Rare terms that occur only in δ documents or fewer are eliminated by replacing them with the average vectors of the documents that contain them.
assuming amortized constant time for set testing and insertion (Sedgewick, 2002), that the update is smaller than the corpus, i.e., U < D , and that the number of documents is greater than the number of terms, i.e., n > m, the update algorithm can be executed with a complexity of O(( D + U ) nnz k + mk ), where nnz is the expected number of non-zero elements in any document vector.