2
$\begingroup$

The motivation of this question is to show that two probabilities on $C_{0}^{n}(0,1)$ (the space of continuous $\mathbb R^{n}$ valued process on $[0,1]$ starting from zero) induced by two non-degenerate $n$ dimensional Brownian motions $W_{1}$ and $W_{2}$ are equivalent to each other.

Intuitively, it may be correct, if we think of these two probability measures by Gaussian measures on the space $C_{0}^{n}(0,1)$ in terms of abstract Wiener space. However, due to the lack of knowledge in this area, I could not find a rigorous proof.

I appreciate if one can provide a detailed proof. Thanks.

In this below, I will given one example to make the question clear: Suppose $W_1$ is 2-D standard BM, $W_2$ is given by $W_2 (t) = [1, 0; \rho, \sqrt{1 - \rho^2}] W_1(t)$ for some $\rho\in (0,1)$. Are $W_1$ and $W_2$ are singular or equivalent?

Motivated from the comments below, $W_1$ and $W_2$ seems to be mutually singular. The argument is that $W_1$ is concentrated to the set $A_1 = \{x\in C_0^2[0,1]: [x](t) = I t\}$, but $W_2$ is concentrated on another disjoint set $A_2 = \{x \in C_0^2[0,1]: [x](t) = [1, \rho; \rho, 1] t\}$, where $[x](t)$ is quadratic variation of $x$ on $[0,t]$.

However, consider a set $B= \{x\in C_0^2: x^1(1)>0\}$, then $P(W_1\in B) = P(W_2\in B) = 1/2$, which contradicts to singularity? It's not contradiction to singularity, since $B\cap A_1$ and $B\cap A_2$ are both not empty.

$\endgroup$
7
  • 3
    $\begingroup$ I'm not sure I understand the question. What is supposed to be the difference between $W_1$ and $W_2$? If they are both standard Brownian motions then they induce the same probability measure on $C_0^n([0,1])$. If they have, e.g., different variances, then their measures will be mutually singular. $\endgroup$ Commented Feb 14, 2015 at 5:55
  • $\begingroup$ @NateEldredge $W_1$ and $W_2$ are two BMs with two different variance processes. Say $W_1$ is standard BM, and $W_2(t) = \int_0^t \sigma(t) dW_1(t)$ for some $\sigma\neq I$. Then, I don't see why they are singular? $\endgroup$
    – kenneth
    Commented Feb 14, 2015 at 7:08
  • 1
    $\begingroup$ You can recover $\sigma^2$ as an almost sure property of the sample path of $W_2$, therefore the two measures are mutually singular. $\endgroup$ Commented Feb 14, 2015 at 12:01
  • $\begingroup$ @NateEldredge I added some lines according to your comment, and makes me confused. Thanks if you would clarify. $\endgroup$
    – kenneth
    Commented Feb 14, 2015 at 14:52
  • 1
    $\begingroup$ You have a correct argument in your second-to-last paragraph that the measures are mutually singular. Your last paragraph does not contradict it, if you carefully examine the definition of "singular". $\endgroup$ Commented Feb 14, 2015 at 16:59

0

You must log in to answer this question.