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Sci Rep. 2016; 6: 27992.

Published online 2016 June 20. doi: 10.1038/srep27992

PMCID: PMC4913272

Received 2016 April 4; Accepted 2016 May 25.

Copyright © 2016, Macmillan Publishers Limited

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Deep connections are known to exist between scale-free networks and non-Gibbsian statistics. For example, typical degree distributions at the thermodynamical limit are of the form , where the *q*-exponential form optimizes the nonadditive entropy *S*_{q} (which, for *q*→1, recovers the Boltzmann-Gibbs entropy). We introduce and study here *d*-dimensional geographically-located networks which grow with preferential attachment involving Euclidean distances through . Revealing the connection with *q*-statistics, we numerically verify (for *d*=1, 2, 3 and 4) that the *q*-exponential degree distributions exhibit, for both *q* and *k*, universal dependences on the ratio *α*_{A}/*d*. Moreover, the *q*=1 limit is rapidly achieved by increasing *α*_{A}/*d* to infinity.

Networks emerge spontaneously in many natural, artificial and social systems. Their study is potentially important for physics, biology, economics, social sciences, among other areas. For example, many empirical studies have identified peculiar properties in very different networks such as the Internet and online social networks (e.g., Facebook), citations networks, neurons networks^{1}^{,2}^{,3}, to quote but a few. An ubiquitous class of such networks is constituted by the scale-free ones (more precisely, asymptotically scale-free). As we shall soon verify, these networks can be seen as a particular application of nonextensive statistical mechanics, based on the nonadditive entropy , where *BG* stands for *Boltzmann-Gibbs*)^{4}^{,5}^{,6}. This current generalization of the BG entropy and corresponding statistical mechanics has been widely successful in clarifying the foundations of thermal statistics as well as for applications in complex systems in high-energy collisions at LHC/CERN (CMS, ALICE, ATLAS and LHCb detectors) and at RHIC/Brookhaven (PHENIX detector)^{7}^{,8}^{,9}^{,10}^{,11}^{,12}^{,13}^{,14}^{,15}^{,16}, cold atoms^{17}, dusty plasmas^{18}, spin-glasses^{19}, trapped ions^{20}, astrophysical plasma^{21}^{,22}, biological systems^{23}, type-II superconductors^{24}, granular matter^{25}, the Kuramoto model at the edge of chaos^{26}, low-dimensional maps, for instance the (area-preserving) standard map^{27} (see Bibliography in http://tsallis.cat.cbpf.br/biblio.htm). Many other physical situations are described which are analogous, such as long-range-interacting Hamiltonians, for example, gravitational problems like globular clusters, spins systems, like the Ising, XY and Heisenberg long-range models. We may also point out random-walk anomalous diffusion where the jumps obey a power low probability distribution function. Some (naturally not all) of the properties of long-range-interacting systems may be described as forming complex network where the sites are linked according to power-law preferential attachment. In the present work we address a wide class of this kind of problems focusing on some basic universality relations.

The deep relationship between scale-free networks and *q*-statistics started being explored in 2005^{28}^{,29}^{,30}, and is presently very active^{31}^{,32}^{,33}^{,34}^{,35}. The basic connection comes (along the lines of the BG canonical ensemble) from the fact that, if we optimize the functional with the constraint or analogous (*k* being the degree of a generic site, i.e., the number of links connected to a given site; *P*(*k*) denotes the degree or connectivity distribution), we straightforwardly obtain , which turns out to be the generic degree distribution for virtually all kinds of scale-free networks. The *q*-exponential function is defined as . We verify that, for *q*>1 and *k*→∞, *P*(*k*)~1/*k*^{γ} with *γ*1/(*q*−1). The classical result *γ*=3^{36} corresponds to *q*=4/3.

In the present work we address the question of how universal such results might be, and more specifically, how *P*(*k*) varies with the dimension *d* of the system?

