Runshan Wang | Searcharxiv

arXiv2026

On the rainbow Cameron-Erd\H{o}s problem with respect to generalized Sidon sets of multidimensional grids

For positive integers $n$, $d$, $k$ and $h$, let $[n]^d$ be the $d$-dimensional grid of order $n$, and we refer to the equation $\sum_{i=1}^{h}x_{1,i}=\cdots =\sum_{i=1}^{h}x_{k,i}$ as the {\it $B_{k,h}$-equation}, where $x_{1,1}, \ldots, x_{1,h}, \ldots, x_{k,1}, \ldots, x_{k,h}$ are $kh$ points in $[n]^d$. In this paper, we study the rainbow Cameron-Erd\H{o}s problem with respect to the $B_{k,h}$-equation. We obtain the asymptotic number of $r$-colorings of $[n]^d$ without rainbow solutions to the $B_{k,h}$-equation, and we show that the typical colorings with this property are $(kh-1)$-colorings. We also prove that among all subsets of $[n]^d$, $[n]^d$ is the unique subset admitting the maximum number of $r$-colorings without rainbow solutions to the $B_{k,h}$-equation. The case $d=1$ and $k=h=2$ of our result confirms a conjecture on Sidon sets by Lin, Wang and Zhou~[{\it European J. Combin.}, 2022]; the case $d=1$, $k=2$ and $h\geq 2$ of our result partly solves a problem concerning linear equations proposed by Cheng, Jing, Li, Wang and Zhou~[{\it J. Combin. Theory Ser. A}, 2023]; the case $d\geq 2$ and $k=h=2$ corresponds to colorings without rainbow (possibly degenerate) parallelograms, and this geometric perspective might be of independent interest. Our proof combines the hypergraph container method with a stability analysis and a deviation gain argument.

math.CO↗

arXiv2025

Edge-colored 3-uniform hypergraphs without rainbow paths of length 3 and applications to Ramsey theory

Motivated by problems in Ramsey theory, we study edge-colorings of 3-uniform hypergraphs that contain no rainbow paths of length 3. We consider the following three natural 3-uniform paths of length 3: the tight path $\mathcal{T}=\{v_1v_2v_3, v_2v_3v_4, v_3v_4v_5\}$, the messy path $\mathcal{M}=\{v_1v_2v_3, v_2v_3v_4, v_4v_5v_6\}$ and the loose path $\mathcal{L}=\{v_1v_2v_3,$ $v_3v_4v_5, v_5v_6v_7\}$. In this paper, we characterize the structures of rainbow $\mathcal{T}$-free, rainbow $\mathcal{M}$-free and rainbow $\mathcal{L}$-free edge-colorings of complete 3-uniform hypergraphs $K_n^{(3)}$, respectively. This extends a result of Thomason and Wagner (2007) on edge-colored complete graphs $K_n$ without rainbow paths of length 3. As applications, we obtain several Ramsey-type results. Given two $3$-uniform hypergraphs $H$ and $G$, the {\it constrained Ramsey number} $f(H,G)$ is defined as the minimum integer $n$ such that in every edge-coloring of $K^{(3)}_n$ with any number of colors, there is either a monochromatic copy of $H$ or a rainbow copy of $G$. For $G\in \{\mathcal{T}, \mathcal{M}, \mathcal{L}\}$ and infinitely many 3-uniform hypergraphs $H$, we show that $f(H, G)=R_2(H)$, where $R_2(H)$ is the 2-colored Ramsey number of $H$. Given a $3$-uniform hypergraph $G$ and an integer $n\geq |V(G)|$, the {\it anti-Ramsey number} $ar(n, G)$ is the minimum integer $k$ such that in every edge-coloring of $K^{(3)}_n$ with at least $k$ colors, there is a rainbow copy of $G$. We show that $ar(n, \mathcal{T})=\left\lfloor\frac{n}{3}\right\rfloor+2$ for $n\geq 5$, $ar(n, \mathcal{M})=3$ for $n\geq 7$, and $ar(n, \mathcal{L})=n$ for $n\geq 7$. Our Ramsey-type results extend results of Gy\'{a}rf\'{a}s, Lehel and Schelp (2007) and of Liu (2024) on constrained Ramsey numbers, and improve a result of Tang, Li and Yan (2022) on anti-Ramsey numbers.

math.CO↗