Erdős Problem #958

Let $A\subset \mathbb{R}^2$ be a finite set of size $n$, and let $\{d_1,\ldots,d_k\}$ be the set of distances determined by $A$. Let $f(d)$ be the multiplicity of $d$, that is, the number of ordered pairs from $A$ of distance $d$ apart.

Is it true that $k=n-1$ and $\{f(d_i)\}=\{n-1,\ldots,1\}$ if and only if $A$ is a set of equidistant points on a line or a circle?

#958: [Er84c]

distances | geometry

Erdős conjectured that the answer is no, and other such configurations exist.

This was proved by Clemen, Dumitrescu, and Liu [CDL25], who observed that equidistant points on a short circular arc on a circle of radius $1$, together with the centre, are also an example.

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