17, we can see that is used to balance the prize for the divergence from history epoch and penalty on the divergence from the update epoch.
when is set as the value of 0, the scoring of sentences is only determined by docset H. a sentence is likely to be selected into summary, only when it has a large divergence of aspect distribution from docset H. when is set a larger value, penalty on the divergence from docset U is more considered.