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Lei Fu

Publications and source records attributed to Lei Fu.

50 records · Page 3Linked to original sources
arXiv2016

$\ell$-adic GKZ hypergeometric sheaf and exponential sums

To a torus action on a complex vector space, Gelfand, Kapranov and Zelevinsky introduce a system of differential equations, called the GKZ hypergeometric system. Its solutions are GKZ hypergeometric functions. We study the $\ell$-adic counterpart of the GKZ hypergeometric system, which we call the $\ell$-adic GKZ hypergeometric sheaf. It is an object in the derived category of $\ell$-adic sheaves on the affine space over a finite field. Traces of Frobenius on stalks of this object at rational points of the affine space define the hypergeometric functions over the finite field introduced by Gelfand and Graev. We prove that the $\ell$-adic GKZ hypergeometric sheaf is perverse, calculate its rank, and prove that it is irreducible under the non-resonance condition. We also study the weight filtration of the GKZ hypergeometric sheaf, determine its lisse locus, and apply our result to the study of weights of twisted exponential sums.

math.AG↗
arXiv2013

A Thom-Sebastiani Theorem in Characteristic p

Let $k$ be a perfect field of characteristic $p$, let $f_i:X_i\to\mathbb A_k^1$ $(i=1,2)$ be two $k$-morphism of finite type, and let $f:X_1\times_k X_2\to \mathbb A_k^1$ be the morphism defined by $f(z_1,z_2)=f_1(z_1)+f_2(z_2)$. For each $i\in\{1,2\}$, let $x_i$ be a $k$-rational point in the fiber $f_i^{-1}(0)$ such that $f_i$ is smooth on $X_i-\{x_i\}$. Using the $\ell$-adic Fourier transformation and the stationary phase principle of Laumon, we prove that the vanishing cycle of $f$ at $x=(x_1,x_2)$ is the convolution product of the vanishing cycles of $f_i$ at $x_i$ $(i=1,2)$.

math.AG↗
arXiv2013

Population synthesis of young isolated neutron stars: the effect of fallback disk accretion and magnetic field evolution

The spin evolution of isolated neutron stars (NSs) is dominatd by their magnetic fields. The measured braking indices of young NSs show that the spin-down mechanism due to magnetic dipole radiation with constant magnetic fields is inadequate. Assuming that the NS magnetic field is buried by supernova fallback matter and re-emerges after accretion stops, we carry out Monte-Carlo simulation of the evolution of young NSs, and show that most of the pulsars have the braking indices ranging from -1 to 3. The results are compatible with the observational data of NSs associated with supernova remnants. They also suggest that the initial spin periods of NSs might occupy a relatively wide range.

astro-ph.HE↗
arXiv2012

Could SXP 1062 be an Accreting Magnetar?

In this work we explore the possible evolutionary track of the neutron star in the newly discovered Be/X-ray binary SXP 1062, which is believed to be the first X-ray pulsar associated with a supernova remnant. Although no cyclotron feature has been detected to indicate the strength of the neutron star's magnetic field, we show that it may be $\ga 10^{14} $G. If so SXP 1062 may belong to the accreting magnetars in binary systems. We attempt to reconcile the short age and long spin period of the pulsar taking account of different initial parameters and spin-down mechanisms of the neutron star. Our calculated results show that, to spin down to a period $\sim 1000 $s within $10-40 $kyr requires efficient propeller mechanisms. In particular, the model for angular momentum loss under energy conservation seems to be ruled out.

astro-ph.HE↗
arXiv2011

On Pulsar-Driven Mass Ejection in Low-Mass X-ray Binaries

There is accumulating evidence for mass ejection in low-mass X-ray binaries (LMXBs) driven by radio pulsar activity during X-ray quiescence. In this paper we consider the condition for mass ejection by comparing the radiation pressure from a millisecond pulsar, and the gas pressure at the inner Lagrange point or at the surrounding accretion disk. We calculate the critical spin period of the pulsar below which mass ejection is allowed. Combining with the evolution of the mass transfer rate, we present constraints on the orbital periods of the systems. We show that mass ejection could happen in both wide and compact LMXBs. It may be caused by transient accretion due to thermal instability in the accretion disks in the former, and irradiation-driven mass-transfer cycles in the latter.

astro-ph.HE↗
arXiv2010

Calculation of l-adic local Fourier transformations

We calculate the local Fourier transformations for a class of $\bar{\mathbb Q}_\ell$-sheaves. In particular, we verify a conjecture of Laumon and Malgrange. As an application, we calculate the local monodromy of $\ell$-adic hypergeometric sheaves introduced by Katz. We also discuss the characteristic $p$ analogue of the Turrittin-Levelt Theorem for $D$-modules. The method used in this paper can be used to show a conjecture of Ramero which states that the Fourier transformation of an analytic sheaf with meromorphic ramification still has meromorphic ramification.

