4 : "bibliographic coupling" - if document id Y appears in the bibliographic
coupling subvector for document X with a weight of w, it means X
and Y have w common references in their bibliographies; the weight
of did X in the vector for X is the number of items in X's bibliography.
5 : "links" - documents X and Y are linked if X cites Y, Y cites X, or
X == Y.
6 : "co-citations" - if document id Y appears in the co-citation subvector
for document X with weight w, it means X and Y are cited together in
w documents; the weight of did X in the vector for X is the number
of documents that cite X.