Topological and Diophantine properties of lattice subset projections

Abstract

Fix $1 \leq n < m, k = m-n.$ The Grassmannian $Gr(n,m)$ is a compact $kn$-dimensional manifold with a unique rotation invariant probability measure $σ_n.$ For $W \in Gr(n,m)$, $P_W : \mathbb R^m \mapsto W$ is orthogonal projection. A lattice subset $L \subset \mathbb Z^m \subset \mathbb R^m$ is called $k$-dense if it intersects $C(O) := \bigcup_{V \in O} V\backslash \{0\}$ for every nonempty open $O \subset Gr(k,m)$. We use Baire&#39;s category theorem [4] to prove that $L$ is $k$-dense iff $L_{n,lim} := \{W \in Gr(n,m) : 0 \mbox{ is a limit point of } P_W(L) \}$ is a $G_δ$ set. We use Khintchine-Groshev&#39;s theorem [5,13,20] to characterize Diophantine properties of $L_{n,lim}$ by lacunary properties of $L$ and construct $k$-dense $L$ with $σ_n(L_{n,lim}) = 0$ and with $σ_n(L_{n,lim}) = 1.$ We pose related questions about the construction of multidimensional crystalline measures and Fourier quasicrystals.