Editing Scale-free network (section)

====Fitness model====
The idea is that the link between two vertices is assigned not randomly with a probability ''p'' equal for all the couple of vertices. Rather, for 
every vertex ''j'' there is an intrinsic ''fitness'' ''x''<sub>''j''</sub> and a link between vertex ''i'' and ''j'' is created with a probability 
<math> p(x_i,x_j)</math>.<ref name=Cal>{{cite journal | last1 = Caldarelli | first1 = G. | display-authors = etal | year = 2002 | title =  Scale-free networks from varying vertex intrinsic fitness| doi = 10.1103/physrevlett.89.258702 | journal = Phys. Rev. Lett. | volume = 89 | issue = 25| page = 258702 | pmid=12484927| bibcode = 2002PhRvL..89y8702C | url = http://eprints.imtlucca.it/1137/1/PhysRevLett_Caldarelli_02.pdf }}</ref> 
In the case of World Trade Web it is possible to reconstruct all the properties by using as fitnesses of the country their GDP, and taking

: <math> p(x_i,x_j)=\frac {\delta x_ix_j}{1+\delta x_ix_j}. </math><ref name=Gar>{{cite journal | last1 = Garlaschelli | first1 = D. | display-authors = etal | year = 2004 | title =  Fitness-Dependent Topological Properties of the World Trade Web| doi =10.1103/physrevlett.93.188701 | journal = Phys. Rev. Lett. | volume = 93 | issue = 18| page = 188701 | pmid = 15525215 | bibcode=2004PhRvL..93r8701G| arxiv = cond-mat/0403051 | s2cid = 16367275 }}</ref>