1
$\begingroup$

Let $W$ be a one dimensional standard Brownian motion, and $\sigma: [0, \infty) \to \mathbb R$ a Borel function with $c < \sigma < C$ for some constants $c, C > 0$.

Does there exist some $M > 0$ such that, conditional on $W_t > -1$ for all $0 \leq t \leq 1$ and $W_1 > M$,

$\int_0^1 \sigma(t) \, dW_t \geq 0$, almost surely?

Remark: The statement would be true if the integrator had uniformly bounded variation, but I suspect it does not hold for Brownian motion which has unbounded variation.

$\endgroup$

1 Answer 1

1
$\begingroup$

No such $M$ exists for the following $\sigma$. Partition $[0,1]$ into countably many intervals, with endpoints $t_0=0,t_1,t_2,...$ and let $\sigma$ take value 1 on the odd ones and value 2 on the even ones. Given any $M$, the following event $A_M$ has positive probability:

$A_M$ requires that the increments $W_{t_k}-W_{t_{k-1}}$ are in $(9,10)$ for the first $M$ odd values of $k$, and in $(-6,-7)$ for the first $M$ even values of $k$, with $W_t-W_{t_{k-1}}>-1$ for $t \in [t_{k-1},t_k]$ when $0<k<2M$.

Then given $A_M$, we have $W_{t_{2M}}>2M$, so the event $W_1>M$ holds with high probability, yet on $A_M$, the stochastic integral considered is $<-2M$ if you integrate up to $t_{2M}$, and the integral over $[t_{2M},1]$ is likely to be $<2M$.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.