Multi-frequency Polarization Imaging of Blazars with Cyclic Activity
  Results of multi-frequency VLBA polarization imaging for sources showing evidence of quasi-periodic variations in their total flux density are discussed. Possible origins of the observed behavior are proposed. 
T. B. Pyatunina, D. C. Gabuzda, S. G. Jorstad, M. F. Aller, H. D.   Aller, H. Terasranta