摘要
The apparent superluminal motions observed in some compact radio sources are an important astrophysical phenomenon. Recent VLBI (Very Long Baseline Interfer- ometer) observations have found that a new knot C4 which was just ejected from the nucleus of the superluminal source 3C345 was both moving and expanding su- perluminally. So this is an apparent dual-superluminal phenomenon. If such a phe-
