The purity of set-systems related to Grassmann necklaces

Abstract: Studying the problem of quasicommuting quantum minors, Leclerc and Zelevinsky introduced in 1998 the notion of weakly separated sets in $[n]:={1,\ldots, n}$. Moreover, they raised several conjectures on the purity for this symmetric relation, in particular, on the Boolean cube $2^{[n]}$. In 0909.1423[math.CO] we proved these purity conjectures for the Boolean cube $2^{[n]}$, the discrete Grassmanian ${[n]\choose r}$, and some other set-systems. Oh, Postnikov, and Speyer in arxiv:1109.4434 proved the purity for weakly separated collections inside a positroid which contain a Grassmann necklace $\mathcal N$ defining the positroid. We denote such set-systems as $\mathcal{I}nt(\mathcal N )$. In this paper we give an alternative (and shorter) proof of the purity of $\mathcal{I}nt(\mathcal N )$ and present a stronger result. More precisely, we introduce a set-system $\mathcal{O}ut(\mathcal N )$ complementary to $\mathcal{I}nt(\mathcal N )$, in a sense, and establish its purity. Moreover, we prove (Theorem~3) that these two set-systems are weakly separated from each other. As a consequence of Theorem~3, we obtain the purity of set-systems related to pairs of weakly separated necklaces (Proposition 4 and Corollaries 1 and 2). Finally, we raise a conjecture on the purity of both the interior and exterior of a generalized necklace.