The singleton graph is the graph consisting of a single isolated node with no edges. It is therefore the empty
graph on one node. It is commonly denoted 
(i.e., the complete graph
on one node).
The Singleton graph is implemented in the Wolfram Language as GraphData["SingletonGraph"].
By convention, the singleton graph is considered to be Hamiltonian (B. McKay, pers. comm., Mar. 22, 2007).
