In graph theory, a graceful labeling of a graph with m edges is a labeling of its vertices with some subset of the integers between 0 and m inclusive, such that no two vertices share a label, and such that each edge is uniquely identified by the positive, or absolute difference between its endpoints. A graph which admits a graceful labeling is called a graceful graph.
A major unproven conjecture in graph theory is the Ringel–Kotzig conjecture, named after Gerhard Ringel and Anton Kotzig, which hypothesizes that all trees are graceful. The Ringel-Kotzig conjecture is also known as the "graceful labeling conjecture". Kotzig once called the effort to prove the conjecture a "disease".
- In his original paper, Rosa proved that an Eulerian graph with number of edges m ≡ 1 (mod 4) or m ≡ 2 (mod 4) can't be graceful.
- Also in his original paper, Rosa proved that the cycle Cn is graceful if and only if n ≡ 0 (mod 4) or n ≡ 3 (mod 4).
- All path graphs and caterpillar graphs are graceful.
- All lobster graphs with a perfect matching are graceful.
- All trees with at most 27 vertices are graceful; this result was shown by Aldred and McKay in 1998 using a computer program. An extension of this (using a different computational method) up to trees with 35 vertices was claimed in 2010 by the Graceful Tree Verification Project, a distributed computing project led by Wenjie Fang.
- All wheel graphs, web graphs, Helm graphs, gear graphs, and rectangular grids are graceful.
- All n-dimensional hypercubes are graceful.
- 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 the butterfly graph.
- Virginia Vassilevska, "Coding and Graceful Labeling of trees." SURF 2001. PostScript
- Rosa, A. (1967), "On certain valuations of the vertices of a graph", Theory of Graphs (Internat. Sympos., Rome, 1966), New York: Gordon and Breach, pp. 349–355, MR 0223271.
- Huang, C.; Kotzig, A.; Rosa, A. (1982), "Further results on tree labellings", Utilitas Mathematica 21: 31–48, MR 668845.
- Morgan, David (2008), "All lobsters with perfect matchings are graceful", Bulletin of the Institute of Combinatorics and its Applications 53: 82-85.
- Gallian, Joseph A. (1998), "A dynamic survey of graph labeling", Electronic Journal of Combinatorics 5: Dynamic Survey 6, 43 pp. (electronic), MR 1668059.
- Aldred, R. E. L.; McKay, Brendan D. (1998), "Graceful and harmonious labellings of trees", Bulletin of the Institute of Combinatorics and its Applications 23: 69–72, MR 1621760.
- Fang, Wenjie (2010), A Computational Approach to the Graceful Tree Conjecture, arXiv:1003.3045. See also Graceful Tree Verification Project
- Kotzig, Anton (1981), "Decompositions of complete graphs into isomorphic cubes", Journal of Combinatorial Theory. Series B 31 (3): 292–296, doi:10.1016/0095-8956(81)90031-9, MR 638285.
- Weisstein, Eric W., "Graceful graph", MathWorld.
- (K.Eshghi) Introduction to Graceful Graphs, Sharif University of Technology, 2002.
- (U. N. Deshmukh and Vasanti N. Bhat-Nayak), New families of graceful banana trees - Proceedings Mathematical Sciences, 1996 - Springer