Help with a Probability Question

mandy123

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Mar 9, 2020
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Hi,

Just wondering if anyone would be able to help me with a question I'm stuck on:

Assume that initially, at time t_0 = 0 the particle is located at a point x_0 ∈ R.

At time t_1 = t_0 + δt for some δt > 0 the particle’s location is given by

X_1= x_0 + νδt + σW_1√(δt)

where W_1 is a standard normal variable and ν ∈ R is a constant drift term and σ > 0 is the diffusion constant that depends on the ambient temperature among other things. Notice that generally, the movement of the particle in equidistant time steps, i.e. at times t_1, . . . , t_n where t_n = t_(n−1) + δt, can be described recursively by

X_j =X_(j−1) + νδt + σW_j√(δt)

where W_1, W_2, . . . , W_n are independent standard normal random variables and X_(j−1) denotes the location of the tracked particle at time t_(j−1)

Based on the above info, how do I find an expression for X_j that is a function of W_1, . . . , W_j and the parameters but does not depend on the previous position X_(j−1)?

Thanks in advance!
 
Based on the above info, how do I find an expression for X_j that is a function of W_1, . . . , W_j and the parameters but does not depend on the previous position X_(j−1)?

Hi, welcome to FMH!

given this
A) X_j =X_(j−1) + νδt + σW_j√(δt)

can you see that the following also holds?
B) X_(j-1) =X_(j−2) + νδt + σW_(j-1)√(δt)

Sub the value of X_(j-1) from equation B into equation A and what do you get? Perhaps it can be simplified?


The above method eliminates X_(j−1) as requested, but it does not require the use of all the values W_1, . . . , W_j. Therefore please double check the exact wording of the question.
 
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