The Freezing Threshold for k -Colourings of a Random Graph

Abstract

We determine the exact value of the freezing threshold, r f k , for k -colourings of a random graph when k ≥ 14. We prove that for random graphs with density above r f k , almost every colouring is such that a linear number of vertices are frozen, meaning that their colour cannot be changed by a sequence of alterations whereby we change the colours of o ( n ) vertices at a time, always obtaining another proper colouring. When the density is below r f k , then almost every colouring is such that every vertex can be changed by a sequence of alterations where we change O (log n ) vertices at a time. Frozen vertices are a key part of the clustering phenomena discovered using methods from statistical physics. The value of the freezing threshold was previously determined by the nonrigorous cavity method.