O ct 2 01 2 Criticality in conserved dynamical systems : Experimental observation vs . exact properties

Conserved dynamical systems are generally considered to be critical. We study a class of critical routing models, equivalent to random maps, which can be solved rigorously in the thermodynamic limit. The information flow is conserved for these routing models and governed by cyclic attractors. We consider two classes of information flow, Markovian routing without memory and vertex routing involving a one-step routing memory. Investigating the respective cycle length distributions for complete graphs we find log corrections to power-law scaling for the mean cycle length, as a function of the number of vertices, and a sub-polynomial growth for the overall number of cycles. When observing experimentally a real-world dynamical system one normally samples stochastically its phase space. The number and the length of the attractors are then weighted by the size of their respective basins of attraction. This situation is equivalent to 'on the fly' generation of routing tables for which we find power law scaling for the weighted average length of attractors, for both conserved routing models. These results show that critical dynamical systems are generically not scale-invariant, but may show power-law scaling when sampled stochastically. It is hence important to distinguish between intrinsic properties of a critical dynamical system and its behavior that one would observe when randomly probing its phase space. 2 Power law scaling is observed in many real-world systems, like the distribution of neural avalanches in the brain. In statistical physics all critical systems, at the point of a second-order phase transition, show power law scaling. Power law scaling is hence commonly attributed to criticality, but it is an open question to which extend this relation is satisfied for complex dynamical systems. There is, in addition, a difference between the distribution an observer may be able to sample and the exact properties of the underlying dynamical system. An observer will sample in general the number and the size of attractors as weighted by size of their respective basins of attraction. Here we investigate critical models for information routing and show that the number and the length of attractors does not obey power law scaling, while, on the other hand, an external observer, sampling the weighted distribution, would find power law scaling. We hence conclude that drawing conclusions from experimentally observed power law scaling needs to take into account the implicitly employed sampling procedures.