Chapter 3.10

Linear Transformations in the Space L2L^2

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Problem 3.10.1

Let ξ=(ξt,t∈[0,1])\xi =\left(\xi_{t}, t \in [0,1]\right) be a random process such that all ξt,t∈T\xi_{t}, t \in T, are jointly independent, identically distributed, and nontrivial (not equal to constants). Is the process ξ\xi stochastically continuous at even a single point of [0,1][0,1]?

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Problem 3.10.2

Give an example of a process (Xt,t∈[0,1])\left(X_{t}, t \in [0,1]\right) that is stochastically continuous on [0,1][0,1], but not integrable in probability on [0,1][0,1] (the integral in probability is defined as convergence in probability of the Riemann integral sums).

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Remark.
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This example shows that convergence in probability is rather poor, since continuity does not imply integrability, unlike convergence in L2L^{2}.

Problem 3.10.3

Is the Poisson process (Nt,t⩾0)\left(N_{t}, t \geqslant 0\right) differentiable

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(a)

in probability,

(b)

in mean square?

Problem 3.10.4

Let (Wt,t⩾0)\left(W_{t}, t \geqslant 0\right) be a Wiener process. Find the distribution of the random variable Xt=∫0tWsdsX_{t}=\int_{0}^{t} W_{s} d s. Prove that the process (Xt,t⩾0)\left(X_{t}, t \geqslant 0\right) is Gaussian.

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Problem 3.10.5

Find, for every t≥0t \geq 0, the joint distribution of the quantities WtW_{t} and XtX_{t} from the previous problem.

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Problem 3.10.6

We are given the random process Xt=∫0te−WsdsX_{t}=\int_{0}^{t} e^{-W_{s}} d s, where WsW_{s} is a Wiener process. Find the mean function and covariance function of the process XtX_{t}.

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Problem 3.10.7

Let (Wt,t⩾0)\left(W_{t}, t \geqslant 0\right) be a Wiener process. Compute, for t>0t>0, the limit in L2L^{2} as n→∞n \rightarrow \infty of the expression

∑i=1nWt(i−1)/n(Wti/n−Wt(i−1)/n) \sum _{i=1}^{n} W_{t(i-1) / n}\left(W_{t i / n}-W_{t(i-1) / n}\right)
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