[R] Geometric Brownian Process

[Simon Blomberg]

This function samples from the solution to the stochastic differential 
equation of the geometric wiener process:

rgwiener <- function (n=1, t=1, S0=1000, mu=0, sigma=1) {
    S0 * exp((mu - 1/2 * sigma^2) * t + sigma * sqrt(t) * rnorm(n))
}

To test this, note that the geometric wiener process has log(S_t/S0) ~ 
N((mu -1/2*sigma^2)* t, sigma^2 * t)

So if we let mu = 0, sigma = 1, t =1, S0 = 1000, log(S_t/S0) should be ~ 
Normal(-0.5, 1).

 > samp <- log(rgwiener(100000)/1000)
 > hist(samp)
 > library(MASS)
 > fitdistr(samp, "normal")

       mean            sd    
  -0.500553189    1.000032814
 ( 0.003162381) ( 0.002236141)

which looks good to me.

Because the geometric wiener process is a transformation of the ordinary 
wiener process, we can simulate it easily enough (apologies to the 
authors of package e1071, whose function rwiener I hacked):

gwiener <- function (mu=0, sigma=1, S0=1000, frequency=1000) {
z <- S0 * exp((mu - 1/2 * sigma^2) * seq(0, 1, length=frequency) +
    sigma * cumsum(rnorm(frequency)/sqrt(frequency)))
ts(z, start = 1/frequency, frequency=frequency)
}

 > plot(gwiener(), type="l")

Looks ok.

Cheers,

Simon.

Michael Graber wrote:
> Dear R People,
>
> Consider I have 3 realizations of an Geometric Brownian process.
> Now i want to overlay  the plot of these realizations. In a future point 
> in time a probability density curve  in this specific point of time 
> should overlay this plot ( view rotated 90°).
> I am sorry for not providing any source code. Can anybody point mo to an 
> package, or has anybody an idea how to simulate an geometric brownian 
> process in R?
>
> Thanks in advance,
>
> Michael Graber
>
> ______________________________________________
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> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>
>   


-- 
Simon Blomberg, B.Sc.(Hons.), Ph.D, M.App.Stat. 
Centre for Resource and Environmental Studies
The Australian National University              
Canberra ACT 0200                               
Australia                                       
T: +61 2 6125 7800 email: Simon.Blomberg_at_anu.edu.au
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The combination of some data and an aching desire for 
an answer does not ensure that a reasonable answer 
can be extracted from a given body of data.
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