Protrusion Reduction Techniques

A protrusion is a piece of a graph that has small treewidth and connects to the rest of the graph through only a few vertices. For a lot of problems, that piece can be cut out and swapped for a smaller gadget that behaves identically (as far as the problem is concerned), and doing this repeatedly leads to small kernels. I build up protrusion reduction from kernels for Feedback Vertex Set and connect it to kernels for the broader family of Planar F-Deletion problems, which includes both Vertex Cover and Feedback Vertex Set.