A Study of $k$-dipath Colourings of Oriented Graphs
  We examine $t$-colourings of oriented graphs in which, for a fixed integer $k \geq 1$, vertices joined by a directed path of length at most $k$ must be assigned different colours. A homomorphism model that extends the ideas of Sherk for the case $k=2$ is described. Dichotomy theorems for the complexity of the problem of deciding, for fixed $k$ and $t$, whether there exists such a $t$-colouring are proved. 
Christopher Duffy, Gary MacGillivray, \'Eric Sopena
cs.DM math.CO