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第490行： + − BA模型是一个随机网络模型，用于说明偏好依附效应（优先链接）preferential attachment或“富人越富”效应。 在这个模型中，边最有可能附着在度数较高的节点上。 这个网络从一个 m0节点的初始网络开始。 M0≥2，初始网络中每个节点的度至少为1，否则它将始终与网络的其余部分断开。+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +
→Barabási–Albert (BA) preferential attachment model
However, for <math>m=1</math> it describes the winner takes it all mechanism as we find that almost <math>99\%</math> of the total nodes have degree one and one is super-rich in degree. As <math>m</math> value increases the disparity between the super rich and poor decreases and as <math>m>14</math> we find a transition from rich get super richer to rich get richer mechanism.
However, for <math>m=1</math> it describes the winner takes it all mechanism as we find that almost <math>99\%</math> of the total nodes have degree one and one is super-rich in degree. As <math>m</math> value increases the disparity between the super rich and poor decreases and as <math>m>14</math> we find a transition from rich get super richer to rich get richer mechanism.
=== Barabási–Albert (BA) 优先链接模型 ===
[[BA模型]]是一个随机网络模型，用于说明偏好依附效应（优先链接）preferential attachment或“富人越富”效应。 在这个模型中，边最有可能附着在度数较高的节点上。 这个网络从一个 ''m''<sub>0</sub>节点的初始网络开始。 ''m''<sub>0</sub> ≥ 2，初始网络中每个节点的度至少为 1，否则它将始终与网络的其余部分断开。
In the BA model, new nodes are added to the network one at a time. Each new node is connected to <math>m</math> existing nodes with a probability that is proportional to the number of links that the existing nodes already have. Formally, the probability ''p''<sub>''i''</sub> that the new node is connected to node ''i'' is<ref name=RMP>{{Cite journal
|url = http://www.nd.edu/~networks/Publication%20Categories/03%20Journal%20Articles/Physics/StatisticalMechanics_Rev%20of%20Modern%20Physics%2074,%2047%20(2002).pdf
|author1 = R. Albert
|author2 = A.-L. Barabási
|title = Statistical mechanics of complex networks
|journal = [[Reviews of Modern Physics]]
|volume = 74
|issue = 1
|pages = 47–97
|year = 2002
|doi = 10.1103/RevModPhys.74.47
|bibcode = 2002RvMP...74...47A
|arxiv = cond-mat/0106096
|url-status = dead
|archiveurl = https://web.archive.org/web/20150824235818/http://www3.nd.edu/~networks/Publication%20Categories/03%20Journal%20Articles/Physics/StatisticalMechanics_Rev%20of%20Modern%20Physics%2074,%2047%20(2002).pdf
|archivedate = 2015-08-24
|citeseerx = 10.1.1.242.4753
}}</ref>
: <math>p_i = \frac{k_i}{\sum_j k_j},</math>
where ''k''<sub>''i''</sub> is the degree of node ''i''. Heavily linked nodes ("hubs") tend to quickly accumulate even more links, while nodes with only a few links are unlikely to be chosen as the destination for a new link. The new nodes have a "preference" to attach themselves to the already heavily linked nodes.
[[File:Barabasi-albert model degree distribution.svg|thumb|The degree distribution of the BA Model, which follows a power law. In loglog scale the power law function is a straight line.<ref name=Barabasi1999>{{Cite journal
|url = http://www.nd.edu/~networks/Publication%20Categories/03%20Journal%20Articles/Physics/EmergenceRandom_Science%20286,%20509-512%20(1999).pdf
|author = [[Albert-László Barabási]] & [[Réka Albert]]
|title = Emergence of scaling in random networks
|journal = [[Science (journal)|Science]]
|volume = 286
|pages = 509–512
|date = October 1999
|doi = 10.1126/science.286.5439.509
|issue = 5439
|pmid = 10521342
|arxiv = cond-mat/9910332
|bibcode = 1999Sci...286..509B
|url-status = dead
|archiveurl = https://web.archive.org/web/20120417112354/http://www.nd.edu/~networks/Publication%20Categories/03%20Journal%20Articles/Physics/EmergenceRandom_Science%20286,%20509-512%20(1999).pdf
|archivedate = 2012-04-17
}}</ref>]]
The degree distribution resulting from the BA model is scale free, in particular, it is a power law of the form:
: <math>P(k)\sim k^{-3} \, </math>
Hubs exhibit high betweenness centrality which allows short paths to exist between nodes. As a result, the BA model tends to have very short average path lengths. The clustering coefficient of this model also tends to 0.
While the diameter, D, of many models including the Erdős Rényi random graph model and several small world networks is proportional to log N, the BA model exhibits D~loglogN (ultrasmall world).<ref>{{cite journal|last=Cohen|first=R. |title=Scale-free networks are ultrasmall|journal=Phys. Rev. Lett.|year=2003|volume=90|pages=058701|url=http://havlin.biu.ac.il/Publications.php?keyword=Scale-free+networks+are+ultrasmall&year=*&match=all|doi=10.1103/PhysRevLett.90.058701|pmid=12633404|first2=S.|last2=Havlin|issue=5|bibcode=2003PhRvL..90e8701C |arxiv=cond-mat/0205476}}</ref>
Note that the average path length scales with N as the diameter.
====Mediation-driven attachment (MDA) model====
In the [[mediation-driven attachment model|mediation-driven attachment (MDA) model]] in which a new node coming with <math>m</math> edges picks an existing connected node at random and then connects itself not with that one but with <math>m</math> of its neighbors chosen also at random. The probability <math>\Pi(i)</math> that the node <math>i</math> of the existing node picked is
: <math> \Pi(i) = \frac{k_i} N \frac{ \sum_{j=1}^{k_i} \frac 1 {k_j} }{k_i}.</math>
The factor <math>\frac{\sum_{j=1}^{k_i}{\frac{1}{k_j}}}{k_i}</math> is the inverse of the harmonic mean
(IHM) of degrees of the <math>k_i</math> neighbors of a node <math>i</math>. Extensive numerical investigation suggest that for an approximately <math>m> 14</math> the mean IHM value in the large <math>N</math> limit becomes a constant which means <math>\Pi(i) \propto k_i</math>. It implies that the higher the
links (degree) a node has, the higher its chance of gaining more links since they can be
reached in a larger number of ways through mediators which essentially embodies the intuitive
idea of rich get richer mechanism (or the preferential attachment rule of the Barabasi–Albert model). Therefore, the MDA network can be seen to follow
the PA rule but in disguise.<ref>{{cite journal | last1 = Hassan | first1 = M. K. | last2 = Islam | first2 = Liana | last3 = Arefinul Haque | first3 = Syed | date = March 2017 | title = Degree distribution, rank-size distribution, and leadership persistence in mediation-driven attachment networks | doi = 10.1016/j.physa.2016.11.001 | journal = Physica A | volume = 469 | issue = | pages = 23–30 | arxiv = 1411.3444 | bibcode = 2017PhyA..469...23H }}</ref>
However, for <math>m=1</math> it describes the winner takes it all mechanism as we find that almost <math>99\%</math> of the total nodes have degree one and one is super-rich in degree. As <math>m</math> value increases the disparity between the super rich and poor decreases and as <math>m>14</math> we find a transition from rich get super richer to rich get richer mechanism.
=== Fitness model ===
=== Fitness model ===