Path graph

In the mathematical field of graph theory, a path graph or linear graph is a graph whose vertices can be listed in the order v₁, v₂, …, v_n such that the edges are {v_i, v_i+1} where i = 1, 2, …, n − 1. Equivalently, a path with at least two vertices is connected and has two terminal vertices (vertices that have degree 1), while all others (if any) have degree 2.

Path graph
A path graph on 6 vertices
Vertices	n
Edges	n − 1
Radius	⌊n / 2⌋
Diameter	n − 1
Automorphisms	2
Chromatic number	2
Chromatic index	2
Spectrum	{2 cos(k π / (n + 1)); k = 1, ..., n}
Properties	Unit distance Bipartite graph Tree
Notation	${\displaystyle P_{n}}$
Table of graphs and parameters

Paths are often important in their role as subgraphs of other graphs, in which case they are called paths in that graph. A path is a particularly simple example of a tree, and in fact the paths are exactly the trees in which no vertex has degree 3 or more. A disjoint union of paths is called a linear forest.

Paths are fundamental concepts of graph theory, described in the introductory sections of most graph theory texts. See, for example, Bondy and Murty (1976), Gibbons (1985), or Diestel (2005).