arXiv subjects

Chi Hoi Yip

Publications and source records attributed to Chi Hoi Yip.

At least 19 recordsLinked to original sources

arXiv2026

A sharp inverse theorem for the quadratic large sieve

We prove that if $A\subseteq[N]$, $|A|\gg\sqrt N$, and $|A_p|\le p/2+O(1)$ for every prime $p\ll \log N$, then $A$ contains at least $\exp\left(c\sqrt{\log N}/\log\log N\right)$ elements in the image of a single integral quadratic $m\pm x^2$. This significantly improves Hanson's logarithmic lower bound, while requiring the residue restriction only for primes $p\ll\log N$. Our proof uses a new weighted entropy argument inspired by our previous work \href{https://arxiv.org/abs/2606.17487}{arXiv:2606.17487} with Sheffer. A matching construction shows that this exponential scale is optimal even when the quadratic may be chosen arbitrarily in $\mathbb{Z}[x]$. We also show extending the residue restrictions to primes up to $(\log N)^3$ yields the stronger bound $\exp(c\sqrt{\log N/\log\log N})$ via a Selberg-sieve-type argument, and discuss an application to the inverse Goldbach problem.

math.NT↗

arXiv2026

Additive decompositions of multiplicative subgroups via differential identities

Sárközy conjectured that the nonzero quadratic residues modulo a sufficiently large prime have no nontrivial additive decomposition. Hanson and Petridis proved the conjecture for almost all primes, and Kalmynin completed the proof. Kalmynin also developed a general framework for additive decompositions of multiplicative subgroups. More recently, Rudnev and Tyrrell used this framework to classify all additive decompositions of proper multiplicative subgroups of prime fields, showing that the only nontrivial example is the subgroup of order $4$. We give a new self-contained proof of this classification that streamlines the arguments of Kalmynin and of Rudnev and Tyrrell. At the heart of the proof are two new global differential identities. They give an independent proof of Kalmynin's theorem that the two summands have equal size and ultimately reduce the classification to direct coefficient comparisons, avoiding the residue calculations and subsequent arithmetic analysis in the earlier arguments.

math.NT↗

arXiv2026

Majorization and additive tuples in $\mathbb{Z}_2^n$

Majorization is a fundamental tool for comparing how "spread out" the entries of two vectors are. Key majorization results were obtained for the integers by Hardy, Littlewood and Pólya and for $\mathbb{Z}_p$ by Lev. In this paper, we establish a powerful majorization theorem in $\mathbb{Z}_2^n$ that is an analogue of Lev's result in $\mathbb{Z}_p$. Our proof is based on a novel use of a compression argument that optimizes certain sums of additive representation counts. Our majorization theorem has several applications in additive combinatorics. We resolve the question: given subsets $A_1,\ldots,A_k\subseteq\mathbb{Z}_2^n$ of prescribed sizes, when is the function $r_k(A_1,\ldots,A_k)$, which counts the number of additive $k$-tuples in $A_1\times\cdots\times A_k$, maximized? We establish the corresponding minimization result and a characterization of all extremizers for $r_k(A,\ldots,A)$ when $k$ is odd. When $k=3$, this quantity is the number of Schur triples in $A$; as a special case, we recover a theorem of Samotij and Sudakov on Schur triples. We also obtain a convexity inequality and use it to prove an analogue of Pollard's theorem that strengthens and extends a well-known result of Bollobás and Leader in the $\mathbb{Z}_2^n$ setting.

math.CO↗

arXiv2026

The Prouhet--Tarry--Escott problem for subsets with small doubling in integral domains

The Prouhet--Tarry--Escott (PTE) problem has many generalizations and has been studied in various algebraic domains. In this paper, we prove that finite subsets $S$ of integral domains with small additive doubling constant (but still a power of $|S|$) always contain solutions to Wright's generalization of the PTE problem: there are small subsets $A$ and $B$ of the same size such that $\sum_{a\in A} a^j=\sum_{b\in B} b^j$ for $1\le j\le k$, but not for $j=k+1$. More generally, our method gives simultaneous solutions for $m$ systems, with pairwise distinct $(k+1)$-th power sums. In contrast with the classical case $S\subseteq [N]$, where the problem has been studied by Wooley and others using Vinogradov's mean value theorem, our approach is based on polynomial identities and additive properties of $S$. We also discuss barriers to extending these results to broader settings.

