Selected Topics in Graph Theory, lecture 7: Minors in Planar Graphs

Treewidth - radius relation in planar graphs, excluded grid minors in planar graphs

[alexgunning]

Technical point -- I think you want to minimise Q\P, not the symmetric difference.

The problem is that a left excursion can have arbitrarily large intersection with some previous path in P..

(The optimal P symmetric difference Q set of paths might still be optimal though)