Let $B_t \in \mathbb{R}^2$ and we define $X_t = \ln|B_t|$, we also define $\tau = \inf\{t>0:|B_t| \le \epsilon\}$, where $0 < \epsilon < \infty$.
I am confused about the $||$ symbol in these definition. I think it is absolutely value but $B_t$ is multidimensional. What does it mean of $||$ symbol here.
Thanks
