Show that, if $C$ is the price of an American call option on a futures contract when the strike price is $K$ and the maturity is $T$, and $P$ is the price of an American put on the same futures contract with the same strike price and exercise date, then
$$
F_0 e^{-r T}-K<C-P<F_0-K e^{-r T}
$$
where $F_0$ is the futures price and $r$ is the risk-free rate. Assume that $r>0$ and that there is no difference between forward and futures contracts. (Hint: Use an analogous approach to that indicated for Problem 17.12.)