The Complexity of Colouring by Semicomplete Digraphs

Authors:
  • JBJørgen Bang-Jensen
    Jørgen Bang-Jensen
  • PHPavol Hell
    Pavol Hell
  • GMGary MacGillivray
    Gary MacGillivray

Abstract
The following problem, known as the H-colouring problem, is studied. An H-colouring of a directed graph D is a mapping $f:V( D ) \to V( H )$ such that $( f( x ),f( y ) )$ is an edge of H whenever $( x,y )$ is an edge of D. The H-colouring problem is the following. Instance: A directed graph D. Question: Does there exist an H-colouring of D? In this paper it is shown that for semicomplete digraphs T the T-colouring problem is NP-complete when T has more than one directed cycle, and polynomially decidable otherwise.