  A theoretical framework for analyzing stochastic data from single-particle
tracking in complex or viscoelastic materials and under the influence of a
trapping potential is presented. Starting from a generalized Langevin equation
we found explicit expressions for the two-time dynamics of the tracer particle.
The mean square displacement and the velocity autocorrelation function of the
diffusing particle are given in terms of the time lag. In particular, we
investigate the subdiffusive case. The exact solutions are discussed and the
validity of usual approximations are examined.
