On strengthenings of the intersecting shadow theorem

Abstract

Let $n > k > t \geq j \geq 1$ be integers. Let $X$ be an $n$-element set, ${X\choose k}$ the collection of its $k$-subsets. A family $\mathcal F \subset {X\choose k}$ is called $t$-intersecting if $|F \cap F'| \geq t$ for all $F, F' \in \mathcal F$. The $j$'th shadow $\partial^j \mathcal F$ is the collection of all $(k - j)$-subsets that are contained in some member of~$\mathcal F$. Estimating $|\partial^j \mathcal F|$ as a function of $|\mathcal F|$ is a widely used tool in extremal set theory. A classical result of the second author (Theorem \ref{th:1.3}) provides such a bound for $t$-intersecting families. It is best possible for $|\mathcal F| = {2k - t\choose k}$. Our main result is Theorem \ref{th:1.4} which gives an asymptotically optimal bound on $|\partial^j \mathcal F| / |\mathcal F|$ for $|\mathcal F|$ slightly larger, e.g., $|\mathcal F| > \frac32 {2k - t\choose k}$. We provide further improvements for $|\mathcal F|$ very large as well.