Citation searching is a very useful tool, for finding information on a particular topic (by following up to see who has cited known articles), information on particular authors or articles (how often they are cited by others can be a valuable clue as to their prominence or importance), and even information on journals (citation "impact factors" can help indicate which are the most important journals in a specific field).
Note that citation searching in all its forms must be used with caution: someone's citation of a paper or book doesn't guarantee that they read it (you can find examples of mis-citation that suggest that the citing author has never seen the paper in question!); nor does the lack of a citation mean that the author hasn't read the paper (and they may even have found it to be very important). It is also necessary to be aware of citation differences in different fields: in some fields papers are much more frequently cited than in others, so you should never directly compare "impact factors" or other citation measures across differing fields.
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