Maximal subsets of $n\times n$ board without cycles

Abstract

Define $\nabla(n)$ as the least possible number of cells that must be removed from the $n\times n$ square board in order to leave no cycles. Put $L_n=\left\lceil\left((n-1)^2+1\right)/3\right\rceil$. The results of Luccio, Madelaine-Stewart, and Lien-Fu-Shih imply that $\nabla(n)=L_n$ in case $n\equiv 1\pmod 6$ and $n\equiv 4\pmod 6$, while for the remaining $4$ residue classes $\nabla(n)=L_n$ or $\nabla(n)=L_n+1$. In this paper, we provide optimal constructions in the odd residue classes $n\equiv3,5\pmod6$, (proving $\nabla(n)=L_n$) and a geometric obstruction showing that equality in lower bound is impossible when $n\equiv0,2\pmod6$ (yielding $\nabla(n)=L_n+1$). Starting with $n=3$, the resulting sequence is thus given by \[ \nabla(n)=\bigg{\lceil}\frac{n^2}{3}-\frac{n}{6}+\frac{4}{3}\bigg{\rceil}-\bigg{\lceil}\frac{n}{2}\bigg{\rceil}. \]