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Mei-Chu Chang

Publications and source records attributed to Mei-Chu Chang.

14 recordsLinked to original sources
arXiv2017

On the exponential large sieve inequality for sparse sequences modulo primes

We complement the argument of M. Z. Garaev (2009) with several other ideas to obtain a stronger version of the large sieve inequality with sparse exponential sequences of the form $λ^{s_n}$. In particular, we obtain a result which is non-trivial for monotonically increasing sequences $\cal{S}=\{s_n \}_{n=1}^{\infty}$ provided $s_n\le n^{2+o(1)}$, whereas the original argument of M. Z. Garaev requires $s_n \le n^{15/14 +o(1)}$ in the same setting. We also give an application of our result to arithmetic properties of integers with almost all digits prescribed.

math.NT↗
arXiv2017

Orbits of Polynomial Dynamical Systems Modulo Primes

We present lower bounds for the orbit length of reduction modulo primes of parametric polynomial dynamical systems defined over the integers, under a suitable hypothesis on its set of preperiodic points over $\mathbb C$. Applying recent results of Baker and DeMarco~(2011) and of Ghioca, Krieger, Nguyen and Ye~(2017), we obtain explicit families of parametric polynomials and initial points such that the reductions modulo primes have long orbits, for all but a finite number of values of the parameters. This generalizes a previous lower bound due to Chang~(2015). As a by-product, we also slighly improve a result of Silverman~(2008) and recover a result of Akbary and Ghioca~(2009) as special extreme cases of our estimates.

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arXiv2016

Arithmetic progressions in multiplicative groups of finite fields

Let $G$ be a multiplicative subgroup of the prime field $\mathbb F_p$ of size $|G|> p^{1-κ}$ and $r$ an arbitrarily fixed positive integer. Assuming $κ=κ(r)>0$ and $p$ large enough, it is shown that any proportional subset $A\subset G$ contains non-trivial arithmetic progressions of length $r$. The main ingredient is the Szemerédi-Green-Tao theorem.

math.NT↗
arXiv2016

Nonlinear Roth type theorems in finite fields

We obtain smoothing estimates for certain nonlinear convolution operators on prime fields, leading to quantitative nonlinear Roth type theorems. Compared with the usual linear setting (i.e. arithmetic progressions), the nonlinear nature of the operators leads to different phenomena, both qualitatively and quantitatively.

math.NT↗
arXiv2015

On a paper of Erdös and Szekeres

Propositions 1.1 -- 1.3 stated below contribute to results and certain problems considered in a paper by Erdos and Szekeres, on the behavior of products $\prod^n_1 (1-z^{a_j}), 1\leq a_1\leq \cdots\leq a_n$ integers. In the discussion, $\{a_1, \ldots, a_n \}$ will be either a proportional subset of $\{1, \ldots, n\}$ or a set of large arithmetic diameter.

math.NT↗
arXiv2015

A Remark on Sieving in Biased Coin Convolutions

In this work, we establish a nontrivial level of distribution for densities on $\{1,\ldots, N\}$ obtained by a biased coin convolution. As a consequence of sieving theory, one then derives the expected lower bound for the weight of such densities on sets of pseudo-primes.

math.NT↗
arXiv2014

On the Density of Integer Points on Generalised Markoff-Hurwitz and Dwork Hypersurfaces

We use bounds of mixed character sums modulo a square-free integer $q$ of a special structure to estimate the density of integer points on the hypersurface $$ f_1(x_1) + \ldots + f_n(x_n) =a x_1^{k_1} \ldots x_n^{k_n} $$ for some polynomials $f_i \in {\mathbb Z}[X]$ and nonzero integers $a$ and $k_i$, $i=1, \ldots, n$. In the case of $$ f_1(X) = \ldots = f_n(X) = X^2\quad \text{and} \quad k_1 = \ldots = k_n =1 $$ the above hypersurface is known as the Markoff-Hurwitz hypersurface, while for $$ f_1(X) = \ldots = f_n(X) = X^n\quad \text{and} \quad k_1 = \ldots = k_n =1 $$ it is known as the Dwork hypersurface. Our results are substantially stronger than those known for general hypersurfaces.