Our growing model starts with one site at the origin. We then stochastically locate a second site (and then a third, a fourth, and so on up to *N*) through the *d*-dimensional isotropic distribution

where *r* ≥ 1 is the Euclidean distance from the newly arrived site to the center of mass of the pre-existing system (in one dimension, *r*=|*x*|; in two dimensions, ; in three dimensions , and so on); we assume angular isotropy; *p*(*r*) is zero for 0≤*r*<1; the subindex *G* stands for *growth*. We consider *α*_{G}>0 so that the distribution *P*(*r*) is normalizable; indeed, , which is finite for *α*_{G}>0, and diverges otherwise. See Fig. 1.

Every new site which arrives is then attached to one and only one site of the pre-existing cluster. The choice of the site to be linked with is done through the following preferential attachment probability:

where *k*_{i} is the connectivity of the *i*-th pre-existing site (i.e., the number of sites that are already attached to site *i*), and *r*_{ij} is the Euclidean distance from site *i* to the newly arrived site *j*; subindex *A* stands for *attachment*.

For *α*_{A} approaching zero and arbitrary *d*, the physical distances gradually loose relevance and, at the limit *α*_{A}=0, all distances becomes irrelevant in what concerns the connectivity distribution, and we therefore recover the Barabási-Albert (BA) model^{36}, which has topology but no metrics. The BA model was extended^{37} in such a way that it would be able to yield an exponent *γ* such that 2<*γ*<3, thus making the model more realistic. In this work they showed a topological phase transition which range from scale-free networks to exponential networks through three control parameters (addition of links, redirection of edges, and addition of new sites). In the present paper we show that, for arbitrary dimensionality, *γ* can be controlled in a kind of simpler manner, namely by metric changes through only one control parameter (namely the ratio *α*_{A}/*d*) in the structure of the network. Notice, however, that the BA generalized model is not a particular case of our model, and neither the other way around.

Large-scale simulations have been performed for the (*d*=1, 2, 3, 4) models for fixed (*α*_{G}, *α*_{A}), and we have verified in all cases that the degree distribution *P*(*k*) is completely independent from *α*_{G}: see Fig. 2. Using this fact, we have arbitrarily fixed *α*_{G}=2, and have numerically studied the influence of (*d*, *α*_{A}) on *P*(*k*): see Figs 3 and and4.4. In all cases, the *q*-exponential fittings with *q*>1 and *κ*>0 have been remarkably good. To test the goodness of fit, we performed Kolmogorov-Smirnov test^{38} (see Table 1). To deal with the problem that the data are very sparse in the tail, we excluded data points with sample probability less than 10^{−6}. The best fitting values for (*q*, *κ*) are indicated in Fig. 5. From normalization of *P*(*k*), *P*(0) can be expressed as a straightforward function of (*q*, *κ*).

Our most remarkable results are presented in Fig. 6, namely the fact that both the index *q* and the characteristic degree (or “effective temperature”) *κ* do not depend from (*α*_{A}, *d*) in an independent manner but *only from the ratio α*_{A}/*d*. This nontrivial fact puts the growing *d*-dimensional geographically located models that have been introduced here for scale-free networks, on similar footing as long-range-interacting many-body classical Hamiltonian systems such as the inertial XY planar rotators^{39}^{,40}^{,41}^{,42} (possibly the generic inertial *n*-vector rotators as well^{43}^{,44}) and Fermi-Pasta-Ulam oscillators, assuming that the strength of the two-body interaction decreases with distance as 1/(*distance*)^{α}. Moreover, as first pointed out generically by Gibbs himself^{45}, we have the facts that the BG canonical partition function of these classical systems anomalously diverges with size for 0≤*α*/*d*≤1 (long-range interactions, e.g., gravitational and dipole-monopole interactions) and converges for *α*/*d*>1 (short-range interactions, e.g., Lennard-Jones interaction), and the internal energy per particle is, in the thermodynamical limit, constant for short-range interactions whereas it diverges like *N*^{1−α/d} for long-range interactions, *N* being the total number of particles.