math.NT↗

arXiv2026

Paley-type matrices and $1$-factorizations of complete graphs

Ball, Ortega--Moreno, and Prodromou asked two questions about whether, for every odd prime $p$, one can find a $1$-factor of the complete graph $K_{p+1}$ with some arithmetic restrictions related to quadratic residues. These problems are motivated by two natural compatibility conditions between $1$-factorizations and the sign patterns of certain Paley-type matrices. Recently, Afifurrahman et al. made some partial progress on the second problem. In this paper, we completely resolve both problems. We prove that the first problem has a solution precisely when $p\equiv3\pmod4$, while the second problem has a solution for every odd prime $p$. We also solve a further problem of Ball et al. for cyclic groups of odd order, and more generally for all finite abelian groups of odd order.

math.CO↗

arXiv2026

Asymptotic Bounds on Generalized Covering Radii of Binary Primitive BCH Codes

Fix integers $e\ge2$ and $r\ge1$. In this paper we study the $r$-th generalized covering radius $ρ_r\left(BCH(e,m)\right)$ of the binary primitive $e$-error-correcting BCH code $BCH(e,m)$. By using an algebraic-geometric reformulation of the covering problem together with an explicit Lang-Weil estimate, we prove that \[ρ_r\bigl(\BCH(e,m)\bigr)\le(r+1)e-1\] for all sufficiently large $m$. For $e\ge7$, this improves a recent result of Belinsky--Zabokritskiy. Our proof gives a substantially simpler geometric approach to this upper bound. In particular it implies that \[ρ_2\bigl(BCH(e,m)\bigr)=3e-1\] for all sufficiently large $m$. Previously it was only known that \[ρ_2\bigl(\BCH(e,m)\bigr) \in \left\{3e-1,3e\right\}\] for all sufficiently large $m$.

cs.IT↗

arXiv2026

Positivity preservers over finite fields II

We say that a matrix over a finite field $\mathbb{F}_q$ is positive definite if it is symmetric and each of its leading principal minors is a nonzero square in $\mathbb{F}_q$. In previous work of the authors [J. Algebra, 2025], the entrywise positivity preservers on $M_n(\mathbb{F}_q)$ were classified for every $n\geq 2$, with one remaining case: $n=2$, $q\equiv 1\pmod 4$, and $q$ not a square. We settle this case by proving that every positivity preserver on $M_2(\mathbb{F}_q)$ is injective on the set $\mathbb{F}_q^+$ of nonzero squares whenever $q\equiv 1\pmod 4$. The proof combines an idempotent reduction of positivity preservers with a well-known property of quadratic characters. This yields the complete classification of entrywise positivity preservers over every finite field and in every fixed dimension.

math.RA↗

arXiv2026

Intersecting families and nonvanishing multivariate polynomials over finite fields

Let $\mathcal{P}_{n,d}$ be the space of polynomials in $n$ variables over $\mathbb{F}_q$ of degree at most $d$. Two polynomials $f,g\in\mathcal{P}_{n,d}$ intersect if $f(\mathbf a)=g(\mathbf a)$ for some $\mathbf a\in\mathbb{F}_q^n$. A star consists of all polynomials $f\in\mathcal{P}_{n,d}$ satisfying $f(\mathbf a)=b$ for fixed $\mathbf a\in\mathbb{F}_q^n$ and $b\in\mathbb{F}_q$. We completely classify the maximum intersecting families in $\mathcal{P}_{n,d}$. When $n=1$ and $d\geq 2$, it was previously shown that all maximum intersecting families are stars. We prove that the same conclusion holds for all $n\geq 2$ and $d\geq 2$ when $q$ is odd. When $q$ is even, however, the situation is more subtle, and a new phenomenon emerges: for $q\geq 4$, maximum non-star examples exist precisely when $d\leq n$. Along the way, we prove two further results of independent interest. First, we determine the span of nonvanishing polynomials in $\mathcal{P}_{n,d}$. Second, we characterize all linear functionals $Ψ\colon\mathcal{P}_{n,d}\to\mathbb{F}_q$ whose kernels are disjoint from the set of nonvanishing polynomials. The first result plays a crucial role in the proof of our main result; the second is a Gleason--Kahane--Żelazko theorem for polynomials of bounded degree over finite fields.

