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## On the distance between $\langle X \rangle$ and $L^{\infty}$ in the space of continuous BMO-martingales

### Tom 168 / 2005

Studia Mathematica 168 (2005), 129-134 MSC: 60G44, 60G46. DOI: 10.4064/sm168-2-3

#### Streszczenie

Let $X=(X_t,{\mathcal F}_t)$ be a continuous BMO-martingale, that is, $$\|X\|_{\rm BMO}\equiv \sup_T\|E[|X_\infty-X_T|\,|\,{\mathcal F}_T]\|_\infty<\infty,$$ where the supremum is taken over all stopping times $T$. Define the critical exponent $b(X)$ by $$b(X)=\{b>0:\sup_T\|E[\exp(b^2(\langle X \rangle_\infty-\langle X \rangle_T))\,|\,{\mathcal F}_T]\|_\infty<\infty\},$$ where the supremum is taken over all stopping times $T$. Consider the continuous martingale $q(X)$ defined by $$q(X)_t=E[\langle X \rangle_\infty\,|\,{\mathcal F}_t]-E[\langle X\rangle_\infty\,|\, {\mathcal F}_0].$$ We use $q(X)$ to characterize the distance between $\langle X \rangle$ and the class $L^\infty$ of all bounded martingales in the space of continuous BMO-martingales, and we show that the inequalities $$\frac1{4d_1(q(X),L^\infty)}\leq b(X)\leq \frac4{d_1(q(X),L^\infty)}$$ hold for every continuous BMO-martingale $X$.

#### Autorzy

• Litan YanDepartment of Mathematics
College of Science
Donghua University
1882 West Yan'an Rd.
Shanghai 200051, P.R. China
e-mail
• Norihiko KazamakiDepartment of Mathematics
Toyama University
3190 Gofuku,Toyama 930-8555, Japan
e-mail

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