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Which arrangement class each tissue belongs to is not established.","posed_since":"2014","precise":"For the diffusion coefficient $D(t)$ in tissue approaching its long-time value as D(t) - D_inf ~ t^(-theta), with $\\theta = (p + d)/2$ where d is the spatial dimension of the relevant disorder and p the structural exponent of its density correlations at long wavelengths, measure $\\theta$ and assign p for white matter along and across axons, grey matter and muscle. An answer is a classification of tissues by p with measured exponents and uncertainty.","problem_ref":null,"references":"","settled_by":"Oscillating and pulsed-gradient measurements over diffusion times spanning at least a decade, fitted against the competing power laws with model-selection statistics.","status_note":"Short-range disorder along axons ($p = 0, \\theta = 1/2$ in one dimension) has support in brain and muscle data, while assignments for grey matter and transverse diffusion remain debated.","title":"Structural universality class from time-dependent 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