Fun fact: The Kruskal Tree Theorem has been extended to undirected graphs (they form a quasi-wellordered set wrt the graph minor relationship), giving rise to a

Fun fact: The Kruskal Tree Theorem has been extended to undirected graphs (they form a quasi-wellordered set wrt the graph minor relationship), giving rise to an even faster growing sequence called the "Friedman's SSCG function". The theorem is interesting for other reasons though: It allows us to define graph families by forbidding certain substructures.