# Two Colors - All Distances

Points in the plane are each colored with one of two colors: red or blue. The set of distances between the blue points is blue and the set of distances between the red points is red. Prove that either one or the other contains all the positive reals.

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Copyright © 1996-2017 Alexander Bogomolny

Points in the plane are each colored with one of two colors: red or blue. The set of distances between the blue points is blue and the set of distances between the red points is red. Prove that either one or the other contains all the positive reals.

### Solution

Assume to the contrary that the blue set lacks distance *b* and the red set lacks distance *r*. Choose a blue point *B* and form an isosceles triangle *ABC* with *AB* = *BC* = *b**AC* = *r*.*A* and *C* are red or one of them is blue. If they are both red we have two red points at distance *r*. If one of them blue, we have two blue points at distance *b*.

- Ramsey's Theorem
- Party Acquaintances
- Ramsey Number R(3, 3, 3)
- Ramsey Number R(4, 3)
- Ramsey Number R(5, 3)
- Ramsey Number R(4, 4)
- Geometric Application of Ramsey's Theory
- Coloring Points in the Plane and Elsewhere
- Two Colors - Two Points
- Three Colors - Two Points
- Two Colors - All Distances
- Two Colors on a Straight Line
- Two Colors - Three Points
- Three Colors - Bichromatic Lines
- Chromatic Number of the Plane
- Monochromatic Rectangle in a 2-coloring of the Plane
- Two Colors - Three Points on Circle
- Coloring a Graph
- No Equilateral Triangles, Please

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Copyright © 1996-2017 Alexander Bogomolny

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