Recurrent Activity in Active Galactic Nuclei
  There has been a growing body of evidence to suggest that AGN activity, which
is powered by mass accretion on to a supermasive black hole, could be episodic,
although the range of time scales involved needs to be explored further. The
structure and spectra of radio emission from radio galaxies, whose sizes range
up to $\sim$5 Mpc, contain information on the history of AGN activity in the
source. They thus provide a unique opportunity to study the time scales of
recurrent AGN activity. The most striking examples of recurrent activity in
radio galaxies and quasars are the double-double or triple-double radio sources
which contain two or three pairs of distinct lobes on opposite sides of the
parent optical object. Spectral and dynamical ages of these lobes could be used
to constrain time scales of episodic activity. Inverse-Compton scattered cosmic
microwave background radiation could in principle probe lower Lorentz-factor
particles than radio observations of synchrotron emission, and thereby reveal
an older population. We review briefly the radio continuum as well as molecular
and atomic gas properties of radio sources which exhibit recurrent or episodic
activity, and present a few cases of quasars which require further observations
to confirm their episodic nature. We also illustrate evidence of episodic AGN
activity in radio sources in clusters of galaxies.
