Lattice Embeddings of Planar Point Sets

Abstract

Let $$\mathcal {M}$$M be a finite non-collinear set of points in the Euclidean plane, with the squared distance between each pair of points integral. Considering the points as lying in the complex plane, there is at most one positive square-free integer D, called the "characteristic" of $$\mathcal {M}$$M, such that a congruent copy of $$\mathcal {M}$$M embeds in $$\mathbb {Q}(\sqrt{-D})$$Q(-D). We generalize the work of Yiu and Fricke on embedding point sets in $$\mathbb {Z}^2$$Z2 by providing conditions that characterize when $$\mathcal {M}$$M embeds in the lattice corresponding to $$\mathcal {O}_{-D}$$O-D, the ring of integers in $$\mathbb {Q}(\sqrt{-D})$$Q(-D). In particular, we show that if the square of every ideal in $$\mathcal {O}_{-D}$$O-D is principal and the distance between at least one pair of points in $$\mathcal {M}$$M is integral, then $$\mathcal {M}$$M embeds in $$\mathcal {O}_{-D}$$O-D. Moreover, if $$\mathcal {M}$$M is primitive, so that the squared distances between pairs of points are relatively prime, and $$\mathcal {O}_{-D}$$O-D is a principal ideal domain, then $$\mathcal {M}$$M embeds in $$\mathcal {O}_{-D}$$O-D.