On the anisotropy and Lefschetz property for PL-spheres

Abstract

A simplicial sphere $\Delta$ is said to be generically anisotropic over a field $\mathbb{F}$ if, for a certain purely transcendental field extension $\mathbf{k}$ of $\mathbb{F}$, a certain Artinian reduction $A$ of the face ring $\mathbf{k}[\Delta ]$ has the following property: For every nonzero homogeneous element $\alpha \in A$ of degree at most $(\dim \Delta +1)/2$, its square ${\alpha }^2$ is also nonzero. The importance of this property is that the hard Lefschetz property for simplicial spheres can be derived from it. A recent result of Papadakis and Petrotou shows that every simplicial sphere is generically anisotropic over any field of characteristic $2$. In this paper, we give an equivalent condition of being generically anisotropic, and use it to present a simplified proof of Papadakis-Petrotou theorem for PL-spheres. We also prove that the simplicial spheres of dimension $2$ are generically anisotropic over any field $\mathbb{F}$.

16 pages. Updated version with minor typos corrected