This is a series problem and I'm struggling with the last part. I assume that the last part has nothing to do with previous ones, so i won't put up the other parts. Question is :
Let $\tau$ be a stopping time and $X=(X_n,{\cal F_n})$ be a martingale. Show that stopped sequence $X_{n}^{\tau}=( X_{\tau_{n}},{\cal F_n})$ where $\tau_{n}=\min\{\tau,n\}$ is a martingale.
I started with $E(X_{\tau_{n}}|F_{n-1})=E(X_{\min\{\tau,n\}}|F_{n-1})$ which then would have two cases, but all ends up with $E(X_{n-1})$, or should that be $E(X_{\tau_{n-1}})$?