Criterion for the Buchstaber invariant of simplicial complexes to be equal to two (1212.3970v1)

Abstract: In this paper we study the Buchstaber invariant of simplicial complexes, which comes from toric topology. With each simplicial complex $K$ on $m$ vertices we can associate a moment-angle complex $\mathcal Z_K$ with a canonical action of the compact torus $T^m$. Then $s(K)$ is the maximal dimension of a toric subgroup that acts freely on $\mathcal Z_K$. We develop the Buchstaber invariant theory from the viewpoint of the set of minimal non-simplices of $K$. It is easy to show that $s(K)=1$ if and only if any two and any three minimal non-simplices intersect. For $K=\partial P^*$, where $P$ is a simple polytope, this implies that $P$ is a simplex. The case $s(P)=2$ is such more complicated. For example, for any $k\geqslant 2$ there exists an $n$-polytope with $n+k$ facets such that $s(P)=2$. Our main result is the criterion for the Buchstaber invariant of a simplicial complex $K$ to be equal to two.