If all these meaningful scalings are put together, we obtain a highly plausible scenario for the respective domains of validity of the Boltzmann-Gibbs (additive) entropy and associated statistical mechanics, and that of the nonadditive entropies *S*_{q} (with *q*≠1) and associated statistical mechanics.

Finally, we notice in Fig. 6 that both *q* and *κ* approach quickly their BG limits (*q*=1) for *α*_{A}/*d*→∞. Moreover, the same exponential *e*^{1−α/d} appears in both heuristic expressions for *q* and *κ*. Consequently, the following *linear* relation can be straightforwardly established:

In fact, this simple relation is numerically quite well satisfied as can be seen in Fig. 7. Its existence reveals an interesting peculiarity of the nature of *q*-statistics. If in the celebrated BG factor *e*^{−energy/kT}, corresponding to *q*=1, we are free to consider an arbitrary value for *T*, how come in the present problem, *κ* is not a free parameter but has instead a fixed value for each specific model that we are focusing on? This is precisely what occurs in the high-energy applications of *q*-statistics, e.g., in quark-gluon soup^{46} where *q*=1.114 and *T*=135.2*Mev*, as well as in all the LHC/CERN and RHIC/Brookhaven experiments^{7}. Another example which is reminiscent of this type of behavior is the sensitivity to the initial conditions at the edge of chaos (Feigenbaum point) of the logistic map; indeed, the inverse *q*-generalized Lyapunov exponent satisfies the linear relation 1/*λ*_{q}=1−*q*^{47}^{,48}. The cause of this interesting and ubiquitous feature comes from the fact that *q*-statistics typically emerges at critical-like regimes and is deeply related to an hierarchical occupation of phase space (or Hilbert space or Fock space), which in turn points towards asymptotic power-laws (see also^{49}). In other words, *κ* plays a role analogous to a critical temperature, which is of course not a free parameter but is instead fixed by the specific model.

**How to cite this article**: Brito, S. *et al*. Role of dimensionality in complex networks. *Sci. Rep.*
**6**, 27992; doi: 10.1038/srep27992 (2016).

We have benefitted from fruitful discussions with D. Bagchi, E.M.F. Curado, F.D. Nobre, P. Rapcan and G. Sicuro. We also appreciate the suggestions of an anonymous referee which helped us to improve this work. We gratefully acknowledge partial financial support from CNPq and Faperj (Brazilian agencies) and from the John Templeton Foundation-USA.

**Author Contributions** C.T. conceived the research, analyzed the results, contributed to the manuscript text and revised it. S.B. developed and carried out the numerics, prepared figures, analyzed the results, contributed to the manuscript text and revised it. L.R.d.S. offered constructive suggestions, analyzed the results, contributed to the manuscript text and revised it.

