Large subsets of discrete hypersurfaces in ${\mathbb{Z}}^d$ contain arbitrarily many collinear points

In 1977 L.T. Ramsey showed that any sequence in ${\mathbb{Z}}^2$ with bounded gaps contains arbitrarily many collinear points. Thereafter, in 1980, C. Pomerance provided a density version of this result, relaxing the condition on the sequence from having bounded gaps to having gaps bounded on average. We give a higher dimensional generalization of these results. Our main theorem is the following. Theorem: Let $d\in \mathbb{N}$, let $f:{\mathbb{Z}}^d\to {\mathbb{Z}}^{d+1}$ be a Lipschitz map and let $A\subset {\mathbb{Z}}^d$ have positive upper Banach density. Then $f(A)$ contains arbitrarily many collinear points. Note that Pomerance’s theorem corresponds to the special case $d=1$. In our proof, we transfer the problem from a discrete to a continuous setting, allowing us to take advantage of analytic and measure theoretic tools such as Rademacher’s theorem.