  In the paper "The relativistic Doppler effect: when a zero-frequency shift or
a red shift exists for sources approaching the observer, Ann. Phys. (Berlin)
523, No. 3, 239-246 (2011), DOI 10.1002/andp.201000099 by C. Wang the use of an
erroneous equation ended up at a number of faulty conclusions which are
corrected in the present Comment.
