1. J.M. Smith, E. Szäthmáry, The Major Transitions in Evolution (Oxford University Press, Oxford, 1997)
2. M.A. Montemurro, D.H. Zanette, arXiv:1503.01129v1 (2015)
3. L. Lü, Z.K. Zhang, T. Zhou, Sci. Rep. 3, 1082 (2013)
4. G. Zipf, Human Behavior and the Principle of Least Effort: An introduction to Human Ecology (Addison-Wesley Press, Cambridge, 1949)
5. H.S. Heaps, Information Retrieval: Computational and Theoretical Aspects (Academic Press, New York, 1978)