The Tutte (3,8)-cage.

In the mathematical field of graph theory, a cage is a regular graph that has as few vertices as possible for its girth.

Formally, an (r, g)-graph is defined to be a graph in which each vertex has exactly r neighbors, and in which the shortest cycle has length exactly g. An (r, g)-cage is an (r, g)-graph with the smallest possible number of vertices, among all (r, g)-graphs. A (3, g)-cage is often called a g-cage.

It is known that an (r, g)-graph exists for any combination of r ≥ 2 and g ≥ 3. It follows that all (r, g)-cages exist.

If a Moore graph exists with degree r and girth g, it must be a cage. Moreover, the bounds on the sizes of Moore graphs generalize to cages: any cage with odd girth g must have at least

${\displaystyle 1+r\sum _{i=0}^{(g-3)/2}(r-1)^{i}}$

vertices, and any cage with even girth g must have at least

${\displaystyle 2\sum _{i=0}^{(g-2)/2}(r-1)^{i}}$

vertices. Any (r, g)-graph with exactly this many vertices is by definition a Moore graph and therefore automatically a cage.

There may exist multiple cages for a given combination of r and g. For instance there are three nonisomorphic (3, 10)-cages, each with 70 vertices: the Balaban 10-cage, the Harries graph and the Harries–Wong graph. But there is only one (3, 11)-cage: the Balaban 11-cage (with 112 vertices).

Known cages

A 1-regular graph has no cycle, and a connected 2-regular graph has girth equal to its number of vertices, so cages are only of interest for r ≥ 3. The (r,3)-cage is a complete graph K_r+1 on r+1 vertices, and the (r,4)-cage is a complete bipartite graph K_r,r on 2r vertices.

Notable cages include:

• (3,5)-cage: the Petersen graph, 10 vertices
• (3,6)-cage: the Heawood graph, 14 vertices
• (3,7)-cage: the McGee graph, 24 vertices
• (3,8)-cage: the Tutte–Coxeter graph, 30 vertices
• (3,10)-cage: the Balaban 10-cage, 70 vertices
• (3,11)-cage: the Balaban 11-cage, 112 vertices
• (4,5)-cage: the Robertson graph, 19 vertices
• (7,5)-cage: The Hoffman–Singleton graph, 50 vertices.
• When r − 1 is a prime power, the (r,6) cages are the incidence graphs of projective planes.
• When r − 1 is a prime power, the (r,8) and (r,12) cages are generalized polygons.

The numbers of vertices in the known (r,g) cages, for values of r > 2 and g > 2, other than projective planes and generalized polygons, are:

Asymptotics

For large values of g, the Moore bound implies that the number n of vertices must grow at least singly exponentially as a function of g. Equivalently, g can be at most proportional to the logarithm of n. More precisely,

${\displaystyle g\leq 2\log _{r-1}n+O(1).}$

It is believed that this bound is tight or close to tight (Bollobás & Szemerédi 2002). The best known lower bounds on g are also logarithmic, but with a smaller constant factor (implying that n grows singly exponentially but at a higher rate than the Moore bound). Specifically, the construction of Ramanujan graphs defined by Lubotzky, Phillips & Sarnak (1988) satisfy the bound

${\displaystyle g\geq {\frac {4}{3}}\log _{r-1}n+O(1).}$

This bound was improved slightly by Lazebnik, Ustimenko & Woldar (1995).

It is unlikely that these graphs are themselves cages, but their existence gives an upper bound to the number of vertices needed in a cage.

References

• Biggs, Norman (1993), Algebraic Graph Theory (2nd ed.), Cambridge Mathematical Library, pp. 180–190, ISBN 0-521-45897-8.
• Bollobás, Béla; Szemerédi, Endre (2002), "Girth of sparse graphs", Journal of Graph Theory, 39 (3): 194–200, doi:10.1002/jgt.10023, MR 1883596.
• Exoo, G; Jajcay, R (2008), "Dynamic Cage Survey", Dynamic Surveys, Electronic Journal of Combinatorics, DS16, archived from the original on 2015-01-01, retrieved 2012-03-25.
• Erdős, Paul; Rényi, Alfréd; Sós, Vera T. (1966), "On a problem of graph theory" (PDF), Studia Sci. Math. Hungar., 1: 215–235, archived from the original (PDF) on 2016-03-09, retrieved 2010-02-23.
• Hartsfield, Nora; Ringel, Gerhard (1990), Pearls in Graph Theory: A Comprehensive Introduction, Academic Press, pp. 77–81, ISBN 0-12-328552-6.
• Holton, D. A.; Sheehan, J. (1993), The Petersen Graph, Cambridge University Press, pp. 183–213, ISBN 0-521-43594-3.
• Lazebnik, F.; Ustimenko, V. A.; Woldar, A. J. (1995), "A new series of dense graphs of high girth", Bulletin of the American Mathematical Society, New Series, 32 (1): 73–79, arXiv:math/9501231, doi:10.1090/S0273-0979-1995-00569-0, MR 1284775.
• Lubotzky, A.; Phillips, R.; Sarnak, P. (1988), "Ramanujan graphs", Combinatorica, 8 (3): 261–277, doi:10.1007/BF02126799, MR 0963118.
• Tutte, W. T. (1947), "A family of cubical graphs", Proc. Cambridge Philos. Soc., 43 (4): 459–474, Bibcode:1947PCPS...43..459T, doi:10.1017/S0305004100023720.

External links

• Brouwer, Andries E. Cages
• Royle, Gordon. Cubic Cages and Higher valency cages
• Weisstein, Eric W. "Cage Graph". MathWorld.