Graceful labeling
From Wikipedia, the free encyclopedia
In graph theory, a graceful labeling of a graph with n vertices and e edges is a labeling of its vertices with distinct integers between 0 and e inclusive, such that each edge is uniquely identified by the positive, or absolute difference between its endpoints.[1] A graph which admits a graceful labeling is called a graceful graph.
The name "graceful labeling" is due to Solomon W. Golomb; this class of labelings was originally given the name β-labelings by Alex Rosa in a 1967 paper on graph labelings.[2]
A major unproven conjecture in graph theory is the Ringel-Kotzig conjecture, which hypothesizes that all trees are graceful. (The Ringel-Kotzig conjecture is also known as "Von Koch's conjecture"[2] and the "graceful labeling conjecture".) Kotzig once called the effort to prove the conjecture a "disease".[3]
[edit] Selected results
- In his original paper, Rosa proved that no Eulerian graph with order equivalent to 1 or 2 (mod 4) is graceful.[2]
- All paths and caterpillar graphs are graceful.
- All lobster graphs with a perfect matching are graceful.[4]
- All trees with at most 27 vertices are graceful; this result was shown by Aldred and McKay using a computer program.[5][6]
- All wheel graphs, web graphs, Helm graphs, gear graphs, and rectangular grids are graceful.[5]
- All n-dimensional hypercubes are graceful.[7]
- All simple graphs with four or fewer vertices are graceful. The only non-graceful simple graphs with five vertices are the 5-cycle (pentagon); the complete graph K5; and a "bow-tie" graph on five vertices, consisting of two triangles joined at one vertex.[8]
[edit] See also
[edit] References
- ^ Virginia Vassilevska, "Coding and Graceful Labeling of trees." SURF 2001. PostScript
- ^ a b Alex Rosa, "On certain valuations of the vertices of a graph." Theory of Graphs (International Symposium, Rome, July 1966), Gordon and Breach, N.Y. and Dunod Paris. (1967) 349–355
- ^ C. Huang, A. Kotzig, and A. Rosa, "Further results on tree labellings". Utilitas Mathematica, 21c (1982) 31–48; cited in Gallian, 1998
- ^ "Von Koch's conjecture", Usenet post to sci.math.research by Jim Nastos, 2003. [1]
- ^ a b Joseph A. Gallian, "A Dynamic Survey of Graph Labeling." The Electronic Journal of Combinatorics 5 (1998). MR1668059 PDF, updated 2008
- ^ R. E. L. Aldred, B. D. McKay, "Graceful and harmonious labellings of trees", Bulletin of the Institute of Combinatorics and Its Applications 23 (1998), 69–72 MR1621760
- ^ Anton Kotzig. "Decomposition of complete graphs into isomorphic cubes", Journal of Combinatoric Theory, Series B, 31 (1981) 292–296 MR0638285; cited in Gallian, 1998
- ^ Eric W. Weisstein, Graceful graph at MathWorld.
