Smoothing N-gram language models with Zr = Nr / 0.5 (t - q); What to do with the final frequency? (q, r, and t are successive non-zero frequences)

Question

Question about language modeling, I\'m trying to implement a Katz back-off model.

Many PDFs mention the equation I\'m asking about: Zr = Nr / 0.5 (t - q)