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A power law is a special mathematical relationship between two quantities in which one quantity varies as a power of the other. Numerous examples of power laws occur in nature, for example, the frequency of any word in a language is inversely proportional to its ranking in a frequency table.
Are scale-free networks rare or universal? Important or not? We present the recent research about degree distributions of networks. This is a controversial topic, but, we argue, with some adjustments of the terminology, it does not have to be.
The concept of preferential attachment is behind the hubs and power laws seen in many networks. New results fuel an old debate about its origin, and beg the question of whether it is based on randomness or optimization. See Letter p.537
Multi-scale modelling of the deformation of magnesium oxide reveals the need for a re-examination of the way in which laboratory data are used to estimate the strength of Earth's lower mantle. See Letter p.177
