Chemist_MZ 发表于 2013-10-13 21:08 
very odd question, if you know you are to get nothing in the end, why do you not sell it or exercise ...
In order that an American call option may be held optimally until expiry and expires
in-the-money, we must have S*(0+) > X and ST > X. When q >=r, it is known
that
S*(0+) = max(1,r/q)X=X.
At time close to expiry, if the American call remains alive, then it must be out-of-
the-money. That is, the asset price at T = 0+ is less than X. If not, it should have
been exercised since S*(0+) = X. Hence, when q >= r, the American call that is
held optimally until expiry cannot expire in-the-money.
以上是老师给的答案,我没有看太懂。