math.NT↗
arXiv2014

Short character sums for composite moduli

We establish new estimates on short character sums for arbitrary composite moduli with small prime factors. Our main result improves on the Graham-Ringrose bound for square free moduli and also on the result due to Gallagher and Iwaniec when the core $q'=\prod_{p|q}p$ of the modulus $q$ satisfies $\log q'\sim \log q$. Some applications to zero free regions of Dirichlet L-functions and the $\rm{P\acute{o}lya}$ and Vinogradov inequalities are indicated.

math.NT↗
arXiv2014

Double Character Sums over Subgroups and Intervals

We estimate double sums $$ S_χ(a, I, G) = \sum_{x \in I} \sum_{λ\in G} χ(x + aλ), \qquad 1\le a < p-1, $$ with a multiplicative character $χ$ modulo $p$ where $I= \{1,\ldots, H\}$ and $G$ is a subgroup of order $T$ of the multiplicative group of the finite field of $p$ elements. A nontrivial upper bound on $S_χ(a, I, G)$ can be derived from the Burgess bound if $H \ge p^{1/4+\varepsilon}$ and from some standard elementary arguments if $T \ge p^{1/2+\varepsilon}$, where $\varepsilon>0$ is arbitrary. We obtain a nontrivial estimate in a wider range of parameters $H$ and $T$. We also estimate double sums $$ T_χ(a, G) = \sum_{λ, μ\in G} χ(a + λ+ μ), \qquad 1\le a < p-1, $$ and give an application to primitive roots modulo $p$ with $3$ non-zero binary digits.

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arXiv2012

Points on curves in small boxes en applications

We introduce several new methods to obtain upper bounds on the number of solutions of the congruences $f(x) \equiv y \pmod p$ and $f(x) \equiv y^2 \pmod p,$ with a prime $p$ and a polynomial $f$, where $(x,y)$ belongs to an arbitrary square with side length $M$. We use these results and methods to derive non-trivial upper bounds for the number of hyperelliptic curves $Y^2=X^{2g+1} + a_{2g-1}X^{2g-1} +...+ a_1X+a_0$ over the finite field $\F_p$ of $p$ elements, with coefficients in a $2g$-dimensional cube $ (a_0,..., a_{2g-1})\in [R_0+1,R_0+M]\times...\times [R_{2g-1}+1,R_{2g-1}+M]$ that are isomorphic to a given curve and give an almost sharp lower bound on the number of non-isomorphic hyperelliptic curves with coefficients in that cube. Furthermore, we study the size of the smallest box that contain a partial trajectory of a polynomial dynamical system over $\F_p$.

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arXiv2004

The Erdős-Szemerédi problem on sum set and product set

The basic theme of this paper is the fact that if $A$ is a finite set of integers, then the sum and product sets cannot both be small. A precise formulation of this fact is Conjecture 1 below due to Erd\H os-Szemerédi [E-S]. (see also [El], [T], and [K-T] for related aspects.) Only much weaker results or very special cases of this conjecture are presently known. One approach consists of assuming the sum set $A + A$ small and then deriving that the product set $AA$ is large (using Freiman's structure theorem). (cf [N-T], [Na3].) We follow the reverse route and prove that if $|AA| < c|A|$, then $|A+A| > c^\prime |A|^2$ (see Theorem 1). A quantitative version of this phenomenon combined with Plünnecke type of inequality (due to Ruzsa) permit us to settle completely a related conjecture in [E-S] on the growth in $k$. If $$ g(k) \equiv \text{min}\{|A[1]| + |A\{1\}|\} $$ over all sets $A\subset \Bbb Z$ of cardinality $|A| = k$ and where $A[1]$ (respectively, $A\{1\}$) refers to the simple sum (resp., product) of elements of $A$. (See (0.6), (0.7).) It was conjectured in [E-S] that $g(k)$ grows faster than any power of $k$ for $k\to\infty$. We will prove here that $\ell n g(k)\sim\frac{(\ell n k)^2}{\ell n \ell n k}$ (see Theorem 2) which is the main result of this paper.

math.CO↗
arXiv2003

On the size of $k$-fold sum and product sets of integers

We prove the following theorem: for all positive integers $b$ there exists a positive integer $k$, such that for every finite set $A$ of integers with cardinality $|A| > 1$, we have either $$ |A + ... + A| \geq |A|^b$$ or $$ |A \cdot ... \cdot A| \geq |A|^b$$ where $A + ... + A$ and $A \cdot ... \cdot A$ are the collections of $k$-fold sums and products of elements of $A$ respectively. This is progress towards a conjecture of Erdös and Szemerédi on sum and product sets.

math.CO↗