VIKKi - Ringel Circles Problem

In 1959 Gerhard Ringel posed the following problem which remained open for over 60 years.
Is it possible to always color a given set of circles in the plane with five colors so that tangent circles get distinct colors?
James Davis, Chaya Keller, Linda Kleist, Shakhar Smorodinsky and Bartosz Walczak provided a strong negative answer to Ringel's 1959 open problem.

explaining the problem: (00:00)
The solution: (1:24)

Видео VIKKi - Ringel Circles Problem канала Vikki Academy