The van der Waerden Simplicial Complex and its Lefschetz Properties

Abstract

The van der Waerden simplicial complex, denoted ${\tt vdw}(n,k)$, is the simpicial complex whose facets correspond to the arithmetic progressions of length $k$ in the set $\{1,\ldots,n\}$. We study the Lefschetz properties of the Artinian ring $A({\tt vdw}(n,k)) = K[x_1,\ldots,x_n]/(I_{{\tt vdw}(n,k)} + \langle x_1^2,\ldots,x_n^2\rangle)$ where $I_{{\tt vdw}(n,k)}$ is the associated Stanley--Reisner ideal. If $k=1,2$ or $n-1$, the ring $A({\tt vdw}(n,k))$ will have the Weak Lefschetz Property for all $n > k$. When $k=3$, we classify the rings $A({\tt vdw}(n,3))$ that have the Weak Lefschetz Property when the characteristic is zero. We conjecture that $A({\tt vdw}(n,k))$ fails to have the Weak Lefschetz Property if $n \gg k \geq 3$ and $k$ odd. We also classify when ${\tt vdw}(n,k)$ is a pseudo-manifold, which allows us to show that $A({\tt vdw}(n,k))$ satisfies the Weak Lefschetz Property in some degrees by using a result of Dao and Nair.