  Several classes of physical systems exhibit ultraslow diffusion for which the
mean squared displacement at long times grows as a power of the logarithm of
time ("strong anomaly") and share the interesting property that the probability
distribution of particle's position at long times is a double-sided
exponential. We show that such behaviors can be adequately described by a
distributed-order fractional Fokker-Planck equations with a power-law
weighting-function. We discuss the equations and the properties of their
solutions, and connect this description with a scheme based on continuous-time
random walks.
