A Note on Obstructions to Clustered Planarity
Jamie Sneddon, C. Paul Bonnington · The Electronic Journal of Combinatorics · 2011
A planar digraph $D$ is clustered planar if in some planar embedding of $D$ we have at each vertex the in-arcs occurring sequentially in the local rotation. By supplementing the operations used to form the usual minors in Kuratowski's theorem, clustered planar digraphs are characterised.