Graph labeling
In the mathematical discipline of graph theory, a graph labelling is the assignment of labels, traditionally represented by integers, to edges and/or vertices of a graph. Formally, given a graph , a vertex labelling is a function of  to a set of labels; a graph with such a function defined is called a vertex-labeled graph. Likewise, an edge labelling is a function of  to a set of labels. In this case, the graph is called an edge-labeled graph. When the edge labels are members of an ordered set (e.g., the real numbers), it may be called a weighted graph. When used without qualification, the term labeled graph generally refers to a vertex-labeled graph with all labels distinct. Such a graph may equivalently be labeled by the consecutive integers , where  is the number of vertices in the graph. For many applications, the edges or vertices are given labels that are meaningful in the associated domain. For example, the edges may be assigned weights representing the "cost" of traversing between the incident vertices. In the above definition a graph is understood to be a finite undirected simple graph. However, the notion of labelling may be applied to all extensions and generalizations of graphs. For example, in automata theory and formal language theory it is convenient to consider labeled multigraphs, i.e., a pair of vertices may be connected by several labeled edges. History
Most graph labellings trace their origins to labellings presented by Alexander Rosa in his 1967 paper. Rosa identified three types of labellings, which he called -, -, and -labellings. -labellings were later renamed as "graceful" by Solomon Golomb, and the name has been popular since. Special cases

Graceful labelling

A graph is known as graceful when its vertices are labeled from 0 to , the size of the graph, and this labelling induces an edge labelling from 1 to . For any edge , the label of  is the positive difference between the two vertices incident with . In other words, if  is incident with vertices labeled  and ,  will be labeled . Thus, a graph  is graceful if and only if there exists an injection that induces a bijection from  to the positive integers up to . In his original paper, Rosa proved that all Eulerian graphs with size equivalent to 1 or 2 (mod 4) are not graceful. Whether or not certain families of graphs are graceful is an area of graph theory under extensive study. Arguably, the largest unproven conjecture in graph labelling is the Ringel–Kotzig conjecture, which hypothesizes that all trees are graceful. This has been proven for all paths, caterpillars, and many other infinite families of trees. Anton Kotzig himself has called the effort to prove the conjecture a "disease". Edge-graceful labelling

An edge-graceful labelling on a simple graph without loops or multiple edges on  vertices and  edges is a labelling of the edges by distinct integers in  such that the labelling on the vertices induced by labelling a vertex with the sum of the incident edges taken modulo  assigns all values from 0 to  to the vertices. A graph  is said to be "edge-graceful" if it admits an edge-graceful labelling. Edge-graceful labellings were first introduced by Sheng-Ping Lo in 1985. A necessary condition for a graph to be edge-graceful is "Lo's condition":

Harmonious labelling
A "harmonious labelling" on a graph  is an injection from the vertices of  to the group of integers modulo , where  is the number of edges of , that induces a bijection between the edges of  and the numbers modulo  by taking the edge label for an edge  to be the sum of the labels of the two vertices . A "harmonious graph" is one that has a harmonious labelling. Odd cycles are harmonious, as are Petersen graphs. It is conjectured that trees are all harmonious if one vertex label is allowed to be reused. The seven-page book graph  provides an example of a graph that is not harmonious. Graph colouring

A graph colouring is a subclass of graph labellings. Vertex colourings assign different labels to adjacent vertices, while edge colourings assign different labels to adjacent edges. Lucky labelling
A lucky labelling of a graph  is an assignment of positive integers to the vertices of  such that if  denotes the sum of the labels on the neighbours of , then  is a vertex coloring of . The "lucky number" of  is the least  such that  has a lucky labelling with the integers

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Extensions and generalizations of graphs