Generalised Flatness Constants: A Framework Applied in Dimension $2$

Let $A \in \{ \mathbb{Z}, \mathbb{R} \}$ and $X \subset \mathbb{R}^d$ be a bounded set. Affine transformations given by an automorphism of $\mathbb{Z}^d$ and a translation in $A^d$ are called (affine) $A$-unimodular transformations. The image of $X$ under such a transformation is called an $A$-unimodular copy of $X$. It was shown in [Averkov, Hofscheier, Nill, 2019] that every convex body whose width is "big enough" contains an $A$-unimodular copy of $X$. The threshold when this happens is called the generalised flatness constant $\mathrm{Flt}_d^A(X)$. It resembles the classical flatness constant if $A=\mathbb{Z}$ and $X$ is a lattice point. In this work, we introduce a general framework for the explicit computation of these numerical constants. The approach relies on the study of $A$-$X$-free convex bodies generalising lattice-free (also known as hollow) convex bodies. We then focus on the case that $X=P$ is a full-dimensional polytope and show that inclusion-maximal $A$-$P$-free convex bodies are polytopes. The study of those inclusion-maximal polytopes provides us with the means to explicitly determine generalised flatness constants. We apply our approach to the case $X=\Delta_2$ the standard simplex in $\mathbb{R}^2$ of normalised volume $1$ and compute $\mathrm{Flt}^{\mathbb{R}}_2(\Delta_2)=2$ and $\mathrm{Flt}^{\mathbb{Z}}_2(\Delta_2)=\frac{10}3$.