Hadwiger–Nelson problem
In geometric graph theory , the Hadwiger–Nelson problem , named after Hugo Hadwiger and Edward Nelson , asks for the minimum number of colors required to color the plane such that no two points at distance 1 from each other have the same color . 
<triplet> Hadwiger–Nelson problem <subj> Hugo Hadwiger <obj> named after <subj> Edward Nelson <obj> named after