  Exponential relaxation to equilibrium is a typical property of physical
systems, but inhomogeneities are known to distort the exponential relaxation
curve, leading to a wide variety of relaxation patterns. Power law relaxation
is related to fractional derivatives in the time variable. More general
relaxation patterns are considered here, and the corresponding semi-Markov
processes are studied. Our method, based on Bernstein functions, unifies three
different approaches in the literature.

Probability