math.NT↗
arXiv2010

A Tauberian Theorem for $\ell$-adic Sheaves on $\mathbb A^1$

Let $K\in L^1(\mathbb R)$ and let $f\in L^\infty(\mathbb R)$ be two functions on $\mathbb R$. The convolution $$(K\ast f)(x)=\int_{\mathbb R}K(x-y)f(y)dy$$ can be considered as an average of $f$ with weight defined by $K$. Wiener's Tauberian theorem says that under suitable conditions, if $$\lim_{x\to \infty}(K\ast f)(x)=\lim_{x\to \infty} (K\ast A)(x)$$ for some constant $A$, then $$\lim_{x\to \infty}f(x)=A.$$ We prove the following $\ell$-adic analogue of this theorem: Suppose $K,F, G$ are perverse $\ell$-adic sheaves on the affine line $\mathbb A$ over an algebraically closed field of characteristic $p$ ($p\not=\ell$). Under suitable conditions, if $$(K\ast F)|_{η_\infty}\cong (K\ast G)|_{η_\infty},$$ then $$F|_{η_\infty}\cong G|_{η_\infty},$$ where $η_\infty$ is the spectrum of the local field of $\mathbb A$ at $\infty$.

math.AG↗
arXiv2008

Functional Equations of $L$-Functions for Symmetric Products of the Kloosterman Sheaf

We determine the (arithmetic) local monodromy at 0 and at $\infty$ of the Kloosterman sheaf using local Fourier transformations and Laumon's stationary phase principle. We then calculate $ε$-factors for symmetric products of the Kloosterman sheaf. Using Laumon's product formula, we get functional equations of $L$-functions for these symmetric products, and prove a conjecture of Evans on signs of constants of functional equations.

math.NT↗
arXiv2007

L-functions of Symmetric Products of the Kloosterman Sheaf over Z

The classical $n$-variable Kloosterman sums over the finite field ${\bf F}_p$ give rise to a lisse $\bar {\bf Q}_l$-sheaf ${\rm Kl}_{n+1}$ on ${\bf G}_{m, {\bf F}_p}={\bf P}^1_{{\bf F}_p}-\{0,\infty\}$, which we call the Kloosterman sheaf. Let $L_p({\bf G}_{m,{\bf F}_p}, {\rm Sym}^k{\rm Kl}_{n+1}, s)$ be the $L$-function of the $k$-fold symmetric product of ${\rm Kl}_{n+1}$. We construct an explicit virtual scheme $X$ of finite type over ${\rm Spec} {\bf Z}$ such that the $p$-Euler factor of the zeta function of $X$ coincides with $L_p({\bf G}_{m,{\bf F}_p}, {\rm Sym}^k{\rm Kl}_{n+1}, s)$. We also prove similar results for $\otimes^k {\rm Kl}_{n+1}$ and $\bigwedge^k {\rm Kl}_{n+1}$.

math.AG↗
arXiv2007

Twisted Exponential Sums

Let $k$ be a finite field of characteristic $p$, $l$ a prime number distinct to $p$, $ψ:k\to \bar {\bf Q}_l^\ast$ a nontrivial additive character, and $χ:{k^\ast}^n\to \bar{\bf Q}_l^\ast$ a character on ${k^\ast}^n$. Then $ψ$ defines an Artin-Schreier sheaf ${\cal L}_ψ$ on the affine line ${\bf A}_k^1$, and $χ$ defines a Kummer sheaf ${\cal K}_χ$ on the $n$-dimensional torus ${\bf T}_k^n$. Let $f\in k[X_1,X_1^{-1},..., X_n,X_n^{-1}]$ be a Laurent polynomial. It defines a $k$-morphism $f:{\bf T}_k^n\to {\bf A}_k^1$. In this paper, we calculate the dimensions and weights of $H_c^i({\bf T}_{\bar k}^n, {\cal K}_χ\otimes f^\ast {\cal L}_ψ)$ under some non-degeneracy conditions on $f$. Our results can be used to estimate sums of the form $$\sum_{x_1,..., x_n\in k^\ast} χ_1(f_1(x_1,..., x_n))... χ_m(f_m(x_1,..., x_n))ψ(f(x_1,..., x_n)),$$ where $χ_1,..., χ_m:k^\ast\to {\bf C}^\ast$ are multiplicative characters, $ψ:k\to {\bf C}^\ast$ is a nontrivial additive character, and $f_1,..., f_m, f$ are Laurent polynomials.

math.NT↗
arXiv2005

Mirror Congruence for Rational Points on Calabi-Yau Varieties

Wan conjectures that if $X$ and $Y$ form a strong mirror pair of Calabi-Yau varieties over a finite field $F_q$ with $q$ elements, then X and Y have the same number of $F_{q^k}$-rational points modulo $q^k$. We prove this conjecture under the condition that $Y$ can be obtained from $X$ through quotient construction.

math.AG↗
arXiv2004

L-Functions for Symmetric Products of Kloosterman Sums

The classical Kloosterman sums give rise to a Galois representation of the function field unramfied outside 0 and $\infty$. We study the local monodromy of this representation at $\infty$ using $l$-adic method based on the work of Deligne and Katz. As an application, we determine the degrees and the bad factors of the $L$-functions of the symmetric products of the above representation. Our results generalize some results of Robba obtained through $p$-adic method.

math.NT↗