arXiv subjects

Hongbin Zhao

Publications and source records attributed to Hongbin Zhao.

10 recordsLinked to original sources

arXiv2026

Asymptotically optimal packings of arithmetic progressions with prime differences

For a positive integer $n$, put $A_d=\{id:1\le i\le\lfloor n/d\rfloor\}$ for $1\le d\le n$ and $B_d=\{id:1\le i\le n\}$ for $d\in\mathbb{N}$. For $D\subseteq\{1,\ldots,n\}$, let $m_D(n)$ be the minimum length of an integer interval containing pairwise disjoint shifted copies of $A_d$ for all $d\in D$. For a finite set $E\subseteq\mathbb{N}$, define $M_E(n)$ analogously using $B_e$, $e\in E$. Let $\mathcal{P}(x)=\{p\le x:p\text{ is prime}\}$. We prove $m_{\mathcal{P}(\sqrt n)}(n)=\left(\frac43+o(1)\right)\frac{n^{3/2}}{\ln n}$ and $M_{\mathcal{P}(n)}(n)=\left(\frac16+o(1)\right)\frac{n^3}{\ln n}$ as $n\to\infty$. These asymptotic formulas attain the known lower bounds and settle two conjectures of Alon, D\k{e}bski, Grytczuk and Przyby\l{}o concerning prime differences. The proof combines a cyclic phase-selection principle with lattice covering estimates and a decomposition into regular blocks of primes.

math.NT↗

arXiv2026

Local and global average degree in bipartite graphs

Let $F_{\mathrm{bip}}(n)$ denote the maximum, over all $n$-vertex bipartite graphs without isolated vertices, of the ratio of the minimum local average degree to the global average degree. We prove that $F_{\mathrm{bip}}(n)=\frac14\sqrt n+\frac38+o(1)$. This answers a problem posed by Tuza.

math.CO↗

arXiv2026

An Improved Upper Bound for Finite Sidon Sets via Vector-Valued Smoothing

Let $F(N)$ denote the largest cardinality of a Sidon subset of $\{0,1,\ldots,N-1\}$. We prove \[ F(N)\le N^{1/2}+\gamma_0N^{1/4}+O(1), \qquad \gamma_0=0.94349\ldots<0.9435. \] This improves the previously published coefficient $0.98183$. Our argument develops a vector-valued smoothing method that combines several discrete smoothing kernels, each accompanied by boundary weights that compensate for endpoint effects, so that their weighted combination satisfies the required finite covering inequalities. We also show that averaging systems that satisfy these inequalities individually cannot improve upon the best constituent. Numerical optimization is used to find an eight-component candidate system, which is then certified by exact rational arithmetic.

math.CO↗

arXiv2026

Tur\'an numbers of $4$-uniform tight even cycles minus one edge

For every integer $k \ge 1$ and sufficiently large $n$, we show that the extremal construction for the Tur\'{a}n number of the $4$-uniform tight cycle of length $4k+2$ minus one edge is a complete odd-bipartite $4$-graph. In particular, since $C_{6}^{4-}$ contains the $4$-uniform expanded triangle as a subgraph, our result extends that of Frankl and Keevash--Sudakov on the Tur\'an density and the Tur\'{a}n number of the $4$-uniform expanded triangle. We also show that the Tur\'{a}n density of $C_{4k+2}^{4}$ is $1/2$ for all integers $k \ge 2$, and establish the corresponding stability result. This strengthens the result of Sankar on the Tur\'{a}n density of $C_{4k+2}^{4}$ which holds only for all sufficiently large $k$.

math.CO↗

arXiv2026

On cliques in hypergraphs under bounded $(j,p)$-norm

Let $\mathcal{H}$ be an $r$-uniform hypergraph. For $S\in \binom{V(\mathcal{H})}{j}$, let $\mathrm{deg}(S)$ be the number of edges of $\mathcal{H}$ containing $S$, and define the $(j,p)$-norm of $\mathcal{H}$ by $\|\mathcal{H}\|_{j,p}=\left(\sum_{S\in \binom{V(\mathcal{H})}{j}}\mathrm{deg}(S)^p\right)^{1/p}$. Motivated by a problem of Chao, Dong, Shen and Yang, we determine the maximum number of $t$-cliques in an $n$-vertex $r$-graph with bounded $(j,p)$-norm in the range $p>(t-j)/(r-j)$. The proof uses an entropy argument adapted to hypergraphs, together with a continuous interpolation step. The bound is sharp whenever the corresponding Steiner systems exist.

math.CO↗

arXiv2026

Near-Optimal Algorithms for Maximal Clique Enumeration in Structurally Sparse Graphs

