# Peripheral cycle

In this graph, the red triangle formed by vertices 1, 2, and 5 is a peripheral cycle: the four remaining edges form a single bridge. However, pentagon 1–2–3–4–5 is not peripheral, as the two remaining edges form two separate bridges.

In graph theory, a peripheral cycle (or peripheral circuit) in an undirected graph is, intuitively, a cycle that does not separate any part of the graph from any other part. Peripheral cycles (or, as they were initially called, peripheral polygons) were first studied by Tutte (1963), and play important roles in the characterization of planar graphs and in generating the cycle spaces of nonplanar graphs.[1]

## Definitions

A peripheral cycle $C$ in a graph $G$ can be defined formally in one of several equivalent ways:

• $C$ is peripheral if it is a simple cycle in a connected graph with the property that, for every two edges $e_1$ and $e_2$ in $G\setminus C$, there exists a path in $G$ that starts with $e_1$, ends with $e_2$, and has no interior vertices belonging to $C$.[2]
• $C$ is peripheral if it is an induced cycle with the property that the subgraph $G\setminus C$ formed by deleting the edges and vertices of $C$ is connected.[3]
• If $C$ is any subgraph of $G$, a bridge[4] of $C$ is a minimal subgraph $B$ of $G$ that is edge-disjoint from $C$ and that has the property that all of its points of attachments (vertices adjacent to edges in both $B$ and $G\setminus B$) belong to $C$.[5] A simple cycle $C$ is peripheral if it has exactly one bridge.[6]

The equivalence of these definitions is not hard to see: a connected subgraph of $G\setminus C$ (together with the edges linking it to $C$), or a chord of a cycle that causes it to fail to be induced, must in either case be a bridge, and must also be an equivalence class of the binary relation on edges in which two edges are related if they are the ends of a path with no interior vertices in $C$.[7]

## Properties

Peripheral cycles appear in the theory of polyhedral graphs, that is, 3-vertex-connected planar graphs. For every planar graph $G$, and every planar embedding of $G$, the faces of the embedding that are induced cycles must be peripheral cycles. In a polyhedral graph, all faces are peripheral cycles, and every peripheral cycle is a face.[8] It follows from this fact that (up to combinatorial equivalence, the choice of the outer face, and the orientation of the plane) every polyhedral graph has a unique planar embedding.[9]

In planar graphs, the cycle space is generated by the faces, but in non-planar graphs peripheral cycles play a similar role: for every 3-vertex-connected finite graph, the cycle space is generated by the peripheral cycles.[10] The result can also be extended to locally-finite but infinite graphs.[11] In particular, it follows that 3-connected graphs are guaranteed to contain peripheral cycles. There exist 2-connected graphs that do not contain peripheral cycles (an example is the complete bipartite graph $K_{2,4}$, for which every cycle has two bridges) but if a 2-connected graph has minimum degree three then it contains at least one peripheral cycle.[12]

Peripheral cycles in 3-connected graphs can be computed in linear time and have been used for designing planarity tests.[13] They were also extended to the more general notion of non-separating ear decompositions. In some algorithms for testing planarity of graphs, it is useful to find a cycle that is not peripheral, in order to partition the problem into smaller subproblems. In a biconnected graph of circuit rank less than three (such as a cycle graph or theta graph) every cycle is peripheral, but every biconnected graph with circuit rank three or more has a non-peripheral cycle, which may be found in linear time.[14]

Generalizing chordal graphs, Weaver & Seymour (1984) define a strangulated graph to be a graph in which every peripheral cycle is a triangle. They characterize these graphs as being the clique-sums of chordal graphs and maximal planar graphs.[15]

## Related concepts

Peripheral cycles have also been called non-separating cycles,[2] but this term is ambiguous, as it has also been used for two related but distinct concepts: simple cycles the removal of which would disconnect the remaining graph,[16] and cycles of a topologically embedded graph such that cutting along the cycle would not disconnect the surface on which the graph is embedded.[17]

In matroids, a non-separating circuit is a circuit of the matroid (that is, a minimal dependent set) such that deleting the circuit leaves a smaller matroid that is connected (that is, that cannot be written as a direct sum of matroids).[18] These are analogous to peripheral cycles, but not the same even in graphic matroids (the matroids whose circuits are the simple cycles of a graph). For example, in the complete bipartite graph $K_{2,3}$, every cycle is peripheral (it has only one bridge, a two-edge path) but the graphic matroid formed by this bridge is not connected, so no circuit of the graphic matroid of $K_{2,3}$ is non-separating.

