 Vertex (graph theory)

For other uses, see Vertex (disambiguation).
In graph theory, a vertex (plural vertices) or node is the fundamental unit out of which graphs are formed: an undirected graph consists of a set of vertices and a set of edges (unordered pairs of vertices), while a directed graph consists of a set of vertices and a set of arcs (ordered pairs of vertices). From the point of view of graph theory, vertices are treated as featureless and indivisible objects, although they may have additional structure depending on the application from which the graph arises; for instance, a semantic network is a graph in which the vertices represent concepts or classes of objects.
The two vertices forming an edge are said to be the endpoints of this, and the edge is said to be incident to the vertices. A vertex w is said to be adjacent to another vertex v if the graph contains an edge (v,w). The neighborhood of a vertex v is an induced subgraph of the graph, formed by all vertices adjacent to v.
The degree of a vertex in a graph is the number of edges incident to it. An isolated vertex is a vertex with degree zero; that is, a vertex that is not an endpoint of any edge. A leaf vertex (also pendant vertex) is a vertex with degree one. In a directed graph, one can distinguish the outdegree (number of outgoing edges) from the indegree (number of incoming edges); a source vertex is a vertex with indegree zero, while a sink vertex is a vertex with outdegree zero.
A cut vertex is a vertex the removal of which would disconnect the remaining graph; a vertex separator is a collection of vertices the removal of which would disconnect the remaining graph into small pieces. A kvertexconnected graph is a graph in which removing fewer than k vertices always leaves the remaining graph connected. An independent set is a set of vertices no two of which are adjacent, and a vertex cover is a set of vertices that includes the endpoint of each edge in the graph. The vertex space of a graph is a vector space having a set of basis vectors corresponding with the graph's vertices.
A graph is vertextransitive if it has symmetries that map any vertex to any other vertex. In the context of graph enumeration and graph isomorphism it is important to distinguish between labeled vertices and unlabeled vertices. A labeled vertex is a vertex that is associated with extra information that enables it to be distinguished from other labeled vertices; two graphs can be considered isomorphic only if the correspondence between their vertices pairs up vertices with equal labels. An unlabeled vertex is one that can be substituted for any other vertex based only on its adjacencies in the graph and not based on any additional information.
Vertices in graphs are analogous to, but not the same as, vertices of polyhedra: the skeleton of a polyhedron forms a graph, the vertices of which are the vertices of the polyhedron, but polyhedron vertices have additional structure (their geometric location) that is not assumed to be present in graph theory. The vertex figure of a vertex in a polyhedron is analogous to the neighborhood of a vertex in a graph.
In a directed graph, the forward star of a vertex u is defined as its outgoing edges. In a Graph G with the set of vertices V and the set of edges E, the forward star of u can be described as
 ^{[1]}
Notes
 ^ (Gallo & Pallotino 1988, p. 4)
References
 Gallo, Giorgio; Pallotino, Stefano (1988). "Shortest Path Algorithms". Annals of Operations Research 13 (1): 1–79. doi:10.1007/BF02288320.
 Berge, Claude, Théorie des graphes et ses applications. Collection Universitaire de Mathématiques, II Dunod, Paris 1958, viii+277 pp. (English edition, Wiley 1961; Methuen & Co, New York 1962; Russian, Moscow 1961; Spanish, Mexico 1962; Roumanian, Bucharest 1969; Chinese, Shanghai 1963; Second printing of the 1962 first English edition. Dover, New York 2001)
 Chartrand, Gary (1985). Introductory graph theory. New York: Dover. ISBN 0486247759.
 Biggs, Norman; Lloyd, E. H.; Wilson, Robin J. (1986). Graph theory, 17361936. Oxford [Oxfordshire]: Clarendon Press. ISBN 0198539169.
 Harary, Frank (1969). Graph theory. Reading, Mass.: AddisonWesley Publishing. ISBN 0201410338.
 Harary, Frank; Palmer, Edgar M. (1973). Graphical enumeration. New York, Academic Press. ISBN 0123242452.
External links
 Weisstein, Eric W., "Graph Vertex" from MathWorld.
Categories: Graph theory objects
Wikimedia Foundation. 2010.
Look at other dictionaries:
Graph theory — In mathematics and computer science, graph theory is the study of graphs : mathematical structures used to model pairwise relations between objects from a certain collection. A graph in this context refers to a collection of vertices or nodes and … Wikipedia
graph theory — Math. the branch of mathematics dealing with linear graphs. [1965 70] * * * Mathematical theory of networks. A graph consists of nodes (also called points or vertices) and edges (lines) connecting certain pairs of nodes. An edge that connects a… … Universalium
Degeneracy (graph theory) — In graph theory, a k degenerate graph is an undirected graph in which every subgraph has a vertex of degree at most k: that is, some vertex in the subgraph touches k or fewer of the subgraph s edges. The degeneracy of a graph is the smallest… … Wikipedia
Connectivity (graph theory) — In mathematics and computer science, connectivity is one of the basic concepts of graph theory: it asks for the minimum number of elements (nodes or edges) which need to be removed to disconnect the remaining nodes from each other[1]. It is… … Wikipedia
Clique (graph theory) — A graph with 23 1 vertex cliques (its vertices), 42 2 vertex cliques (its edges), 19 3 vertex cliques (the light blue triangles), and 2 4 vertex cliques (dark blue). Six of the edges and 11 of the triangles form maximal cliques. The two dark blue … Wikipedia
Tree (graph theory) — Trees A labeled tree with 6 vertices and 5 edges Vertices v Edges v 1 Chromatic number … Wikipedia
Path (graph theory) — In graph theory, a path in a graph is a sequence of vertices such that from each of its vertices there is an edge to the next vertex in the sequence. The first vertex is called the start vertex and the last vertex is called the end vertex . Both… … Wikipedia
Degree (graph theory) — A graph with vertices labeled by degree In graph theory, the degree (or valency) of a vertex of a graph is the number of edges incident to the vertex, with loops counted twice.[1] The degree of a vertex … Wikipedia
Neighbourhood (graph theory) — A graph consisting of 6 vertices and 7 edges For other meanings of neighbourhoods in mathematics, see Neighbourhood (mathematics). For non mathematical neighbourhoods, see Neighbourhood (disambiguation). In graph theory, an adjacent vertex of a… … Wikipedia
Matching (graph theory) — In the mathematical discipline of graph theory, a matching or independent edge set in a graph is a set of edges without common vertices. It may also be an entire graph consisting of edges without common vertices. Covering packing dualities… … Wikipedia