
0
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0answers
7 views

does the $\zeta_K$ function of a function field determine the genus of that function field?

Let $K_1$ and $K_2$ both be function fields over a finite field (or algebraic curves, if you like) with zeta functions $\zeta_{K_1}$ and $\zeta_{K_2}$. Say that $\zeta_{K_1} = \zeta_{K_2}$ - so that ...
1
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0answers
30 views

kernel of the artin map when dealing with S-ideles and S-divisors for function fields

Let $L/K$ be an abelian extension of function fields and let $\vartheta_{L/K}:\mathcal{D} \to \text{Gal}(L/K)$ be the Artin map from the divisors of $K$ to the galois group. What can be said about ...
0
votes
1answer
42 views

Relation between ramification locus of a tower and of it's constant field extension

I am trying to understand Remark 7.2.22 (Page 256) of Algebraic Function Fields and Codes (Second Edition) by Henning Stichtenoth. In that remark he considers a tower $\mathcal{F} = (F_0,F_1,F_2,\...
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0answers
36 views

How does Galois theory over function fields compare to that of number fields?

Let $M(S)$ be the function field of a surface. We can then consider field extensions, auto morphia groups, ... How does Galois theory in this setting compare to that of number fields? What plays the ...
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votes
0answers
16 views

Compute $\ell(W+P)$ and $\ell(W-P)$ for a place $P\in\mathbb{P}(F/K)$ of degree $1$ and any $W$ canonical divisor.

Let $F/K$ be a function field of genus $g$. Let $P\in\mathbb{P}(F/K)$ be a place of degree $1$ and $W$ be any canonical divisor. Determine $\ell(W+P)$ and $\ell(W-P)$. If $\ell(P)=\deg(P)+1$, then we ...
2
votes
1answer
95 views

profinite completion of a ring of integers in a global field

It is well known that the profinite completion of the integers, $\hat{\mathbb{Z}} = \varprojlim \mathbb{Z}/n\mathbb{Z}$ is, by the Chinese remainder theorem, isomorphic to $\prod_p \mathbb{Z}_p$. I ...
3
votes
0answers
44 views

Cokernel of map, function field.

Let $F$ be a function field in one variable with total constant field $k$, let $X$ be the set of all places of $F$, and let $S$ be a nonempty finite subset of $X$. We are interested in the dimension ...
2
votes
0answers
26 views

Primes Splitting in the Gaussian Integers - Function Field Analogue

The function field analogue of the ring $\mathbb{Z}[i]$ are functions of the form $A(T)+\alpha B(T)$, where $\alpha$ is a solution to the equation $x^{2}+T=0$, over $F_{q}[T]$. We know that a prime $...
2
votes
0answers
38 views

How computationally difficult is it to determine if two function fields are isomorphic?

Determining if two number fields are isomorphic is a hard problem, according to Cohen in his book A Course in Computational Algebraic Number Theory. Is determining if two function fields are ...
3
votes
1answer
74 views

Irreducibility in $k((t))[y]$

Let $k$ be an algebraically closed field of char $0$ and suppose $f(y) \in k[y]$ (need not be monic). Let $t$ be an indeterminate and consider the fraction field $k((t))$ of power series ring $k[[t]]$....
0
votes
1answer
21 views

function field of open curve and compactificaton

Let $U$ be a smooth curve over some field $k$ and $C$ the only smooth projective curve containing $U$ as a dense open subset. Can someone help me understading why the function fields of $U$ and $C$ ...
0
votes
1answer
137 views

poles and zeros of function field of $\mathbb{P}^1$.

In which condition: an element of function field of $\mathbb{P}^1$ has zero or pole or no-zero&no-pole. I am thinking that: since $\mathbb{P}^1$ and $\mathbb{A}^1$ is birrationally equivalent (i....
1
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0answers
26 views

Representation Matrix computation time estimation

Abstract/general version of the question If we have a matrix $$ M_z = \begin{pmatrix} \sum_{j=1}^{n}\lambda_j \lambda_{j,1,1} &\dots & \sum_{j=1}^{n}\lambda_j \lambda_{j,n,1}\\ \sum_{j=1}^...
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0answers
25 views

Product of monic polynomials in finite fields

I am trying to show that the product of monic polynomials of degree $n$ in $\mathbb{F}_p[T]$ is given by $\prod_{i=0}^{n}(T^{p^n}-T^{p^i})$. I tried generating function but with no luck. Any hint?
3
votes
1answer
72 views

computing the full constant field of an algebraic function field

Let $K$ be a field such that char$(K) \neq 2$. Let $F=K(x,y)$ be an algebraic function field of one variable $x$ where $$y^2 = f(x) \in K[x].$$ We want to compute the full constant field of $F$ (i.e. ...
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0answers
53 views

Unramified Galois extension of a function field of a curve - definition

Reading some papers I've encountered the following: Let $X$ be a smooth curve over a field $K$ with function field $K(X)$. Consider the maximal Galois extension of $K(X)$ which is unramified over $X$. ...
3
votes
1answer
125 views

When a holomorphy ring is a PID?

I will use the notation and language of Stichtenoth, Algebraic Function Fields and Codes. Let $F$ be a function field over a finite field $\mathbb F_q$, $S$ a non empty set of places (possibly ...
1
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0answers
52 views

When is a holomorphy ring a PID? [duplicate]

I will use the notation and language of Stichtenoth, Algebraic Function Fields and Codes. Let $F$ be a function field over a finite field $\mathbb F_q$, $S$ a non empty set of places (possibly ...
1
vote
0answers
56 views

valuation on function fields

Let $p$ be an irreducible polynomial in $k[x]$, for some characteristic 0 field $k$. So we have a valuation $v_p$ corresponding to $p$. Now take $a$ to be a root of $p(x)$. Then $(x-a)$ is irreducible ...
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votes
0answers
44 views

genus of the field $\mathbb{R}(x,y)$ where $x^2+y^2+1=0$

I have come across a problem while reading Chevelley's "Introduction to the theory of algebraic functions of one variable", in which he says that the genus of the field $L=\mathbb{R}(x,y)$ is 0, where ...
2
votes
1answer
83 views

Explicit examples of higher genus Drinfeld modules?

Let $C$ is a smooth, projective, geometrically irreducible curve over $\mathbb{F}_q$ and $\infty$ a closed point of $C$. A Drinfel'd module over $A = H^0(C, \mathcal{O}_C)$ is an injective ...
3
votes
2answers
84 views

$\dim (D-P)=\dim (D)-1$

I'm trying to prove this question: Let $D$ be a divisor in $F|K$ such that $\dim (D)\gt 0$ and $0 \neq f\in \mathscr L(D)$. Thus $f\notin \mathscr L(D-P)$ for almost all $P$. Then show that $$\...