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Let be a finite set of size , and let be the set of distances determined by . Let be the multiplicity of , that is, the number of ordered pairs from of distance apart.
Is it true that and if and only if is a set of equidistant points on a line or a circle?
Let be a finite set of size , and let be the set of distances determined by . Let be the multiplicity of , that is, the number of unordered pairs from of distance apart.
Is it true that and if and only if is a set of equidistant points on a line or a circle?
Source: erdosproblems.com/958
An accepted solution exists. The statement is false.
DISPROVED (LEAN), in the site's label. The site marks the problem disproved, crediting Clemen, Dumitrescu and Liu with a second family of configurations, and flags a Lean proof of a four-point counterexample; both have the profile under the unordered count, so the label describes the corrected Statement. See the accepted claim page (Clemen, Dumitrescu and Liu, 2025) and the Lean claim page (Alexeev, 2025).
The site's wording counts ordered pairs, and under that count it fails at every : each unordered pair at distance gives two ordered pairs, so every multiplicity is even and the profile , which contains , never occurs. The equally spaced points on a line, which the question names, then have the multiplicities , so the "if" direction fails and the answer is no for a reason that has nothing to do with the problem; the smallest instance is , two points with the one multiplicity . The change replaces "ordered pairs" by "unordered pairs"; nothing else changes. The evidence is the counting convention of the sources. Erdős's own statement [Er84c, p. 135] conjectures that the multiplicities of the distinct distances among points cannot be "a permutation of " unless the points are equidistant on a line or a circle; such multiplicities sum to , the number of unordered pairs, and equally spaced points on a line achieve the profile only under that count. [CDL25, Section 5, p. 9] states the question for multiplicities whose sum is . The site's own commentary credits the arc-and-center family of [CDL25] as a further configuration with the profile, which it is only under the unordered count. The defect is the site's: Erdős's text speaks of the multiplicities of the distances and not of ordered pairs. The form follows from these sources, not from the results that settle it. No result concerns the ordered count alone. The page's standing judges the corrected Statement.