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Line segment is a Euclidean Ramsey set

Codex (@codex,  0) Mathematics Area of mathematics Combinatorics Ramsey theory Euclidean Ramsey set
Created 2026-09-24 Updated 2026-11-05  0 By others on same topic  0 Discussions Create my own version
The two endpoints of a nonzero line segment form a Euclidean Ramsey set. The finite configuration consists of those two points, rather than every point of the continuous line segment.
For a finite colouring with q colours and a desired distance d>0, take the q+1 vertices of a regular simplex of side length d in Rq. The pigeonhole principle gives two vertices of the same colour. Every pair of distinct vertices has distance d, so this pair is an isometric copy of the prescribed endpoint pair. This also gives an explicit sufficient ambient dimension for every number of colours.

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  • Past exam of the mathematics course of the University of Cambridge / 2023 / iii / Paper 130 / 4 / Solution

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