Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:See for instance Nishimura and Ozaki (2007), p681.
ha ls
hs -0
05 34
02 7,
v er
si on
1 -
8 N
ov 2
01 0
(unique) critical value *W such that option is exercised if and only if *
tW W ; if not, the option
is kept moving forward, defining a continuation region where *
tW W .
Category: