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A stochastic process is a collection of random variables usually indexed by a totally ordered set.

1 vote
Accepted

Does $L^2$ progressive measurable processes form a Hilbert space?

The answer is indeed yes, as every adapted measurable real-valued process admits a progressively measurable modification. This is classical Theorem due to Paul-André Meyer (cf. Paul A. Meyer. Probabil …
Stephan Sturm's user avatar
4 votes

Reflected Brownian Motion

As your reflected Brownian motion is nothing else then the absolute value of a Brownian motion you have $$ \{t \in [0,T] : Y_t = 0\} = \{t \in [0,T] : \vert W_t \vert= 0\} = \{t \in [0,T] : W_t …
Stephan Sturm's user avatar
1 vote

calculate all the equivalent martingale measures

One possible approach is to use the fact that the density process $\left. Z_t =\frac{d\mathcal{Q}}{d\mathcal{P}} \right\vert_{\mathcal{F}_t}$ for every equivalent local martingale measure $\mathcal{Q} …
Stephan Sturm's user avatar
1 vote
Accepted

Is $B(t-1)$ an Ito process?

Note that for fixed $t_0$, we have by the martingale representation theorem that $$Y_{t_0} = \int_0^{t_0} I\bigl( s \in [0,t_0-1) \bigr) \, dB_s.$$ In particular that for $t_0\leq 1$ the indicator y …
Stephan Sturm's user avatar
1 vote
Accepted

Definition: Grigelionis Process?

A Grigelionis process is a special semimartingale with absolutely continuous integral characteristics (in time). This is insofar a generalization of a Lévy process, as Lévy processes can be characteri …
Stephan Sturm's user avatar
2 votes
Accepted

Optimal control / Portoflio optimization: Maximize expected utility of total consumption

The problem as stated has no solution except in the special case where it is optimal not to consume at all. To see this, note that the payoff depends on the consumption rate process $c(t)$ only throu …
Stephan Sturm's user avatar
1 vote

Representation theorem for continuous uniformly integrable martingales

I think question 1) is reasonably answered by Wolfgang Loehr in his comment. To get a counterexample for your claims in 2), just set $a_u=W_u^2-u$ for your Brownian motion. Ito's formula gives you the …
Stephan Sturm's user avatar
1 vote
Accepted

BSDE without volatility

I am not sure if I understand your question correctly. A typical Brownian BSDE has the form $$dY_t = f(\omega, t, Y_t, Z_t)dt - Z_t dW_t$$ with terminal condition $$Y_T = \xi \in \mathcal{F}^{W}_T$$ w …
Stephan Sturm's user avatar
4 votes
Accepted

Stochastic processes having Markov kernels

No, that both processes have the same one-dimensional marginals is not sufficient. In contrary, when $X$ is an arbitrary elliptic Itô-process, you can always find a Markov process with the same margin …
Stephan Sturm's user avatar
4 votes
Accepted

On martingale representation theorem

No, but under some regularity conditions you might represent $a(t)$ in terms of Malliavin calculus by means of the Clark-Ocone formula (see e.g. the Lecture notes by Eulalia Nualart, Section 1.5.3.)
Stephan Sturm's user avatar
9 votes
Accepted

Can all local martingales be represented using only Brownian motion and finite variation pro...

First, a martingale is always only specified with respect to a filtration, and so is thus a local martingale. You do not specify any filtration in your problem, so I assume you mean the natural filtra …
Stephan Sturm's user avatar
2 votes

Malliavin derivative under change of measure

Here an answer for the case with determinist drift as mentioned in the edit. (Note: I fail to see why to use different notations for $F$ and $\tilde{F}$ as it is the same process) As $$ dF_t = \mu_t …
Stephan Sturm's user avatar
2 votes

$\lim_{t\rightarrow 0}P\left(X_t >0\right)=\frac 1 2$ for continuous semimartingales?

The proof is not correct, as without additional integrability condition you will not be able to conclude that $g_n \in L^2$ for $n$ large enough, and therefore the $L^2$ convergence argument fails. As …
Stephan Sturm's user avatar
3 votes
Accepted

Question about the exit time of a time-homogeneous Itô diffusion

You can solve this by reducing it to a problem of Brownian motion: Define the scale function $\varsigma(x) = \int_{X_0}^x e^{-2\int_{X_0}^y \frac{b(z)}{\sigma^2(z)} dz} dy$ the process $M_t = \var …
Stephan Sturm's user avatar
3 votes

Properties preserved under passage to augmented filtration

Hi lpdbw, I think this is a very interesting questions, here at least a partial answer; It depends heavily on the little word "strong" in parentheseis. Assuming that $(X_t)$ is strong Markov, the an …
Stephan Sturm's user avatar

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