For a prepared state exact expressions for the time dependent mean fidelity
as well as for the mean inverse paricipation ratio are obtained analytically.
The distribution function of fidelity in the long time limit and of inverse
participation ratio are studied numerically and analytically. Surprising
features like fidelity revival and enhanced non--ergodicity are observed. The
role of the coupling coefficients and of complexity of background is studied as
well.Comment: 4 pages 3 figure
