Editing Simulated annealing (section)

===Sufficiently near neighbour===
Simulated annealing may be modeled as a random walk on a search graph, whose vertices are all possible states, and whose edges are the candidate moves. An essential requirement for the {{code|neighbour()}} function is that it must provide a sufficiently short path on this graph from the initial state to any state which may be the global optimum{{snd}} the diameter of the search graph must be small. In the traveling salesman example above, for instance, the search space for n = 20 cities has n! = 2,432,902,008,176,640,000 (2.4 quintillion) states; yet the number of neighbors of each vertex is <math>\sum_{k=1}^{n-1} k=\frac{n(n-1)}{2}=190</math> edges (coming from <math>n \choose 2</math>), and the diameter of the graph is <math>n-1</math>.