- Strogatz S. H. Exploring complex networks. Nature 410(6825), 268 (2001). [PubMed]
- Newman M. E. J. The structure and function of complex networks. SIAM Review 45(2), 167 (2003).
- Costa L. D. F. et al. . Analyzing and modeling real-world phenomena with complex networks: a survey of applications. Advances in Physics 60(3), 329–412 (2011).
- Tsallis C. Possible generalization of Boltzmann-Gibbs statistics. J. Stat. Phys. 52, 479–487 (1988).
- Gell-Mann M. & Tsallis C. Nonextensive entropy - Interdisciplinary applications (ed. Oxford University Press) (New York, 2004).
- Tsallis C. Introduction to nonextensive statistical mechanics - Approaching a complex world. (ed. Springer) (New York, 2009).
- Khachatryan V.
et al. . Transverse-Momentum and Pseudorapidity Distributions of Charged Hadrons in
*pp*Collisions at . Phys. Rev. Lett. 105, 022002 (2010). [PubMed] - Chatrchyan S. et al. . Search for new physics with jets and missing transverse momentum in pp collisions at . J. High Energy Phys. 8, 1–46 (2011).
- Aamodt K. et al. . Transverse momentum spectra of charged particles in proton-proton collisions at with ALICE at the LHC. Phys. Lett. B 693(2), 53–68 (2010).
- Abelev B. et al. . Measurement of electrons from semileptonic heavy-flavor hadron decays in p p collisions at s=7 TeV. Phys. Rev. D 86(11), 112007 (2012).
- Aad G. et al. . Charged-particle multiplicities in pp interactions measured with the ATLAS detector at the LHC. New J. Phys. 13, 053033 (2011).
- Adare A. et al. . Measurement of neutral mesons in p+ p collisions at s=200 GeV and scaling properties of hadron production. Phys. Rev. D 83(5), 052004 (2011).
- Adare A.
et al. . Production of
*ω*mesons in p+ p, d+ Au, Cu+ Cu, and Au+ Au collisions at s N N=200 GeV. Phys. Rev. C 84(4), 044902 (2011). - Wong C. Y. & Wilk G. Tsallis fits to p T spectra and multiple hard scattering in p p collisions at the LHC. Phys. Rev. D 87(11), 114007 (2013).
- Marques L., Andrade-II E. & Deppman A. Nonextensivity of hadronic systems. Phys. Rev. D 87(11), 114022 (2013).
- Aaij R.
et al. . Study of the production of ∧ b 0 and B 0 hadrons in pp collisions and first measurement of the ∧ b 0→
*J*/*ψp*K-branching fraction. Chinese Physics C 40(1), 11001–011001 (2016). - Douglas P., Bergamini S. & Renzoni F. Tunable Tsallis distributions in dissipative optical lattices. Phys. Rev. Lett. 96(11), 110601 (2006). [PubMed]
- Liu B. & Goree J. Superdiffusion and non-Gaussian statistics in a driven-dissipative 2D dusty plasma. Phys. Rev. Lett. 100(5), 055003 (2008). [PubMed]
- Pickup R. M., Cywinski R., Pappas C., Farago B. & Fouquet P. Generalized spin-glass relaxation. Phys. Rev. Lett 102(9), 097202 (2009). [PubMed]
- DeVoe R. G. Power-law distributions for a trapped ion interacting with a classical buffer gas. Phys. Rev. Lett. 102(6), 063001 (2009). [PubMed]
- Burlaga L. F., Vinas A. F., Ness N. F. & Acuna M. H. Tsallis statistics of the magnetic field in the heliosheath. Astrophys J. 644(1), L83 (2006).
- Livadiotis G. & McComas D. J. Invariant kappa distribution in space plasmas out of equilibrium. Astrophys. J. 741(2), 88 (2011).
- Upadhyaya A., Rieu J. -P., Glazier J. A. & Sawada Y. Anomalous diffusion and non-Gaussian velocity distribution of Hydra cells in cellular aggregates. Physica A 293(3), 549–558 (2001).
- Andrade J. S., da Silva G. F. T. Jr., Moreira A. A., Nobre F. D. & Curado E. M. F. Thermostatistics of overdamped motion of interacting particles. Phys. Rev. Lett 105(26), 260601 (2010). [PubMed]
- Combe G., Richefeu V., Stasiak M. & Atman A. P. F. Experimental Validation of a Nonextensive Scaling Law in Confined Granular Media. Phys. Rev. Lett. 115(23), 238301 (2015). [PubMed]
- Miritello G., Pluchino A. & Rapisarda A. Central limit behavior in the Kuramoto model at the “edge of chao”. Physica A 388(23), 4818–4826 (2009).
