arXiv subjects

Yulu Feng

Publications and source records attributed to Yulu Feng.

4 recordsLinked to original sources

arXiv2021

On the sums of squares of exceptional units in residue class rings

Let $n\ge 1, e\ge 1, k\ge 2$ and $c$ be integers. An integer $u$ is called a unit in the ring $\mathbb{Z}_n$ of residue classes modulo $n$ if $\gcd(u, n)=1$. A unit $u$ is called an exceptional unit in the ring $\mathbb{Z}_n$ if $\gcd(1-u,n)=1$. We denote by $\mathcal{N}_{k,c,e}(n)$ the number of solutions $(x_1,...,x_k)$ of the congruence $x_1^e+...+x_k^e\equiv c \pmod n$ with all $x_i$ being exceptional units in the ring $\mathbb{Z}_n$. In 2017, Mollahajiaghaei presented a formula for the number of solutions $(x_1,...,x_k)$ of the congruence $x_1^2+...+x_k^2\equiv c\pmod n$ with all $x_i$ being the units in the ring $\mathbb{Z}_n$. Meanwhile, Yang and Zhao gave an exact formula for $\mathcal{N}_{k,c,1}(n)$. In this paper, by using Hensel's lemma, exponential sums and quadratic Gauss sums, we derive an explicit formula for the number $\mathcal{N}_{k,c,2}(n)$. Our result extends Mollahajiaghaei's theorem and that of Yang and Zhao.

math.NT↗

arXiv2021

On the number of zeros to the equation $f(x_1)+...+f(x_n)=a$ over finite fields

Let $p$ be a prime, $k$ a positive integer and let $\mathbb{F}_q$ be the finite field of $q=p^k$ elements. Let $f(x)$ be a polynomial over $\mathbb F_q$ and $a\in\mathbb F_q$. We denote by $N_{s}(f,a)$ the number of zeros of $f(x_1)+\cdots+f(x_s)=a$. In this paper, we show that $$\sum_{s=1}^{\infty}N_{s}(f,0)x^s=\frac{x}{1-qx} -\frac{x { M_f^{\prime}}(x)}{qM_f(x)},$$ where $$M_f(x):=\prod_{m\in\mathbb F_q^{\ast}\atop{S_{f, m}\ne 0}}\Big(x-\frac{1}{S_{f,m}}\Big)$$ with $S_{f, m}:=\sum_{x\in \mathbb F_q}ζ_p^{{\rm Tr}(mf(x))}$, $ζ_p$ being the $p$-th primitive unit root and ${\rm Tr}$ being the trace map from $\mathbb F_q$ to $\mathbb F_p$. This extends Richman's theorem which treats the case of $f(x)$ being a monomial. Moreover, we show that the generating series $\sum_{s=1}^{\infty}N_{s}(f,a)x^s$ is a rational function in $x$ and also present its explicit expression in terms of the first $2d+1$ initial values $N_{1}(f,a), ..., N_{2d+1}(f,a)$, where $d$ is a positive integer no more than $q-1$. From this result, the theorems of Chowla-Cowles-Cowles and of Myerson can be derived.

math.NT↗

arXiv2021

On the number of zeros of diagonal quartic forms over finite fields

Let $\mathbb{F}_q$ be the finite field of $q=p^m\equiv 1\pmod 4$ elements with $p$ being an odd prime and $m$ being a positive integer. For $c, y \in\mathbb{F}_q$ with $y\in\mathbb{F}_q^*$ non-quartic, let $N_n(c)$ and $M_n(y)$ be the numbers of zeros of $x_1^4+...+x_n^4=c$ and $x_1^4+...+x_{n-1}^4+yx_n^4=0$, respectively. In 1979, Myerson used Gauss sum and exponential sum to show that the generating function $\sum_{n=1}^{\infty}N_n(0)x^n$ is a rational function in $x$ and presented its explicit expression. In this paper, we make use of the cyclotomic theory and exponential sums to show that the generating functions $\sum_{n=1}^{\infty}N_n(c)x^n$ and $\sum_{n=1}^{\infty}M_{n+1}(y)x^n$ are rational functions in $x$. We also obtain the explicit expressions of these generating functions. Our result extends Myerson's theorem gotten in 1979.

math.NT↗

arXiv2018

A generalization of a theorem of Nagell

Let $n$ be a positive integer. In 1915, Theisinger proved that if $n\ge 2$, then the $n$-th harmonic sum $\sum_{k=1}^n\frac{1}{k}$ is not an integer. Let $a$ and $b$ be positive integers. In 1923, Nagell extended Theisinger's theorem by showing that the reciprocal sum $\sum_{k=1}^{n}\frac{1}{a+(k-1)b}$ is not an integer if $n\ge 2$. In 1946, Erdős and Niven proved a theorem of a similar nature that states that there is only a finite number of integers $n$ for which one or more of the elementary symmetric functions of $1,1/2, ..., 1/n$ is an integer. In this paper, we present a generalization of Nagell's theorem. In fact, we show that for arbitrary $n$ positive integers $s_1, ..., s_n$ (not necessarily distinct and not necessarily monotonic), the following reciprocal power sum $$\sum\limits_{k=1}^{n}\frac{1}{(a+(k-1)b)^{s_{k}}}$$ is never an integer if $n\ge 2$. The proof of our result is analytic and $p$-adic in character.

math.NT↗