[Math] Simulation of brownian motion and fractional brownian motion

brownian motionfractalsstochastic-processes

It's easy to simulate a path of a brownian motion with the method explained in
Wiener process as a limit of random walk:

import numpy as np
import matplotlib.pyplot as plt
X = 2 * np.random.binomial(1, 0.5, 2*1000*1000) - 1
cumsumX = np.cumsum(X)
n = 1000*1000
x = np.linspace(0, 1, num=n)
Y = 1/np.sqrt(n) * np.array([cumsumX[int(n*t)] for t in x])
plt.plot(x,Y)
plt.show()

enter image description here

Is it possible to simulate a fractional brownian motion with such a simple method, based on a random walk? (none of the methods explained in this article seem to be random-walk based)

Best Answer

You can do but it is not so easy since the increments of fBm are dependence. In fact, it is well known that \begin{eqnarray}\label{Bht} B^{H}_{t}=\int^{t}_{0}z_{H}(t,s)dB_{s}\,, \end{eqnarray}where $B_{t}$ is a standard Brownian motion and the deterministic kernel is given by \begin{eqnarray}\label{zh} z_{H}(t,s)=c_{H}\left( \left(\frac{t}{s}\right)^{H-\frac{1}{2}}\left(t-s\right)^{H-\frac{1}{2}}- \left(H-\frac{1}{2}\right)s^{\frac{1}{2}-H}\int_{s}^{t}u^{H-\frac{3}{2}} \left(u-s\right)^{H-\frac{1}{2}}du\right){\bf 1}_{(0,t)}(s), \end{eqnarray} with the constant \begin{eqnarray}\label{ch} c_{H}^2 =\frac{2H\Gamma(\frac{3}{2}-H)}{\Gamma(H+\frac{1}{2})\Gamma(2-2H)}\;, \end{eqnarray} and ${\bf 1}_{(0,t)}(s)$ is an indicator function that takes the value 1 iff $s\in(0,t)$ and is zero otherwise.

Then Inspired by sottinen (2001, FS jounral), for $H\in(0,1)$, we define: \begin{equation}\label{Bthn} B_{t}^{H,n}=\sum\nolimits_{i=1}^{\lfloor nt\rfloor} \sqrt{n} (\int_{\frac{i-1}{n}}^{\frac{i}{n}} z_{H}(\frac{\lfloor nt\rfloor}{n},s)ds)\xi_{i} \;, \end{equation} where $\xi_{1},\ldots,\xi_{n}$ is an $n$-tuple of independent symmetric Bernoulli random variables with $\mathbb{P}(\xi_{i}=\pm 1)=\frac{1}{2}$, living on a probability space $(\Omega_{n}, \mathcal {F}_{n}, \mathbb{P}_{n})$. Now you can similate fbm using $B_{t}^{H,n}$.

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