Per Bak and Kim Sneppen Model

from Wikipedia:
``We consider N species, which are associated with a fitness factor f(i). They are indexed by integers i around a ring. The algorithm consists in choosing the least fit species, and then replacing it and its two closest neighbors (previous and next integer) by new species, with a new random fitness. After a long run there will be a minimum required fitness, below which species don't survive. These "long-run" events are referred to as avalanches, and the model proceeds through these avalanches until it reaches a state of relative stability where all species' fitness are above a certain threshold.''



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