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The esistence of a martingale with given diffusion functional
M. P. Ershov Moscow
Abstract:
Let
$\mathbf R_+=[0,\infty)$ and
$C$ be the space of continuous functions on
$\mathbf R_+$ “starting” from zero with the topology of uniform convergence on compacts.
Let
$A\colon \mathbf R_+\times C\mapsto \mathbf R_+$ be a Borel functional such that
(i) for each
$\mathbf x\in C$,
$A(\,\cdot\,,\mathbf x)\in C$ and is non-decreasing,
(ii) the set
$$
\{\{A(t,\mathbf x)\}_{t\in \mathbf R_+}\mid\mathbf x\in C\}
$$
is relatively compact in
$C$,
(iii) for each
$t\in \mathbf R_+$,
$A(t,\,\cdot\,)$ is continuous, and
(iv) for each
$t\in \mathbf R_+$,
$x_s=y_s$ $(0\le s\le t)$ implies
$$
A(t,\mathbf x)=A(t,y)\quad(\mathbf x=\{x_s\}_{s\in \mathbf R_+},y=\{y_s\}_{s\in \mathbf R_+}).
$$
Then we prove that (on some probability space) there exists a continuous martingale
$\mathbf X$ such that its Meyer squared variation process
$$
\langle\mathbf X\rangle=A(\,\cdot\,,\mathbf X)\quad\text{a.s.}
$$
In particular, in case
$$
A(t,\mathbf x)=\int_0^ta^2(t,\mathbf x)\,ds
$$
where
$a^2$ is a bounded non-anticipative function, it follows that in the conditions of D. W. Stroock and S. R. S. Varadhan [12] continuity in
$(s,\mathbf x)$ may he replaced by that in
$\mathbf x$ only.
Received: 18.12.1973