Finding a subdivision of a prescribed digraph of order 4.

The problem of when a given digraph contains a subdivision of a fixed digraph F is considered. Bang-Jensen etal. [4] laid out foundations for approaching this problem from the algorithmic point of view. In this article, we give further support to several open conjectures and speculations about algorithmic complexity of finding F-subdivisions. In particular, up to five exceptions, we completely classify for which 4-vertex digraphs F, the F-subdivision problem is polynomial-time solvable and for which it is NP-complete. While all NP-hardness proofs are made by reduction from some version of the 2-linkage problem in digraphs, some of the polynomial-time solvable cases involve relatively complicated algorithms.