Year: 2026
Type: preprint
Abstract: Let $d_{K_{s,t}}(n)$ be the largest fraction of all graphs on $[n]$ in a family containing no two distinct members whose symmetric difference is a copy of $K_{s,t}$. We determine this maximum density for every fixed pair of positive integers $s,t$. If both part sizes are odd, it is known that the density is exactly $1/2$. Otherwise, let $2h$ be the largest even part size. We prove $$ d_{K_{s,t}}(n)=Θ_{s,t}(n^{-h}). $$ For $h>1$, this improves the general $O_{s,t}(1/n)$ upper bound obtained from Alon's two-copy gluing theorem~\cite{Alon2024}. The sharp bounds for even stars were already proved by Alon and are recovered as special cases. The upper bound uses the Frankl--Füredi forbidden-intersection theorem. For the lower bound, we construct binary linear codes using BCH codes. When exactly one part size is even, we assign to each edge the sum of the binary vectors assigned to its endpoints. When both part sizes are even, we identify these vectors with elements of a finite field and assign to each edge the product of its endpoint elements. In both cases, every copy of $K_{s,t}$ has a nonzero syndrome.
Source: arXiv (Cornell University)
Author: Bshouty, Nader H.
Institution: Technion – Israel Institute of Technology
Language: English
Subfield: Discrete Mathematics and Combinatorics
Field: Mathematics
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7223880163