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Brownian Motion

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Definition
StochasticDiffs

A process (Bt)t0(B_{t})_{t\ge{0}} on (Ω,F,P)(\Omega,\mathcal{F},P) is called a standard Brownian motion if 1. B0=0B_{0}=0 2. Increments are normally distributed: 0s<t:BtBsN(0,ts)\forall 0\le s<t:B_{t}-B_{s}\sim \mathcal{N}(0,t-s) 3. Independent Increments: 0s<t:BtBs ⁣ ⁣ ⁣FsB\forall 0\le s<t:B_{t}-B_{s}\perp\!\!\!\perp\mathcal{F}_{s}^{B}where FsB=σ(Bu:0u<s)\mathcal{F}_{s}^{B}=\sigma(B_{u}:0\le u<s)and FtB\mathcal{F}_{t}^{B} is the natural filtration of (Bt)t0(B_{t})_{t\ge 0} 4. Continuity: ωΩ:tBt(ω)\forall\omega \in\Omega:t\mapsto B_{t}(\omega) is continuous in R+\mathbb{R}^{+}

Brownian Motion does exist.

Let (Bt)t0(B_{t})_{t\ge 0} be standard BM on (Ω,F,P)(\Omega,\mathcal{F},P) and let (FtB)t0(\mathcal{F}_{t}^{B})_{t\ge0} (FtB=σ(Bu:0u<t)\mathcal{F}_{t}^{B}=\sigma(B_{u}:0\le u<t)) be natural filtration of (Bt)t0(B_{t})_{t\ge 0}. Then (Bt)t0(B_{t})_{t\ge 0} is a (FtB)t0(\mathcal{F}_{t}^{B})_{t\ge 0}-martingale.

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