math.NT↗

arXiv2026

Product representations of polynomials over finite fields

Erdős, Sárközy, and Sós studied the asymptotics of the maximum size of a subset of $\{1,2,\ldots, N\}$ such that it does not contain $k$ distinct elements whose product is a perfect square. More generally, Verstraëte proposed a conjecture regarding the asymptotic behavior of the same quantity with the set of perfect squares replaced by the value set of a polynomial in $\mathbb{Z}[x]$. In this paper, we study a finite field analogue of Verstraëte's conjecture.

math.CO↗

arXiv2026

Multiplicative subgroups are not restricted sumsets

We determine exactly which proper multiplicative subgroups of a prime field can be represented as a restricted sumset of the form $A\mathbin{\widehat{+}} A=\{a+a':a,a'\in A,\ a\ne a'\}$. We prove that a proper multiplicative subgroup $H\le\mathbb F_p^*$ cannot satisfy $H=A\mathbin{\widehat{+}} A$ whenever $|H|\ge7$, and that this threshold is sharp. In fact, such a decomposition exists precisely when $|H|\in\{1,3,6\}$, and we classify all decompositions in these exceptional cases. This gives a sharp, complete resolution of the restricted-sumset analogue of the generalized Sárközy conjecture over prime fields. This significantly extends and refines previous results of Shkredov and Yip.

math.NT↗

arXiv2026

Two-Distance Sets over Finite Fields

We study two-distance sets in standard $d$-dimensional quadratic spaces over finite fields. In characteristic $3$, we construct sets attaining the full Larman--Rogers--Seidel bound, showing that Blokhuis' Euclidean bound $\binom{d+2}{2}$ need not hold over finite fields. We prove a rank-sensitive replacement which precisely measures the failure of positive definiteness. We also show that the Blokhuis bound $\binom{d+2}{2}$ is attained over a suitable finite field exactly for each $d\neq6$; in the exceptional dimension $d=6$, the exact maximum is $27$.

math.CO↗

arXiv2026

Sperner systems with restricted differences

Let $\mathcal{F}$ be a family of subsets of $[n]$ and $L$ be a subset of $[n]$. We say $\mathcal{F}$ is an $L$-differencing Sperner system if $|A\setminus B|\in L$ for any distinct $A,B\in\mathcal{F}$. Let $p$ be a prime and $q$ be a power of $p$. Frankl first studied $p$-modular $L$-differencing Sperner systems and showed an upper bound of the form $\sum_{i=0}^{|L|}\binom{n}{i}$. In this paper, we obtain new upper bounds on $q$-modular $L$-differencing Sperner systems using elementary $p$-adic analysis and polynomial method, extending and improving existing results substantially. Moreover, our techniques can be used to derive new upper bounds on subsets of the hypercube with restricted Hamming distances. One highlight of the paper is the first analogue of the celebrated Snevily's theorem in the $q$-modular setting, which results in several new upper bounds on $q$-modular $L$-avoiding $L$-intersecting systems. In particular, we improve a result of Felszeghy, Hegedűs, and Rónyai, and give a partial answer to a question posed by Babai, Frankl, Kutin, and Štefankovič.

math.CO↗

arXiv2026

An absolute bound for generalized Diophantine tuples over polynomial rings

Let $\mathbb F$ be an algebraically closed field of characteristic $0$. Let $k\geq 2$ be an integer, and let $n\in \mathbb F[x]\setminus\{0\}$. We study generalized Diophantine tuples $A\subset \mathbb F[x]$ with property $D_k(n)$, meaning that $ab+n$ is a $k$-th power in $\mathbb F[x]$ for all distinct elements $a,b\in A$. For $k\ge18$, we prove that every such tuple satisfies $|A|\le6$, except for the necessary exceptional family in which $n=s^2$ is a $k$-th power and $A\subset s\mathbb{F}$. This bound is absolute: it is independent of both $n$ and $\operatorname{deg} n$. Our proof develops a new method for studying polynomial Diophantine tuples, combining a determinant criterion, generalizations of the Mason--Stothers theorem, and the Combinatorial Nullstellensatz. We also record a conditional analogue for generalized Diophantine tuples over the integers.

