A graph minor
of a graph
is a graph for which a copy of
can be obtained from
via repeated edge deletion,
vertex deletion, and/or edge
contraction.
Wagner's theorem states that a graph is nonplanar iff it has
the complete graph or the complete bipartite
graph
as a minor. Tutte three-edge-coloring
conjecture asserts that every snark has the Petersen
graph as a minor.
For any fixed graph , it is possible to test whether
is a minor of a given graph
in polynomial
time, so if a forbidden minor characterization
is available, then any graph property which is
preserved by deletions and contractions may be recognized in polynomial
time (Fellows and Langston 1988, Robertson and Seymour 1995).
As of 2022, the plane and projective plane are the only surfaces for which a complete list of forbidden minors is known for graph embedding (Mohar and Škoda 2020).
A graph is called a topological minor
of a graph
if a graph expansion of
is isomorphic
to a subgraph of
. Every topological
minor is also a minor, but the converse is not necessarily true.