Volume 78, Number 2, April 2007
Article Number 28004
Number of page(s) 5
Section Interdisciplinary Physics and Related Areas of Science and Technology
Published online 03 April 2007
EPL, 78 (2007) 28004
DOI: 10.1209/0295-5075/78/28004

Scale-freeness for networks as a degenerate ground state: A Hamiltonian formulation

P. Minnhagen1, 2, S. Bernhardsson1, 2 and Beom Jun Kim3

1  Department of Physics, Umeå University - 90187 Umeå, Sweden
2  Center for Models for Life - 2100 Copenhagen, Denmark
3  Department of Physics, BK21 Physics Research Division, and Institute of Basic Science, Sungkyunkwan University - Suwon 440-746, Korea

received 7 December 2006; accepted in final form 7 March 2007; published April 2007
published online 3 April 2007

The origin of scale-free degree distributions in the context of networks is addressed through an analogous non-network model in which the node degree corresponds to the number of balls in a box and the rewiring of links to balls moving between the boxes. A statistical mechanical formulation is presented and the corresponding Hamiltonian is derived. The energy, the entropy, as well as the degree distribution and its fluctuations are investigated at various temperatures. The scale-free distribution is shown to correspond to the degenerate ground state, which has small fluctuations in the degree distribution and yet a large entropy. We suggest an implication of our results from the viewpoint of the stability in evolution of networks.

89.75.-k - Complex systems.
89.75.Fb - Structures and organization in complex systems.
89.75.Hc - Networks and genealogical trees.

© Europhysics Letters Association 2007