We study the continuous-time problem of hedging a generalized call
option of the European and of the American type, in the presence of transaction
costs. We show that if the price process of the relevant stock fluctuates with
positive probability, then the only hedge that is possible for the American
option is the trivial one. If the price of the stock, in addition to
fluctuating with positive probability, is also stable with positive
probability, then the same is true for the European option. We also show that
in some sense, stable price with positive probability is a necessary condition
for having only a trivial hedge for the European option. Our basic idea is to
work with an appropriate discrete-time version of the problem which is
transaction costs free. The mathematical tools that we use are elementary. A
related result appears in Soner, Shreve and Cvitanic.