Integral distances from (two) lattice points

Abstract

{\it .}We completely characterize pairs of lattice points $P_1\neq P_2$ in the plane with the property that there are infinitely many lattice points $Q$ whose distance from both $P_1$ and $P_2$ is integral. In particular we show that it suffices that $P_2-P_1\neq (\pm 1,\pm 2), (\pm 2,\pm 1)$, and we show that $|P_1-P_2|>\sqrt{20}$ suffices for having infinitely many such $Q$ outside any finite union of lines. We use only elementary arguments, the crucial ingredient being a theorem of Gauss which does not appear to be often applied. We further include related remarks (and open questions), also for distances from an arbitrary prescribed finite set of lattice points % $P_1,\ldots ,P_r$. }