We study the exact enumeration of maximal cliques in graph classes defined by excluded clique minors and excluded clique immersions. For n-vertex K_t-minor-free graphs, we give an algorithm that lists all maximal cliques in n * 4^(2t/5+o(t)) time, significantly improving the previous n * 2^O(t log log t) bound of Eppstein, L\"offler, and Strash. For n-vertex K_t-immersion-free graphs, we establish the first exact enumeration algorithm parameterized by immersion number, achieving a running time of n * 3^(t/3+o(t)). While both algorithms employ a common degeneracy-based root-assignment scheme, their analyses require distinct structural mechanisms. Crucially, rather than applying generic sparsity bounds, our algorithms deeply integrate the specific structural obstructions -- local density thresholds for minors and minimum-degree branchings for immersions -- directly into the enumeration logic. We also prove matching output-size lower bounds, up to sub-exponential factors in t, using specialized constructions. Consequently, the exponential bases 4^(2/5) and 3^(1/3) are asymptotically optimal.

cs.DS↗

arXiv2025

Odd hypergraph Mantel theorems

A classical result of Sidorenko (1989) shows that the Tur\'{a}n density of every $r$-uniform hypergraph with three edges is bounded from above by $1/2$. For even $r$, this bound is tight, as demonstrated by Mantel's theorem on triangles and Frankl's theorem on expanded triangles. In this note, we prove that for odd $r$, the bound $1/2$ is never attained, thereby answering a question of Keevash and revealing a fundamental difference between hypergraphs of odd and even uniformity. Moreover, our result implies that the expanded triangles form the unique class of three-edge hypergraphs whose Tur\'{a}n density attains $1/2$.

math.CO↗

arXiv2025

Exact Tur\'{a}n densities in triple systems

In this paper, we prove several new Tur\'{a}n density results for $3$-graphs. We show: $\pi(C_4^3, \mathrm{complement\ of\ } F_5) = 2\sqrt{3} - 3$, $\pi(F_{3,2}, C_5^{3-}) = \frac{2}{9}$, and $\pi(F_{3,2}, \mathrm{induced\ complement\ of\ } F_{3,2}) = \frac{3}{8}$. The first result confirms the conjecture of Shi~[On Tur\'an denisties of small triple graphs, European J. Combin. 52 (2016) 95-102]. The other results give several special non-principal family posed by Mubayi and R\"odl~[On the Tur\'an number of triple systems, J. Combin. Theory A. 100 (2002) 135-152].

math.CO↗

arXiv2025

Tur\'{a}n density of tight cycles minus one edge in the $\ell_2$-norm

The $3$-uniform tight $\ell$-cycle minus one edge $C_{\ell}^{3-}$ is the $3$-graph on $\ell$ vertices consisting of $\ell-1$ consecutive triples in the cyclic order. We show that for every integer $\ell \ge 5$ satisfying $\ell\not\equiv 0\pmod3$, every $C_{\ell}^{3-}$-free $3$-graph whose $\ell_2$-norm, that is, the sum of codegree squares, is close to the maximum must be structurally close to the iterative blowup of a single triple. This confirms a conjecture of Balogh--Clemen--Lidick\'{y}~[Surveys in combinatorics 2022, 21-63] in a stronger form.

math.CO↗

arXiv2024

A criterion for Andr\'{a}sfai--Erd\H{o}s--S\'{o}s type theorems and applications

The classical Andr\'{a}sfai--Erd\H{o}s--S\'{o}s Theorem states that for $\ell\ge 2$, every $n$-vertex $K_{\ell+1}$-free graph with minimum degree greater than $\frac{3\ell-4}{3\ell-1}n$ must be $\ell$-partite. We establish a simple criterion for $r$-graphs, $r \geq 2$, to exhibit an Andr\'{a}sfai--Erd\H{o}s--S\'{o}s type property, also known as degree-stability. This leads to a classification of most previously studied hypergraph families with this property. An immediate application of this result, combined with a general theorem by Keevash--Lenz--Mubayi, solves the spectral Tur\'{a}n problems for a large class of hypergraphs. For every $r$-graph $F$ with degree-stability, there is a simple algorithm to decide the $F$-freeness of an $n$-vertex $r$-graph with minimum degree greater than $(\pi(F) - \varepsilon_F)\binom{n}{r-1}$ in time $O(n^r)$, where $\varepsilon_F >0$ is a constant. In particular, for the complete graph $K_{\ell+1}$, we can take $\varepsilon_{K_{\ell+1}} = (3\ell^2-\ell)^{-1}$, and this bound is tight up to some multiplicative constant factor unless $\mathbf{W[1]} = \mathbf{FPT}$. Based on a result by Chen--Huang--Kanj--Xia, we further show that for every fixed $C > 0$, this problem cannot be solved in time $n^{o(\ell)}$ if we replace $\varepsilon_{K_{\ell+1}}$ with $(C\ell)^{-1}$ unless $\mathbf{ETH}$ fails. Furthermore, we apply the degree-stability of $K_{\ell+1}$ to decide the $K_{\ell+1}$-freeness of graphs whose size is close to the Tur\'{a}n bound in time $(\ell+1)n^2$, partially improving a recent result by Fomin--Golovach--Sagunov--Simonov. As an intermediate step, we show that for a specific class of $r$-graphs $F$, the (surjective) $F$-coloring problem can be solved in time $O(n^r)$, provided the input $r$-graph has $n$ vertices and a large minimum degree, refining several previous results.

math.CO↗