Editing Spanning tree (section)

==In directed multigraphs==
The idea of a spanning tree can be generalized to directed multigraphs.<ref name="Levine09">{{citation
 | title = Sandpile groups and spanning trees of directed line graphs
 | journal = Journal of Combinatorial Theory, Series A
 | volume = 118
 | number = 2
 | pages = 350–364
 | year = 2011
 | issn = 0097-3165
 | doi = 10.1016/j.jcta.2010.04.001 |doi-access=free
 | first = Lionel | last = Levine
| arxiv = 0906.2809
 }}</ref> Given a vertex ''v'' on a directed multigraph ''G'', an ''oriented spanning tree'' ''T'' rooted at ''v'' is an acyclic subgraph of ''G'' in which every vertex other than ''v'' has outdegree 1. This definition is only satisfied when the "branches" of ''T'' point towards ''v''.