The starting point is a discrete graph \(G = \langle N, C\rangle\) consisting of a set \(N\) of nodes, \(n_{i}\), and a set \(C\) of connections, \(c_{ij}\), such that no node is connected to itself and nodes \(n_{i}\) and \(n_{j}\) have one connection at most. What seems most obvious is how to define a distance function and in nearly all proposals, this is indeed the strategy followed (similar to the definitions proposed in section 2.5):