For efficiency reasons, in practice, we only consider a small subset of all context words j for which x ij = 0, which is similar in spirit to the use of negative sampling in Skip-gram (Mikolov et al., 2013b).
for efficiency reasons, the counts of the form y ij * , y i * k and y * jk are pre-computed for all word pairs, which can be done efficiently due to the sparsity of co-occurrence counts (i.e.