A process (Bt)t≥0 on (Ω,F,P) is called a standard Brownian motion if 1. B0=0 2. Increments are normally distributed: ∀0≤s<t:Bt−Bs∼N(0,t−s) 3. Independent Increments: ∀0≤s<t:Bt−Bs⊥⊥FsBwhere FsB=σ(Bu:0≤u<s)and FtB is the natural filtration of (Bt)t≥0 4. Continuity: ∀ω∈Ω:t↦Bt(ω) is continuous in R+
Let (Bt)t≥0 be standard BM on (Ω,F,P) and let (FtB)t≥0 (FtB=σ(Bu:0≤u<t)) be natural filtration of (Bt)t≥0. Then (Bt)t≥0 is a (FtB)t≥0-martingale.