Theorem: (Wagner's conjecture)
For every class of graphs , that is closed under taking of minors, there exists a finite set of graphs, ob(), called the obstruction set of , such that for each graph G, if and only if there is no that is a minor of
For every graph H, there exists an time algorithm, that, given a graph G, tests whether H is a minor of G.
For every planar graph H, there exists a constant c(H), such that for every graph G, if H is not a minor of G, then the treewidth of G is at most c(H).
印象中，minor 這傢伙還蠻常出現在 fixed-parameterized problems 裡頭的(??)值得注意一下。