confused about optional stopping theorem (martingales) If $X_n$ is a martingale and $T$ is a stopping time with $P(T \leq n)  = 1$ for some $n$, then the optional stopping theorem applies: $\mathbb{E}(X_T) = \mathbb{E}(X_0)$. Sometimes it's hard to show that the stopping time $T$ is bounded a.s, so instead I believe you can show that $|X_{\min(T,n)}| \leq k$ for some $k$. Is this enough to get the conclusion of the OST (that the expectations at time $T$ and $0$ are equal)? I know that in this case the DCT implies that $EX_{\min(T,n)}$ exists, but so what?
(I have looked everywhere for such a definition but I can't find it)

Since $X_{\min(T,n)}$ is a martingale (see for instance the book of David Williams, "Probability with Martingales"), we have $\mathbf{E}[X_{\min(T,n)}-X_0]=0$ for all $n$.  If your condition holds, we can use dominated convergence to take the limit as $n \rightarrow \infty$ and conclude $\mathbf{E}[X_T-X_0]=0$.