## References

1. ^ Tutte, W. T. (1963), "How to draw a graph", Proceedings of the London Mathematical Society, Third Series 13: 743–767, doi:10.1112/plms/s3-13.1.743, MR 0158387.
2. ^ a b Di Battista, Giuseppe; Eades, Peter; Tamassia, Roberto; Tollis, Ioannis G. (1998), Graph Drawing: Algorithms for the Visualization of Graphs, Prentice Hall, pp. 74–75, ISBN 978-0-13-301615-4.
3. ^ This is, essentially, the definition used by Bruhn (2004). However, Bruhn distinguishes the case that $G$ has isolated vertices from disconnections caused by the removal of $C$.
4. ^ Not to be confused with bridge (graph theory), a different concept.
5. ^ Tutte, W. T. (1960), "Convex representations of graphs", Proceedings of the London Mathematical Society, Third Series 10: 304–320, MR 0114774.
6. ^ This is the definition of peripheral cycles originally used by Tutte (1963). Seymour & Weaver (1984) use the same definition of a peripheral cycle, but with a different definition of a bridge that more closely resembles the induced-cycle definition for peripheral cycles.
7. ^ See e.g. Theorem 2.4 of Tutte (1960), showing that the vertex sets of bridges are path-connected, see Seymour & Weaver (1984) for a definition of bridges using chords and connected components, and also see Di Battista et al. (1998) for a definition of bridges using equivalence classes of the binary relation on edges.
8. ^ Tutte (1963), Theorems 2.7 and 2.8.
9. ^ See the remarks following Theorem 2.8 in Tutte (1963). As Tutte observes, this was already known to Whitney, Hassler (1932), "Non-separable and planar graphs", Transactions of the American Mathematical Society 34 (2): 339–362, doi:10.2307/1989545, MR 1501641.
10. ^ Tutte (1963), Theorem 2.5.
11. ^ Bruhn, Henning (2004), "The cycle space of a 3-connected locally finite graph is generated by its finite and infinite peripheral circuits", Journal of Combinatorial Theory, Series B 92 (2): 235–256, doi:10.1016/j.jctb.2004.03.005, MR 2099143.
12. ^ Thomassen, Carsten; Toft, Bjarne (1981), "Non-separating induced cycles in graphs", Journal of Combinatorial Theory, Series B 31 (2): 199–224, doi:10.1016/S0095-8956(81)80025-1, MR 630983.
13. ^ Schmidt, Jens M. (2014), The Mondshein Sequence, pp. 967–978, doi:10.1007/978-3-662-43948-7_80.
14. ^ Di Battista et al. (1998), Lemma 3.4, pp. 75–76.
15. ^ Seymour, P. D.; Weaver, R. W. (1984), "A generalization of chordal graphs", Journal of Graph Theory 8 (2): 241–251, doi:10.1002/jgt.3190080206, MR 742878.
16. ^ E.g. see Borse, Y. M.; Waphare, B. N. (2008), "Vertex disjoint non-separating cycles in graphs", The Journal of the Indian Mathematical Society, New Series 75 (1-4): 75–92 (2009), MR 2662989.
17. ^ E.g. see Cabello, Sergio; Mohar, Bojan (2007), "Finding shortest non-separating and non-contractible cycles for topologically embedded graphs", Discrete and Computational Geometry 37 (2): 213–235, doi:10.1007/s00454-006-1292-5, MR 2295054.
18. ^ Maia, Bráulio, Junior; Lemos, Manoel; Melo, Tereza R. B. (2007), "Non-separating circuits and cocircuits in matroids", Combinatorics, complexity, and chance, Oxford Lecture Ser. Math. Appl. 34, Oxford: Oxford Univ. Press, pp. 162–171, doi:10.1093/acprof:oso/9780198571278.003.0010, MR 2314567.