It's an interconnected Christmas scene, but that's not a Christmas Tree! (?) COURTESY: Andrew P. Wheeler.
An example of a Christmas Tree directed graph as shown in . The top two graphs are slim trees of order 3 (left) and 4 (right). A Christmas tree (bottom) includes selected long-range connections (longer than the immediate connection to mother, daughter, or sister nodes).
This tree could have used a bit more fault-tolerance!
 Hsu, L-H and Lin, C-K (2008). Graph Theory and Interconnection Networks. CRC Press, New York.
 Hung, C-N, Hsu, L-H, and Sung, T-Y (1999). Christmas tree: A versatile 1-fault-tolerant design for token rings. Information Processing Letters, 72(1–2), 55-63.
 Wang, J-J, Hung, C-N, Tan, J.J-N, Hsu, L-H, and Sung, T-Y (2000). Construction schemes for fault-tolerant Hamiltonian graphs. Networks, 35(3), 233-245.