Shattering-Extremal Set Systems of VC Dimension at most 2

We say that a set system $\mathcal{F}\subseteq 2^{[n]}$ shatters a given set $S\subseteq [n]$ if $2^S=\{F~\cap~S ~:~F~\in~\mathcal{F}\}$. The Sauer inequality states that in general, a set system $\mathcal{F}$ shatters at least $|\mathcal{F}|$ sets. Here we concentrate on the case of equality. A set system is called shattering-extremal if it shatters exactly $|\mathcal{F}|$ sets. In this paper we characterize shattering-extremal set systems of Vapnik-Chervonenkis dimension $2$ in terms of their inclusion graphs, and as a corollary we answer an open question about leaving out elements from shattering-extremal set systems in the case of families of Vapnik-Chervonenkis dimension $2$.

Shattering-Extremal Set Systems of Small VC-Dimension

We say that a set system ℱ⊆2[n] shatters a given set S⊆[n] if 2S={F∩S:F∈ℱ}. The Sauer inequality states that in general, a set system ℱ shatters at least |ℱ| sets. Here, we concentrate on the case of equality. A set system is called shattering-extremal if it shatters exactly |ℱ| sets. We characterize shattering extremal set systems of Vapnik-Chervonenkis dimension 1 in terms of their inclusion graphs. Also, from the perspective of extremality, we relate set systems of bounded Vapnik-Chervonenkis dimension to their projections.