Absolute Value of Martingale is Submartingale
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Theorem
Let $\struct {\Omega, \Sigma, \sequence {\FF_t}_{t \ge 0}, \Pr}$ be a continuous-time filtered probability space.
Let $\sequence {X_t}_{t \ge 0}$ be a $\sequence {\FF_t}_{t \ge 0}$-martingale.
Then $\sequence {\size {X_t} }_{t \ge 0}$ is a $\sequence {\FF_t}_{t \ge 0}$-submartingale.
Proof
From Characterization of Integrable Functions:
- $\size {X_t}$ is integrable for each $t \in \hointr 0 \infty$.
From Absolute Value Function is Convex, $x \mapsto \size x$ is a convex function.
From Martingale Composed with Convex Function is Submartingale, we have:
- $\sequence {\size {X_t} }_{t \ge 0}$ is a $\sequence {\FF_t}_{t \ge 0}$-submartingale.
$\blacksquare$
Sources
- 2016: Jean-François Le Gall: Brownian Motion, Martingales, and Stochastic Calculus ... (previous) ... (next): $3.3$: Continuous Time Martingales and Supermartingales