A parsec-scale wobbling jet in the high-synchrotron peaked blazar PG
  1553+113
  PG 1553+113 is the first blazar showing an approximately two-year
quasi-periodic pattern in its gamma-ray light curve. Such quasi-periodicity
might have a geometrical origin, possibly related to the precessing nature of
the jet, or could be intrinsic to the source and related to pulsational
accretion flow instabilities. By means of a ~2yr very long baseline array
(VLBA) monitoring at 15, 24, and 43 GHz we investigate the source pc-scale
properties during an entire cycle of gamma-ray activity in the period
2015-2017. In contrast to the well-defined periodicity in the gamma-ray
emission, at radio frequencies no clear periodic pattern can be recognized. The
jet position angle, constrained by means of the total intensity ridge line,
varies across the different observing epochs in the range 40-60 deg. We also
investigate the time evolution of the source polarization properties, including
the rotation measure. The brightness temperature is found to decrease as the
frequency increases with an intrinsic value of ~1.5 x 10^10 K and the estimated
Doppler factor is ~1.4.
