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Short Communications
On a property of the moment at which Brownian motion attains its maximum
and some optimal stopping problems
M. A. Urusov M. V. Lomonosov Moscow State University
Abstract:
Let
$B=(B_t)_{0\le t\le 1}$ be a standard Brownian motion
and
$\theta$ be the moment at which
$B$ attains its maximal value,
i.e.,
$B_\theta=\max_{0\le t\le 1}B_t$.
Denote by
$(\mathscr{F}^B_t)_{0\le t\le 1}$ the filtration generated by
$B$.
We prove that for any
$(\mathscr{F}^B_t)$-stopping time
$\tau$ $(0\le\tau\le 1)$,
the following equality holds:
$$
E(B_\theta-B_\tau)^2=
E|\theta-\tau|+\frac{1}{2}.
$$
Together with the results
of [S. E. Graversen, G. Peskir, and A. N. Shiryaev,
Theory Probab. Appl., 45 (2000), pp. 41–50] this implies
that the optimal stopping time
$\tau_*$ in the problem
$$
\inf_\tau
E|\theta-\tau|
$$
has the form
$$
\tau_*=\inf\big\{0\le t\le 1: S_t-B_t\ge z_*\sqrt{1-t}\,\big\},
$$
where
$S_t=\max_{0\le s\le t}B_s$,
$z_*$ is a unique positive
root of the equation
$4\Phi(z)-2z\phi(z)-3=0$,
$\phi(z)$
and
$\Phi(z)$ are the density and the distribution function
of a standard Gaussian random variable.
Similarly, we solve the optimal stopping problems
$$
\inf_{\tau\in\mathfrak{M}_\alpha}
E(\tau-\theta)^+
\quad
and\quad
\inf_{\tau\in\mathfrak{N}_\alpha}
E(\tau-\theta)^-,
$$
where
$\mathfrak{M}_\alpha=\{\tau\colon\,
E(\tau-\theta)^-\le \alpha\}$,
and $\mathfrak{N}_\alpha=\{\tau\colon\,
E(\tau-\theta)^+\le\alpha\}$.
The corresponding optimal stopping times are of the
same form as above (with other
$z_*$'s).
Keywords:
moment of attaining the maximum, Brownian motion, optimal stopping. Received: 11.12.2003
DOI:
10.4213/tvp245