Therefore, we further model the distributional characteristics of global event chains and encourage the main event chains to have a large number of coreferential mentions and a long stretch (the number of sentences that are present in between the first and last event mention of a chain), to avoid creating partial chains.
we add the following objective function (equation 5) and the new constraints (equation 6 and 7): First, we define an indicator variable σ ij by equation 6 3 , corresponding to each event mention pair, that takes value 1 if (1) the event mentions at index i and j are coreferent; (2) the event mention at index i doesn't corefer to any of the mentions preceding it; and (3) mention at index j doesn't corefer to any event mention following it.