A Carlitz module analogue of the Grunwald--Wang theorem

The classical Grunwald--Wang theorem is an example of a local--global (or Hasse) principle stating that except in some special cases which are precisely determined, an element $m$ in a number field $\mathbb{K}$ is an $a$-th power in $\mathbb{K}$ if and only if it is an $a$-th power in the completion $\mathbb{K}_{v}$ for all but finitely many primes $v$ of $\mathbb{K}$. In this paper, we prove a Carlitz module analogue of the Grunwald--Wang theorem

Carlitz module analogues of Mersenne primes, Wieferich primes, and certain prime elements in cyclotomic function fields

In this paper, we introduce a Carlitz module analogue of Mersenne primes, and prove Carlitz module analogues of several classical results concerning Mersenne primes. In contrast to the classical case, we can show that there are infinitely many composite Mersenne numbers. We also study the acquaintances of Mersenne primes including Wieferich and non-Wieferich primes in the Carlitz module context that were first introduced by Dinesh Thakur

Certain sets over function fields are polynomial families

In 1938, Skolem conjectured that $\mathbf{SL}_n(\mathbb{Z})$ is not a polynomial family for any $n \ge 2$. Carter and Keller disproved Skolem's conjecture for all $n \ge 3$ by proving that $\mathbf{SL}_n(\mathbb{Z})$ is boundedly generated by the elementary matrices, and hence a polynomial family for any $n \ge 3$. Only recently, Vaserstein refuted Skolem's conjecture completely by showing that $\mathbf{SL}_2(\mathbb{Z})$ is a polynomial family. An immediate consequence of Vaserstein's theorem also implies that $\mathbf{SL}_n(\mathbb{Z})$ is a polynomial family for any $n \ge 3$. In this paper, we prove a function field analogue of Vaserstein's theorem: that is, if $\mathbf{A}$ is the ring of polynomials over a finite field of odd characteristic, then $\mathbf{SL}_2(\mathbf{A})$ is a polynomial family in 52 variables. A consequence of our main result also implies that $\mathbf{SL}_n(\mathbf{A})$ is a polynomial family for any $n \ge 3$.Comment: Final versio

Generalized Mordell curves, generalized Fermat curves, and the Hasse principle

A generalized Mordell curve of degree $n \ge 3$ over \bQ is the smooth projective model of the affine curve of the form $Az^2 = Bx^n + C$, where $A, B, C$ are nonzero integers. A generalized Fermat curve of signature $(n, n, n)$ with $n \ge 3$ over \bQ is the smooth projective curve of the form $Ax^n + By^n + Cz^n = 0$ for some nonzero integers $A, B, C$. In this paper, we show that for each prime $p$ with $p \equiv 1 \pmod{8}$ and $p \equiv 2 \pmod{3}$, there exists a threefold \cX_p \subseteq \bP^6 such that certain rational points on \cX_p produce infinite families of non-isomorphic generalized Mordell curves of degree $12n$ and infinite families of generalized Fermat curves of signature $(12n, 12n, 12n)$ for each $n \ge 2$ that are counterexamples to the Hasse principle explained by the Brauer-Manin obstruction. We also show that the set of special rational points on \cX_p producing generalized Mordell curves and generalized Fermat curves that are counterexamples to the Hasse principle is infinite, and can be constructed explicitly.Comment: 45 page

Algebraic families of hyperelliptic curves violating the Hasse principle

In $2000$, Colliot-Th\'el\`ene and Poonen showed how to construct algebraic families of genus one curves violating the Hasse principle. Poonen explicitly constructed an algebraic family of genus one cubic curves violating the Hasse principle using the general method developed by Colliot-Th\'el\`ene and himself. The main result in this paper generalizes the result of Colliot-Th\'el\`ene and Poonen to arbitrarily high genus hyperelliptic curves. More precisely, for $n > 5$ and $n \not\equiv 0 \pmod{4}$, we show that there is an algebraic family of hyperelliptic curves of genus $n$ that is counterexamples to the Hasse principle explained by the Brauer-Manin obstruction