- Tirnakli U. & Borges E. P. The standard map: from Boltzmann-Gibbs statistics to Tsallis statistics. Nature/Scientific Reports 6, 23644 (2016). [PMC free article] [PubMed]
- Soares D. J. B., Tsallis C., Mariz A. M. & da Silva L. R. Preferential attachment growth model and nonextensive statistical mechanics. EPL 70(1), 70 (2005).
- Thurner S. & Tsallis C. Nonextensive aspects of self-organized scale-free gas-like networks. EPL 72(2), 197 (2005).
- Thurner S. Nonextensive statistical mechanics and complex scale-free networks. Europhysics News 36(6), 218–220 (2005).
- Andrade J. S., Herrmann H. J. Jr., Andrade R. F. & da Silva L. R. Apollonian networks: Simultaneously scale-free, small world, Euclidean, space filling, and with matching graphs. Phys. Rev. Lett. 94(1), 018702 (2005). [PubMed]
- Lind P. G., da Silva L. R., Andrade J. S. Jr. & Herrmann H. J. Spreading gossip in social networks. Phys. Rev. E 76(3), 036117 (2007). [PubMed]
- Mendes G. A., da Silva L. R. & Herrmann H. J. Traffic gridlock on complex networks. Physica A 391(1), 362–370 (2012).
- Almeida M. L., Mendes G. A., Viswanathan G. M. & da Silva L. R. Scale-free homophilic network. European Phys. J. B 86(2), 1–6 (2013).
- Macedo-Filho A., Moreira D. A., Silva R. & da Silva L. R. Maximum entropy principle for Kaniadakis statistics and networks. Phys. Lett. A 377(12), 842–846 (2013).
- Barabási A. L. & Albert R. Emergence of scaling in random networks. Science 286(5439), 509–512 (1999). [PubMed]
- Barabási A. L. & Albert R. Topology of evolving networks: local events and universality. Phys. Rev. Lett. 85(24), 5234 (2000). [PubMed]
- Bickel P. J. & Doksum K. A. Mathematical Statistics: Basic Ideas and Selected Topics, volume I (1977).
- Antoni C. M. & Ruffo S. Clustering and relaxation in Hamiltonian long-range dynamics. Phys. Rev. E 52(3), 2361 (1995). [PubMed]
- Anteneodo C. & Tsallis C. Breakdown of exponential sensitivity to initial conditions: Role of the range of interactions. Phys. Rev. Lett. 80(24), 5313 (1998).
- Campa A., Giansanti A., Moroni D. & Tsallis C. Classical spin systems with long-range interactions: universal reduction of mixing. Phys. Lett. A 286(4), 251–256 (2001).
- Cirto L. J. L., Assis V. R. V. & Tsallis C. Influence of the interaction range on the thermostatistics of a classical many-body system. Physica A 393, 286–296 (2014).
- Nobre F. D. & Tsallis C. Classical infinite-range-interaction Heisenberg ferromagnetic model: Metastability and sensitivity to initial conditions. Phys. Rev. E 68(3), 036115 (2003). [PubMed]
- Cirto L. J. L., Lima L. S. & Nobre F. D. Controlling the range of interactions in the classical inertial ferromagnetic Heisenberg model: analysis of metastable states. J. Stat. Mech. P04012 (2015).
- Gibbs J. W. Elementary Principles in Statistical Mechanics – Developed with Especial Reference to the Rational Foundation of Thermodynamics (C. Scribner’s Sons, New York, 1902; Yale University Press, New Haven, 1948); OX Bow Press, Woodbridge, Connecticut, 1981).
- Walton D. B. & Rafelski J. Equilibrium distribution of heavy quarks in Fokker-Planck dynamics. Phys. Rev. Lett. 84(1), 31 (2000). [PubMed]
- Lyra M. L. & Tsallis C. Nonextensivity and multifractality in low-dimensional dissipative systems. Phys. Rev. Lett. 80(1), 53 (1998).
- Baldovin F. & Robledo A. Nonextensive Pesin identity: Exact renormalization group analytical results for the dynamics at the edge of chaos of the logistic map. Phys. Rev. E 69(4), 045202(R) (2004). [PubMed]
- Plastino A., Curado E. M. F. & Nobre F. D. Deriving partition functions and entropic functionals from thermodynamics. Physica A 403, 13–20 (2014).
- Emmerich T., Bunde A. & Havlin S. Structural and functional properties of spatially embedded scale-free networks. Phys. Rev. E 89(6), 062806 (2014). [PubMed]

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