**Fixed-parameter Tractability**

For graph invariants with the property that, for each *k*, the graphs with invariant at most *k* are minor-closed, the same method applies. For instance, by this result, treewidth, branchwidth, and pathwidth, vertex cover, and the minimum genus of an embedding are all amenable to this approach, and for any fixed *k* there is a polynomial time algorithm for testing whether these invariants are at most *k*, in which the exponent in the running time of the algorithm does not depend on *k*. A problem with this property, that it can be solved in polynomial time for any fixed *k* with an exponent that does not depend on *k*, is known as fixed-parameter tractable.

However, this method does not directly provide a single fixed-parameter-tractable algorithm for computing the parameter value for a given graph with unknown *k*, because of the difficulty of determining the set of forbidden minors. Additionally, the large constant factors involved in these results make them highly impractical. Therefore, the development of explicit fixed-parameter algorithms for these problems, with improved dependence on *k*, has continued to be an important line of research.

Read more about this topic: Robertson–Seymour Theorem