Robertson–Seymour theorem

In graph theory, the Robertson–Seymour theorem (also called the graph minors theorem) states that the undirected graphs, partially ordered by the graph minor relationship, form a well-quasi-ordering. Equivalently, every family of graphs that is closed under taking minors can be defined by a finite set of forbidden minors, in the same way that Wagner's theorem characterizes the planar graphs as being the graphs that do not have the complete graph K 5 {\displaystyle K_{5}} or the complete bipartite graph K 3 , 3 {\displaystyle K_{3,3}} as minors.

Source: Wikipedia — Robertson–Seymour theorem (CC BY-SA 4.0)