The monotonicity conjecture for diagonal (m,m)(m,m)-town families modulo kk

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Let [n]={1,2,…,n}[n]=\{1,2,\ldots,n\}, let P(n)\mathscr{P}(n) be the power set of [n][n], and let a family be an (a,b)(a,b)-town modulo kk when all its sets have cardinality congruent to aa modulo kk and every pair of distinct sets has intersection cardinality congruent to bb modulo kk. For i=1,2i=1,2, let Fi⊆P(n)\mathcal{F}_i\subseteq\mathscr{P}(n) be a (mi,mi)(m_i,m_i)-town modulo kk family of maximum size, where 0≤m1,m2≤k−10\leq m_1,m_2\leq k-1. Monotonicity conjecture. If m1<m2m_1<m_2, then

∣F1∣≥∣F2∣.|\mathcal{F}_1|\geq |\mathcal{F}_2|.

This conjecture arises from numerical evidence and concerns the relative sizes of diagonal town families when the modulus kk need not be prime. The linear upper bound nn is already known when the two town parameters differ modulo some prime divisor of kk; the remaining cases, particularly those with a≡ba\equiv b modulo every prime divisor of kk, are not fully understood.

References

Primary source

Nikola Veselinov and Miroslav Marinov, “On the maximal size of (a,b)-townk families”, arXiv:2510.00251 (2025).

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