The treewidth is a measure of the count of original graph vertices mapped onto any tree vertex in an optimal tree decomposition. Determining the treewidth of an arbitrary graph is an NP-hard problem. However, many NP-hard problems on graphs of bounded treewidth can be solved in polynomial time. An empty graph has treewidth 0, a tree or forest has treewidth 1, and graphs with treewidth at most 2 correspond to series-parallel graphs. Every Halin graph has a treewidth of 3. The treewidth of a disconnected graph is equal to the maximum of the treewidths of its connected components.

graph bandwidth | halin graph | pathwidth | tree | tree decomposition