“…In principle, the rate laws v j ( X , k ) can have any form, but in (1) and (2) we effectively assume that in practice rate laws can be approximated locally, around the steady state, by the traditional power law,
in which the reaction orders κ ij are not restricted to the integer values given by the law of mass action. These noninteger values have already been found in situations where the mean-field approximation does not hold, as it happens in diffusion-limited kinetics, either in disordered media with fractal structures or on regular lattices of dimensionality smaller than the critical value d = 2 . Here, the single elementary reaction
displays a noninteger order of reaction, which may be even greater than 3 when occurring on fractal “dust”, with spectral dimension 0 ≤ d s ≤ 1.…”