Representation of units in cyclotomic function fields

Hilbert's Satz 90 tells us that for a given cyclic extension $K/k$, a unit of norm $1$ in $K$ can be written as a quotient of conjugate elements in $K$. For the extensions $\mathbb{Q}(\zeta_p)/\mathbb{Q}$ with $p$ prime $> 3$, Newman proved a refinement of Hilbert's Satz 90 that gives a sufficient and necessary condition for which a unit of norm $1$ in $\mathbb{Q}(\zeta_p)$ can be written as a quotient of conjugate units. In order to obtain this result, Newman proved a stronger result that gives a unique representation of units of norm $1$ as a product of a power of $\dfrac{1 - \zeta_p^e}{1 - \zeta_p}$ with a quotient of conjugate units, where $e$ is a given primitive root modulo $p$. In this paper, we obtain a function field analogue of Newman's result for the $\wp$-th cyclotomic function field extensions $\mathbb{K}_{\wp}/\mathbb{F}_q(T)$, where $\wp$ is a monic prime in $\mathbb{F}_q[T]$. As a consequence, we proved a refinement of Hilbert's Satz 90 for the extensions $\mathbb{K}_{\wp}/\mathbb{F}_q(T)$ that gives a sufficient and necessary condition for which a unit of norm $1$ in $\mathbb{K}_{\wp}$ can be written as a quotient of conjugate units

The classical umbral calculus, and the flow of a Drinfeld module

David Goss developed a very general Fourier transform in additive harmonic analysis in the function field setting. In order to introduce the Fourier transform for continuous characteristic $p$ valued functions on $\mathbb{Z}_p$, Goss introduced and studied an analogue of flows in finite characteristic. In this paper, we use another approach to study flows in finite characteristic. We recast the notion of a flow in the language of the classical umbral calculus, which allows to generalize the formula for flows first proved by Goss to a more general setting. We study duality between flows using the classical umbral calculus, and show that the duality notion introduced by Goss seems a natural one. We also formulate a question of Goss about the exact relationship between two flows of a Drinfeld module in the language of the classical umbral calculus, and give a partial answer to it.Comment: Final versio

Function field analogues of Bang-Zsigmondy's theorem and Feit's theorem

In the number field context, Bang-Zsigmondy's theorem states that for any integers $u, m > 1$, there exists a prime divisor $p$ of $u^m - 1$ such that $p$ does not divide $u^n - 1$ for every integer $0 < n < m$ except in some exceptional cases that can be explicitly determined. A prime $p$ satisfying the conditions in Bang-Zsigmondy's theorem is called a Zsigmondy prime for $(u, m)$. In 1988, Feit introduced the notion of large Zsigmondy primes as follows: A Zsigmondy prime $p$ for $(u, m)$ is called a large Zsigmondy prime if either $p > m + 1$ or $p^2$ divides $u^m - 1$. In the same year, Feit proved a refinement of Bang-Zsigmondy's theorem which states that for any integers $u, m > 1$, there exists a large Zsigmondy prime for $(u, m)$ except in some exceptional cases that can be explicitly determined. In this paper, we introduce notions of Zsigmondy primes and large Zsigmondy primes in the Carlitz module context, and prove function field analogues of Bang-Zsigmondy's theorem and Feit's theorem.Comment: Final versio

Waring's problem for unipotent algebraic groups

In this paper, we formulate an analogue of Waring's problem for an algebraic group $G$. At the field level we consider a morphism of varieties $f\colon \mathbb{A}^1\to G$ and ask whether every element of $G(K)$ is the product of a bounded number of elements $f(\mathbb{A}^1(K)) = f(K)$. We give an affirmative answer when $G$ is unipotent and $K$ is a characteristic zero field which is not formally real. The idea is the same at the integral level, except one must work with schemes, and the question is whether every element in a finite index subgroup of $G(\mathcal{O})$ can be written as a product of a bounded number of elements of $f(\mathcal{O})$. We prove this is the case when $G$ is unipotent and $\mathcal{O}$ is the ring of integers of a totally imaginary number field

Towards one-shot learning for rare-word translation with external experts

Neural machine translation (NMT) has significantly improved the quality of automatic translation models. One of the main challenges in current systems is the translation of rare words. We present a generic approach to address this weakness by having external models annotate the training data as Experts, and control the model-expert interaction with a pointer network and reinforcement learning. Our experiments using phrase-based models to simulate Experts to complement neural machine translation models show that the model can be trained to copy the annotations into the output consistently. We demonstrate the benefit of our proposed framework in outof-domain translation scenarios with only lexical resources, improving more than 1.0 BLEU point in both translation directions English to Spanish and German to EnglishComment: 2nd Workshop on Neural Machine Translation and Generation, ACL 201