# Forbidden graph characterization

(Redirected from Forbidden minor)

In graph theory, a branch of mathematics, many important families of graphs can be described by a finite set of individual graphs that do not belong to the family and further exclude all graphs from the family which contain any of these forbidden graphs as (induced) subgraph or minor. A prototypical example of this phenomenon is Kuratowski's theorem, which states that a graph is planar (can be drawn without crossings in the plane) if and only if it does not contain either of two forbidden graphs, the complete graph K5 and the complete bipartite graph K3,3. For Kuratowski's theorem, the notion of containment is that of graph homeomorphism, in which a subdivision of one graph appears as a subgraph of the other. Thus, every graph either has a planar drawing (in which case it belongs to the family of planar graphs) or it has a subdivision of one of these two graphs as a subgraph (in which case it does not belong to the planar graphs).

More generally, a forbidden graph characterization is a method of specifying a family of graph, or hypergraph, structures, by specifying substructures that are forbidden from existing within any graph in the family. Different families vary in the nature of what is forbidden. In general, a structure G is a member of a family ${\mathcal {F}}$ if and only if a forbidden substructure is not contained in G. The forbidden substructure might be one of:

• subgraphs, smaller graphs obtained from subsets of the vertices and edges of a larger graph,
• induced subgraphs, smaller graphs obtained by selecting a subset of the vertices and using all edges with both endpoints in that subset,
• homeomorphic subgraphs (also called topological minors), smaller graphs obtained from subgraphs by collapsing paths of degree-two vertices to single edges, or
• graph minors, smaller graphs obtained from subgraphs by arbitrary edge contractions.

The set of structures that are forbidden from belonging to a given graph family can also be called an obstruction set for that family.

Forbidden graph characterizations may be used in algorithms for testing whether a graph belongs to a given family. In many cases, it is possible to test in polynomial time whether a given graph contains any of the members of the obstruction set, and therefore whether it belongs to the family defined by that obstruction set.

In order for a family to have a forbidden graph characterization, with a particular type of substructure, the family must be closed under substructures. That is, every substructure (of a given type) of a graph in the family must be another graph in the family. Equivalently, if a graph is not part of the family, all larger graphs containing it as a substructure must also be excluded from the family. When this is true, there always exists an obstruction set (the set of graphs that are not in the family but whose smaller substructures all belong to the family). However, for some notions of what a substructure is, this obstruction set could be infinite. The Robertson–Seymour theorem proves that, for the particular case of graph minors, a family that is closed under minors always has a finite obstruction set.

## List of forbidden characterizations for graphs and hypergraphs

Family Forbidden graphs Relation Reference
Forests loops, pairs of parallel edges, and cycles of all lengths subgraph Definition
a loop (for multigraphs) or a triangle K3 (for simple graphs) graph minor Definition
Claw-free graphs star K1,3 induced subgraph Definition
Comparability graphs induced subgraph
Triangle-free graphs triangle K3 induced subgraph Definition
Planar graphs K5 and K3,3 homeomorphic subgraph Kuratowski's theorem
K5 and K3,3 graph minor Wagner's theorem
Outerplanar graphs K4 and K2,3 graph minor Diestel (2000), p. 107
Outer 1-planar graphs five forbidden minors graph minor Auer et al. (2013)
Graphs of fixed genus a finite obstruction set graph minor Diestel (2000), p. 275
Apex graphs a finite obstruction set graph minor 
Linklessly embeddable graphs The Petersen family graph minor 
Bipartite graphs odd cycles subgraph 
Chordal graphs cycles of length 4 or more induced subgraph 
Perfect graphs cycles of odd length 5 or more or their complements induced subgraph 
Line graph of graphs nine forbidden subgraphs (listed here) induced subgraph 
Graph unions of cactus graphs the four-vertex diamond graph formed by removing an edge from the complete graph K4 graph minor 
Ladder graphs K2,3 and its dual graph homeomorphic subgraph 
split graphs $C_{4},C_{5},{\bar {C_{4}}}(=K_{2}+K_{2})$  induced subgraph 
2-connected series-parallel (treewidth ≤ 2 branchwidth ≤ 2) K4 graph minor Diestel (2000), p. 327
treewidth ≤ 3 K5, octahedron, pentagonal prism, Wagner graph graph minor 
branchwidth ≤ 3 K5, octahedron, cube, Wagner graph graph minor 
Complement-reducible graphs (cographs) 4-vertex path P4 induced subgraph 
Trivially perfect graphs 4-vertex path P4 and 4-vertex cycle C4 induced subgraph 
Threshold graphs 4-vertex path P4, 4-vertex cycle C4, and complement of C4 induced subgraph 
Line graph of 3-uniform linear hypergraphs a finite list of forbidden induced subgraphs with minimum degree at least 19 induced subgraph 
Line graph of k-uniform linear hypergraphs, k > 3 a finite list of forbidden induced subgraphs with minimum edge degree at least 2k2 − 3k + 1 induced subgraph 
Graphs ΔY-reducible to a single vertex a finite list of at least 68 billion distinct (1,2,3)-clique sums graph minor 
General theorems
a family defined by an induced-hereditary property a (not necessarily finite) obstruction set induced subgraph
a family defined by a minor-hereditary property a finite obstruction set graph minor Robertson–Seymour theorem