I agree, also many papers near the begining say > We are exending our previous work in [7] or cite a few relevant papers > This topic has been studied in [3-

I agree, also many papers near the begining say > We are exending our previous work in [7] or cite a few relevant papers > This topic has been studied in [3-8] Where 3 was published by group X, 5 by group Y, 7 by group Z and 4, 6 and 8 by group W. Anyone can guess the author of the paper is in group W. Just looking at the citations, it's easy to guess the group of the author.