Tutte's disjoint tree theorem
From Egres Open
Theorem (Tutte)[1]. An undirected graph G=(V,E) contains k edge-disjoint spanning trees if and only if if it is k-partition-connected.
This can be regarded as a special case of the Matroid union theorem. Sometimes it is referred to as the Tutte-Nash-Williams theorem because it also appears in the paper of Nash-Williams [2].