Large Maximal Intersecting Families with Covering Number Three

Abstract

Let $n>2k$ and $k\ge4$. The largest size of an intersecting family of $k$-subsets of $[n]$ with covering number at least three is \[ g_1(n,k)=\binom{n-1}{k-1}-3\binom{n-k-1}{k-1}+\binom{n-k-2}{k-1}+\binom{n-2k}{k-1}+3, \] attained by a unique family up to isomorphism. We prove that every other maximal intersecting family $\mathcal F$ with covering number three satisfies \[ |\mathcal F|\le M(n,k)=\max\{q(n,k),g_2(n,k)\}, \] where \begin{align*} q(n,k)&=\binom{n-1}{k-1}-3\binom{n-k-1}{k-1}+\binom{n-k-3}{k-1}+\binom{n-2k+1}{k-1}+3,\\ g_2(n,k)&=g_1(n,k)-\binom{n-k-3}{k-3}+1. \end{align*} This bound is attained for every $n>2k$ and $k\ge4$, so $M(n,k)$ is the second largest distinct size. For $k=4$, $g_1(n,4)=13n-69$ and $M(n,4)=12n-61$. For $k\ge5$, we have $M(n,k)=q(n,k)$ when $2k<n\le3k-2$ and $M(n,k)=g_2(n,k)$ when $n\ge3k-2$; the two terms coincide only at $n=3k-2$. We also classify all families attaining $M(n,k)$ that admit a critically shifted labeling, including additional nonisomorphic families at $n=2k+1$. The proof combines local replacements with an inverse-shift estimate and gives a new proof of the known maximum and its uniqueness.