Martingale Poisson

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Can somebody help me with working out: $$E[(N_{t}-\lambda t)^2\mid F_{s}]$$ where $N_{t}$ is a Poisson process and $F_{s}$ the $\sigma$-algebra generated by $N_{s}$, $0 \leq s < t$.

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Hint: The (homogeneous) Poisson process has stationary independent increments, so write: $$N_t=N_s+(N_t-N_s)=N_s+N_{t-s}$$ and use that

  1. $E[N_s \mid F_s]=N_s$, since $N_s$ is known at time $s$
  2. $E[N_{t-s}\mid F_s]=E[N_{t-s}]$, since $N_{t-s}$ is independent from what happened up to time $s$
  3. $N_{t-s} \sim$ Poisson$(λ(t-s))$, in order to calculate $E[N_{t-s}]$ and $E[N_{t-s}^2]$.