where 
is the number of times or frequency in which term i appears
in document j. As one does not expect that each word will appear
in every document, the matrix A is typically sparse with rarely any
noticeable nonzero structure. As discussed in [Dum91],
local and global weightings can be applied to either
increase or decrease the importance of terms within or among documents
so that each element may be cast as
where 
is the local weighting for term i in document j,
and 
is the global weighting for term i.
