Shannon multigraph

In the mathematical discipline of graph theory, Shannon multigraphs, named after Claude Shannon by Vizing (1965), are a special type of triangle graphs, which are used in the field of edge coloring in particular.

A Shannon multigraph is multigraph with 3 vertices for which either of the following conditions holds:

• a) all 3 vertices are connected by the same number of edges.
• b) as in a) and one additional edge is added.

More precisely one speaks of Shannon multigraph Sh(n), if the three vertices are connected by $\lfloor \frac{n}{2}\rfloor$, $\lfloor \frac{n}{2}\rfloor$ and $\lfloor \frac{n+1}{2}\rfloor$ edges respectively. This multigraph has maximum degree n. Its multiplicity (the maximum number of edges in a set of edges that all have the same endpoints) is $\lfloor \frac{n+1}{2}\rfloor$.

Examples

• Shannon multigraphs
• Sh(2)
  Sh(2)
• Sh(3)
  Sh(3)
• Sh(4)
  Sh(4)
• Sh(5)
  Sh(5)
• Sh(6)
  Sh(6)
• Sh(7)
  Sh(7)

Edge coloring

File:Multigraph-edge-coloring.svg

This nine-edge Shannon multigraph requires nine colors in any edge coloring; its vertex degree is six and its multiplicity is three.

According to a theorem of Shannon (1949), every multigraph with maximum degree $\mathrm{\Delta }$ has an edge coloring that uses at most $\frac{3}{2}\mathrm{\Delta }$ colors. When $\mathrm{\Delta }$ is even, the example of the Shannon multigraph with multiplicity $\mathrm{\Delta }/2$ shows that this bound is tight: the vertex degree is exactly $\mathrm{\Delta }$, but each of the $\frac{3}{2}\mathrm{\Delta }$ edges is adjacent to every other edge, so it requires $\frac{3}{2}\mathrm{\Delta }$ colors in any proper edge coloring. A version of Vizing's theorem (Vizing 1964) states that every multigraph with maximum degree $\mathrm{\Delta }$ and multiplicity $\mu$ may be colored using at most $\mathrm{\Delta }+\mu$ colors. Again, this bound is tight for the Shannon multigraphs.

References

• Fiorini, S.; Wilson, Robin James (1977), Edge-colourings of graphs, Research Notes in Mathematics, vol. 16, London: Pitman, p. 34, ISBN 0-273-01129-4, MR 0543798
• Shannon, Claude E. (1949), "A theorem on coloring the lines of a network", J. Math. Physics, 28: 148–151, doi:10.1002/sapm1949281148, hdl:10338.dmlcz/101098, MR 0030203.
• Volkmann, Lutz (1996), Fundamente der Graphentheorie (in German), Wien: Springer, p. 289, ISBN 3-211-82774-9{{citation}}: CS1 maint: unrecognized language (link).
• Vizing, V. G. (1964), "On an estimate of the chromatic class of a p-graph", Diskret. Analiz., 3: 25–30, MR 0180505.
• Vizing, V. G. (1965), "The chromatic class of a multigraph", Kibernetika, 1965 (3): 29–39, MR 0189915.

External links

• Lutz Volkmann: Graphen an allen Ecken und Kanten. Lecture notes 2006, p. 242 (German)