Forbidding Kuratowski Graphs as Immersions

The immersion relation is a partial ordering relation on graphs that is weaker than the topological minor relation in the sense that if a graph $G$ contains a graph $H$ as a topological minor, then it also contains it as an immersion but not vice versa. Kuratowski graphs, namely $K_{5}$ and $K_{3,3}$, give a precise characterization of planar graphs when excluded as topological minors. In this note we give a structural characterization of the graphs that exclude Kuratowski graphs as immersions. We prove that they can be constructed by applying consecutive $i$-edge-sums, for $i\leq 3$, starting from graphs that are planar sub-cubic or of branch-width at most 10

Forbidding Kuratowski graphs as immersions

The immersion relation is a partial ordering relation on graphs that is weaker than the topological minor relation in the sense that if a graph G contains a graph H as a topological minor, then it also contains it as an immersion but not vice versa. Kuratowski graphs, namely K 5 and K 3,3 , give a precise characterization of planar graphs when excluded as topological minors. In this note we give a structural characterization of the graphs that exclude Kuratowski graphs as immersions. We prove that they can be constructed by applying consecutive i-edge-sums, for i ≤ 3, starting from graphs that are planar sub-cubic or of branchwidth at most 10

Effective computation of immersion obstructions for unions of graph classes

In the final paper of the Graph Minors series Robertson and Seymour proved that graphs are well-quasi-ordered under the immersion ordering. A direct implication of this theorem is that each class of graphs that is closed under taking immersions can be fully characterized by forbidding a finite set of graphs (immersion obstruction set). However, as the proof of the well-quasi-ordering theorem is non-constructive, there is no generic procedure for computing such a set. Moreover, it remains an open issue to identify for which immersion-closed graph classes the computation of those sets can become effective. By adapting the tools that were introduced by Adler, Grohe and Kreutzer, for the effective computation of minor obstruction sets, we expand the horizon of computability to immersion obstruction sets. In particular, our results propagate the computability of immersion obstruction sets of immersion-closed graph classes to immersion obstruction sets of finite unions of immersion-closed graph classes. © 2013 Elsevier Inc

Effective computation of immersion obstructions for unions of graph classes

In the final paper of the Graph Minors series [Neil Robertson and Paul D. Seymour. Graph minors XXIII. Nash-Williams' immersion conjecture J. Comb. Theory, Ser. B, 100(2):181-205, 2010.], N. Robertson and P. Seymour proved that graphs are well-quasi-ordered with respect to the immersion relation. A direct implication of this theorem is that each class of graphs that is closed under taking immersions can be fully characterized by forbidding a finite set of graphs (immersion obstruction set). However, as the proof of the well-quasi-ordering theorem is non-constructive, there is no generic procedure for computing such a set. Moreover, it remains an open issue to identify for which immersion-closed graph classes the computation of those sets can become effective. By adapting the tools that where introduced in [Isolde Adler, Martin Grohe and Stephan Kreutzer. Computing excluded minors, SODA, 2008: 641-650.] for the effective computation of obstruction sets for the minor relation, we expand the horizon of the computability of obstruction sets for immersion-closed graph classes. In particular, we prove that there exists an algorithm that, given the immersion obstruction sets of two graph classes that are closed under taking immersions, outputs the immersion obstruction set of their union. © 2012 Springer-Verlag