An alternating colouring function on strings

Abstract

An alternating colouring function is defined on strings over the alphabet $\{0, 1\}$. It divides the strings in colourable and non-colourable ones. The points in the subshift of finite type defined by forbidding all non-colourable strings of a certain length alternate between states of one colour and states of the other colour. In other words, the points in the 2nd power shifts all have the same colour. The number $K_n$ of non-colourable strings of length $n \ge 2$ is shown to be $2 \cdot (J_{n-2} + 1)$ where $J$ is the sequence of Jacobsthal numbers. The number of sources and sinks in the de Bruijn graph of dimension $n \ge 3$ with non-colourable edges removed is shown each to be $K_n - 4$.