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Suppose further that the walk stops if it reaches 0 or m ≥ a; the time at which this first occurs is a stopping time. If it is known that the expected time at which the walk ends is finite (say, from Markov chain theory), the optional stopping theorem predicts that the expected stop position is equal to the initial position a.
A stopping time is often defined by a stopping rule, a mechanism for deciding whether to continue or stop a process on the basis of the present position and past events, and which will almost always lead to a decision to stop at some finite time. Stopping times occur in decision theory, and the optional stopping theorem is an important result ...
The concept of a stopped martingale leads to a series of important theorems, including, for example, the optional stopping theorem which states that, under certain conditions, the expected value of a martingale at a stopping time is equal to its initial value.
The class of semimartingales is closed under optional stopping, localization, change of time and absolutely continuous change of probability measure (see Girsanov's Theorem). If X is an R m valued semimartingale and f is a twice continuously differentiable function from R m to R n, then f(X) is a semimartingale. This is a consequence of Itō's ...
In mathematics, the theory of optimal stopping [1] [2] or early stopping [3] is concerned with the problem of choosing a time to take a particular action, in order to maximise an expected reward or minimise an expected cost.
In mathematics, a continuous-time random walk (CTRW) is a generalization of a random walk where the wandering particle waits for a random time between jumps. It is a stochastic jump process with arbitrary distributions of jump lengths and waiting times.
An alternative terminology uses continuous parameter as being more inclusive. [1] A more restricted class of processes are the continuous stochastic processes; here the term often (but not always [2]) implies both that the index variable is continuous and that sample paths of the process are continuous. Given the possible confusion, caution is ...
In mathematics, progressive measurability is a property in the theory of stochastic processes.A progressively measurable process, while defined quite technically, is important because it implies the stopped process is measurable.