Distance (graph theory)
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In the mathematical field of graph theory, the distance between two vertices in a graph is the number of edges in a shortest path connecting them. This is also known as the geodesic distance [1] because it is the length of the graph geodesic between those two vertices.[2] If there is no path connecting the two vertices, i.e., if they belong to different connected components, then conventionally the distance is defined as infinite.
The vertex set and the distance function form a metric space, if and only if the graph is connected.
There are a number of other measurements defined in terms of distance:
The eccentricity ε of a vertex v is the greatest distance between v and any other vertex.
The radius of a graph is the minimum eccentricity of any vertex.
The diameter of a graph is the maximum eccentricity of any vertex in the graph. That is, it is the greatest distance between any two vertices. A peripheral vertex in a graph of diameter d is one that is distance d from some other vertex—that is, a vertex that achieves the diameter.
A pseudo-peripheral vertex v has the property that for any vertex u, if v is as far away from u as possible, then u is as far away from v as possible. Formally, if the distance from u to v equals the eccentricity of u, then it equals the eccentricity of v.
[edit] Algorithm for finding pseudo-peripheral vertices
Often peripheral sparse matrix algorithms need a starting vertex with a high eccentricity. A peripheral vertex would be perfect, but is often hard to calculate. In most circumstances a pseudo-peripheral vertex can be used. A pseudo-peripheral vertex can easily be found with the following algorithm:
- Choose a vertex u.
- Among all the vertices that are as far from u as possible, let v be one with minimal degree.
- If ε(v) > ε(u) then set u=v and repeat with step 2, else v is a pseudo-peripheral vertex.
[edit] Notes
- ^ Bouttier, Jérémie; Di Francesco,P. ,Guitter, E. (July 2003). "Geodesic distance in planar graphs". Nuclear Physics B 663 (3): 535-567. “By distance we mean here geodesic distance along the graph, namely the length of any shortest path between say two given faces”
- ^ Weisstein, Eric W.. "Graph Geodesic" (HTML). MathWorld--A Wolfram Web Resource. Wolfram Research. Retrieved on 2008-04-23. “The length of the graph geodesic between these points d(u,v) is called the graph distance between u and v”
