Shattering $k$-sets with Permutations

Abstract

Many concepts from extremal set theory have analogues for families of permutations. This paper is concerned with the notion of shattering for permutations. A family $\mathcal{P}$ of permutations of an $n$-element set $X$ shatters a $k$-set from $X$ if it appears in each of the $k!$ possible orders in some permutation in $\mathcal{P}$. The smallest family $\mathcal{P}$ which shatters every $k$-subset of $X$ is known to have size $\Theta(\log n)$. Our aim is to introduce and study two natural partial versions of this shattering problem. Our first main result concerns the case where our family must contain only $t$ out of $k!$ of the possible orders. When $k=3$ we show that there are three distinct regimes depending on $t$: constant, $\Theta(\log\log n)$, $\Theta(\log n)$. We also show that for larger $k$ these same regimes exist although they may not cover all values of $t$. Our second direction concerns the problem of determining the largest number of $k$-sets that can be totally shattered by a family with given size. We show that for any $n$, a family of $6$ permutations is enough to shatter a proportion between $\frac{17}{42}$ and $\frac{11}{14}$ of all triples.