Let and be simple graphs. Then is a graph minor of if it is isomorphic to a graph obtained by applying a sequence of operations of the following sort, starting from :
Removing isolated points;
Contracting edges, where contracting an edge means removing it and identifying its endpoints.
Graph minors can be viewed as certain types of subquotients in an appropriate category of graphs.
We take the point of view that a simple graph is essentially the same thing as a set equipped with a reflexive symmetric relation , and a morphism of simple graphs is a function which respects these relations. Indeed, such a relation determines and is uniquely determined by a simple graph where for given vertices , there is an edge between and in iff both and .
Notice that contraction of edges yields a quotient in this category. For example, if we want to contract a collection of edges by identifying certain points along an equivalence relation , that is via some quotient map , then we simply take the image of the composite
notice this image is a reflexive symmetric relation on . While it might seem more natural to use instead an irreflexive relation to express loop-free graphs (a relation being irreflexive if it has empty intersection with the diagonal in ), the fact of the matter is that irreflexive relations are not closed under the taking of images, and the category of sets with symmetric irreflexive relations and relation-preserving morphisms does not admit the forming of such quotients.
Thus, we let denote the category of sets equipped with a reflexive symmetric relation. This category has many convenient properties:
Essentially the same observations apply to the category of finite simple graphs: it too is an extensive quasitopos.
The category of simple graphs is also topological, and cohesive in the following sense:
The forgetful functor is faithful, in fact exhibits as a topological concrete category, and is cohesive: there is an adjoint string , with product-preserving.
Of course assigns to a set the discrete graph on , and the codiscrete graph, while is the set of connected components of . Let denote the unit of ; the component takes a vertex to the connected component to which it belongs. Notice that is a regular epi.
A contraction map between simple graphs is a morphism that arises by pushing out a map of the form along some map :
is a pushout square. If is a contraction map, we also say is a contraction of .
It is immediate that a contraction map is a regular epi, since every is regular epi.
Contraction maps are closed under composition.
A graph is a minor of a graph if it arises as a subobject of some contraction of :
The subquotient relation is reflexive and transitive.
Reflexivity is clear. For transitivity, we compose subquotients by taking a pushout square as follows.
where we use the simple fact that the pushout of a mono along an arrow in is a mono (because reflects monos and preserves monos and pushouts, plus the fact that the pushout of a mono in is a mono).
For finite graphs, the subquotient relation is also antisymmetric (if and are minors of one another, then they are isomorphic). Indeed, if either the arrow or the arrow is not an isomorphism, then has strictly fewer edges and vertices than . This is clearly not the case for infinite graphs (e.g., the infinite rooted binary tree without leaves contains as a subgraph a disjoint sum of two copies of itself).
(I intend to expand this section, eventually. Hopefully one can develop a categorical story about graph minors in particular.)
The collection of forests is closed under the graph minor relation.
The collection of planar graphs is closed under the graph minor relation.
This celebrated result of Robertson and Seymour says that no collection of finite graphs, ordered by the graph minor relation, has an infinite antichain. Equivalently, that in any collection of finite graphs, there are only finitely many elements which are minimal with respect to the graph minor relation.
As a corollary, any class of finite graphs which is closed under minors admits a forbidden minor characterization: is precisely the class of graphs which do not have any members of a specific finite family of graphs as a minor. (Proof: the complement of in the poset of finite graphs has finitely many minimal members. Then, belongs to iff none of these minimal members occurs as a minor of .
For example, the class of forests is precisely the class of finite graphs which do not have a triangle as a minor. The class of planar graphs is precisely the class of finite graphs which do not have or as a minor.