of the system: variable parameters in the model could be changed widely without affecting the emergence of critical behavior: hence, self-organized criticality. Thus, the key result of BTW's paper was its discovery of a mechanism by which the emergence of complexity from simple local interactions could be spontaneous—and therefore plausible as a source of natural complexity—rather than something that was only possible in artificial situations in which control parameters are tuned to precise critical values. An alternative view is that SOC appears when the criticality is linked to a value of zero of the control parameters. Despite the considerable interest and research output generated from the SOC hypothesis, there remains no general agreement with regards to its mechanisms in abstract mathematical form. Bak Tang and Wiesenfeld based their hypothesis on the behavior of their sandpile model. Models of self-organized criticality In chronological order of development: Stick-slip model of fault failure Bak–Tang–Wiesenfeld sandpile Forest-fire model Olami–Feder–Christensen model Bak–Sneppen model Early theoretical work included the development of a variety of alternative SOC-generating dynamics distinct from the BTW model, attempts to prove model properties analytically (including calculating the critical exponents), and examination of the conditions necessary for SOC to emerge. One of the important issues for the latter investigation was whether conservation of energy was required in the local dynamical exchanges of models: the answer in general is no, but with (minor) reservations, as some exchange dynamics (such as those of BTW) do require local conservation at least on average . It has been argued that this model would actually generate 1/f2 noise rather than 1/f noise. This claim was based on untested scaling assumptions, and a more rigorous analysis showed that sandpile models generally produce 1/fa spectra, with a<2. Other simulation models were proposed later that could produce true 1/f noise. In addition to the nonconservative theoretical model mentioned above , other theoretical models for SOC have been based upon information theory, mean field theory, the convergence of random variables, and cluster formation. A continuous model of self-organised criticality is proposed by using tropical geometry. Key theoretical issues yet to be resolved include the calculation of the possible universality classes of SOC behavior and the question of whether it is possible to derive a general rule for determining if an arbitrary algorithm displays SOC. Self-organized criticality in nature SOC has become established as a strong candidate for explaining a number of natural