math.NT↗

arXiv2026

A combinatorial large sieve for Sidon sets, distances, and norm forms

We develop a new combinatorial large sieve method for sets with bounded algebraic multiplicities. The method exploits algebraic splitting modulo many small primes: local congruence branching produces many modular collisions, while global bounded-multiplicity hypotheses force these collisions to be rare. As a first application, we prove that every Sidon subset $A\subset\{1^2,\ldots,N^2\}$ satisfies \[ |A| \le N\exp\left( -c\frac{\log N}{\log\log N} \right) \] for some absolute constant $c>0$. This gives the first super-polylogarithmic saving for a classical problem of Alon and Erdős. As a second application, we establish new upper bounds for two grid-distance problems. We show that the largest subset of $[N]^2$ with no repeated distance has size at most $N\exp\left(-c\log N/\log\log N\right)$, giving the first progress in over thirty years on a problem of Erdős and Guy. The same method also gives a similar saving for subsets of $[N]^2$ with no isosceles triangles, a problem recently popularized by Ellenberg and by the PatternBoost work of Charton, Ellenberg, Wagner, and Williamson. We then develop an entropic version of the method. This gives bounds for $B_2[g]$-sets in the squares and for analogous bounded-multiplicity problems associated with norm forms over arbitrary number fields. More importantly, this new method also allows us to establish the first nontrivial bounds for $B_3[g]$-sets in the cubes and $B_4[g]$-sets in the fourth powers.

math.NT↗

arXiv2026

The Frankl-Pach upper bound is not tight for any uniformity

For any positive integers $n\ge d+1\ge 3$, what is the maximum size of a $(d+1)$-uniform set system in $[n]$ with VC-dimension at most $d$? In 1984, Frankl and Pach initiated the study of this fundamental problem and provided an upper bound $\binom{n}{d}$ via an elegant algebraic proof. Surprisingly, in 2007, Mubayi and Zhao showed that when $n$ is sufficiently large and $d$ is a prime power, the Frankl-Pach upper bound is not tight. They also remarked that their method requires $d$ to be a prime power, and asked for new ideas to improve the Frankl-Pach upper bound without extra assumptions on $n$ and $d$. In this paper, we provide an improvement for any $d\ge 2$ and $n\ge 2d+2$, which demonstrates that the long-standing Frankl-Pach upper bound $\binom{n}{d}$ is not tight for any uniformity. Our proof combines a simple yet powerful polynomial method and structural analysis.

math.CO↗

arXiv2026

On the fractional parts of certain sequences of $ξα^{n}$

Assume that $α>1$ is an algebraic number and $ξ\neq0$ is a real number. We are concerned with the distribution of the fractional parts of the sequence $(ξα^{n})$. Under various Diophantine conditions on $ξ$ and $α$, we obtain lower bounds on the number $n$ with $1\leq n\leq N $ for which the fractional part of the sequence $(ξα^{n})_{n\geq1}$ fall into a prescribed region $I\subset [0,1]$, extending several results in the literature. As an application, we show that the Fourier decay rate of some self-similar measures is logarithmic, generalizing a result of Varjú and Yu.

math.NT↗

arXiv2026

Entrywise transforms preserving matrix positivity and non-positivity

We characterize real and complex functions which, when applied entrywise to square matrices, yield a positive definite matrix if and only if the original matrix is positive definite. We refer to these transformations as sign preservers. Compared to classical work on entrywise preservers of Schoenberg and others, we completely resolve this problem in the harder fixed dimensional setting, extending a similar recent classification of sign preservers obtained for matrices over finite fields. When the matrix dimension is fixed and at least $3$, we show that the sign preservers are precisely the positive scalar multiples of the continuous automorphisms of the underlying field. This is in contrast to the $2 \times 2$ case where the sign preservers are extensions of power functions. These results are built on our classification of $2 \times 2$ entrywise positivity preservers over broader complex domains. Our results yield a complementary connection with a work of Belton, Guillot, Khare, and Putinar (2023) on negativity-preserving transforms. We also extend our sign preserver results to matrices with a structure of zeros, as studied by Guillot, Khare, and Rajaratnam for the entrywise positivity preserver problem. Finally, in the spirit of sign preservers, we address a natural extension to monotone maps, classically studied by Loewner and many others.

math.CA↗

arXiv2026

An Erdős-Ko-Rado theorem for binary codes

We study intersecting families of words from the Erdős-Ko-Rado perspective. When the alphabet size is $2$, a maximum intersecting family is not necessarily a star. However, we prove that every maximum $3$-wise intersecting family is a star. We also present a new proof of the known result for alphabets of size at least $3$: maximum intersecting families of words are exactly the stars.

math.CO↗