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Let $(\Omega,\mathcal F,\mathbb P)$ be a probability space, and let $\lambda$ be Lebesgue measure. Consider a square integrable continuous martingale $M$ with quadratic variation $\langle M\rangle$. The Doléans measure $\mu_M$ on $([0,\infty)\times \Omega,\mathcal{B}([0,\infty))\otimes\mathcal F)$ is defined by is defined by $$\mu_M(A)=\int_{\Omega}\int_0^{\infty} \mathbf{1}_A(t,\omega)d\langle M\rangle_t \mathbb P(d\omega).$$ Does this measure have the property that $\mu_M(\mathcal N)=0$ for all $\lambda\times \mathbb P$-null sets $$\mathcal N\in \mathcal B[0,\infty))\otimes\mathcal F?$$

My attempt

First, I thought that for every fixed $\omega$ and $\mathcal B([0,\infty))$-null set $N$, we have $\int_0^{\infty}\mathbf{1}_N(t,\omega)d\langle M\rangle_t=0$. This could lead to an affirmative answer to the question. However, I found a counterexample: if $N$ is the Cantor set and $\langle M\rangle_t$ is the Cantor function, then this integral equals $1$. Since the Cantor set is a Lebesgue null-set, we cannot use this line of reasoning.

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  • $\begingroup$ "$\mathcal B[0,\infty)\otimes\mathcal F$-null sets" -- wrt to what measure? $\endgroup$ Commented Mar 26, 2025 at 18:10
  • $\begingroup$ That would be the product measure $\lambda\times \mathbb P$ where $\lambda$ is Lebesgue measure $\endgroup$ Commented Mar 26, 2025 at 18:11
  • $\begingroup$ I have edited my question to make this more clear. $\endgroup$ Commented Mar 26, 2025 at 18:18

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Counterexample: $M_t=B_{c(t\wedge1)}$ for real $t\ge0$, where $c$ is the Cantor function on $[0,1]$, and $N=C\times\Omega$, where $C$ is the Cantor subset of $[0,1]$.

Then $N$ is a null set, whereas $\langle M\rangle_t=c(t\wedge1)$ for real $t\ge0$, so that $\mu_M(N)=1$.

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    $\begingroup$ Thanks! It turns out that I was thinking in the right direction $\endgroup$ Commented Mar 26, 2025 at 18:30

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