  The introduction of a fractional differential operator defined in terms of
the Riemann-Liouville derivative makes it possible to generalize the kinetic
equations used to model relaxation in dielectrics. In this context such
fractional equations are called fractional kinetic relaxation equations and
their solutions, called fractional kinetic relaxation functions, are given in
terms of Mittag-Leffler functions. These fractional kinetic relaxation
functions generalize the kinetic relaxation functions associated with the
Debye, Cole-Cole, Cole-Davidson and Havriliak-Negami models, as the latter
functions become particular cases of the fractional solutions, obtained for
specific values of the parameter specifying the order of the derivative. The
aim of this work is to analyse the behavior of these fractional functions in
the time variable. As theoretical tools we use the theorem by Bernstein on the
complete monotonicity of functions together with Titchmarsh's inversion
formula. The last part of the paper contains the graphics of some of those
functions, obtained by varying the value of the parameter in the fractional
differential operator and in the corresponding Mittag-Leffler functions. The
graphics were made with Mathematica 10.4.
