# Tagged Questions

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### The hitting time $T-\tau^{l}$ has the same distribution as $\min\{\tau^{f},T\}$ regarding an Poisson Process.

Assume we have a probability space $(\Omega,\mathcal{F},\mathbb{P})$ where $\mathcal{F} =(\mathcal{F}_t)_{0 \leq t \leq T}$ is a Filtration, with $T < \infty$. On that prbability space we want to ...
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### Hitting times in two-dimensional case: expectation of Brownian motion at a hitting time

Consider two Brownian motions $$X_{1t}=\mu t+\sigma_1B_{1t}$$ and $$X_{2t}=\mu t+\sigma_2B_{2t}.$$ Here $B_{1t}$ and $B_{2t}$ are uncorrelated. Let $\tau_1$ and $\tau_2$ be the stopping times: \begin{...
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### Reference for *optimal* stopping theorem for supermartingales

Can anyone introduce a good reference about optimal (not optional!) stopping times for submartingales / supermartingales? I am looking for some theorem like the one mentioned in this question. I ...
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### Distribution of Markov Chain at a Stopping Time

Suppose $(X_t)_{t \geq 0}$ is a Markov chain on the state space $S$ with transition probability $p$, and that $\pi$ is a stationary distribution for $p$. If $X_0 \sim \pi$, then we know $X_t \sim \pi$ ...
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### Finding optimal strategy in time series game

Let $P_t$ be a time series such that $P_{t+1} = \alpha P_t+S_{t+1}$, where $\forall t\geq0 : S_t \sim N(0,\sigma)$ Consider the following game: In each round $t$, a player sees $P_t$ and decides ...
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### Expected time until pattern (1,0,0,1)

Let $(X_n)_{n\geq 0}$ be i.i.d. with $\mathbb P(X_n = 0 ) = \mathbb P(X_n = 1) = \frac{1}{2}$. Let $\tau_a$ be the stopping times defined as $$\tau_a = \inf\{n: (X_{n-3}, ... , X_n) = (1,0,0,1)\}$$ I ...
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### Recurrence of a state in a finite state space

Suppose $T_A := \inf\{ n \ge 1 : X_n \in A\}$ where $A \subset \mathcal{S}$ is finite. Assume $\mathbb{P}\{T_A < \infty \; | \; X_0 = x\}= 1$ for $\forall x \in \mathcal{S}-A$. I need to show that ...
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