  The statistical properties of energy and waiting time carry essential
information about the source of repeating fast radio bursts (FRBs). In this
paper, we investigate the randomness of energy and waiting time using four data
samples from three extremely active repeating FRBs observed by the
Five-hundred-meter Aperture Spherical radio Telescope (FAST). We report the
deviation from complete randomness of the burst activity using three
statistics, i.e., Hurst exponent, Pincus index and non-Gaussian probability
density distribution of fluctuations. First, the Hurst exponent greater than
0.5 reveals that there is long-term memory in the time series of energy and
waiting time. Second, the deviation of the Pincus index from 1.0 manifests that
the time series is not completely random. Finally, the fluctuations of energy
and waiting time follow the scale-invariant $q$-Gaussian distribution. All
these statistical properties imply that, although the time series of repeating
FRBs seems to be irregular, they are not completely random, similar to the
features of self-organized criticality.
