2016/10/25 by Benny Sudakov, Pedro Vieira, Sudakov, Benny +1
Computer Science · Mathematics · #Advanced Graph Theory Research #Analytic Number Theory Research #Combinatorics (math.CO) #FOS: Mathematics #Limits and Structures in Graph Theory
paper · pdf · doi:10.48550/arxiv.1610.07907
openalex publication_date 2016/10/25 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/28
A family \mathcal A of subsets of an n-element set is called an eventown (resp. oddtown) if all its sets have even (resp. odd) size and all pairwise intersections have even size. Using tools from linear algebra, it was shown by Berlekamp and Graver that the maximum size of an eventown is 2\lfloor n/2\rfloor. On the other hand (somewhat surprisingly), it was proven by Berlekamp, that oddtowns have size at most n. Over the last four decades, many extensions of this even/oddtown problem have been studied. In this paper we present new results on two such extensions. First, extending a result of Vu, we show that a k-wise eventown (i.e., intersections of k sets are even) has for k ≥ 3 a unique extremal configuration and obtain a stability result for this problem. Next we improve some known bounds for the defect version of an ℓ-oddtown problem. In this problem we consider sets of size \not≡ 0 \pmod ℓ where ℓ is a prime number ℓ (not necessarily 2) and allow a few pairwise intersections to also have size \not≡ 0 \pmod ℓ.