However, it is likely that each time an n-gram is used in a query, the need for finding more occurrences of this particular n-gram decreases.
we introduced a decay factor δ(s | Q), by which the initial ρ(•) value, written ρ 0 (q s ), is multiplied: The decay is exponential: is a small value between zero and one (for instance 0.05), and ∀s∈Q 1 is the number of times the n-gram s has occurred in past queries.