问题1：
Let  (Nt) be a Poisson process with rate lamda (and N0 = 0). Show that for s, t >= 0,
  E[NtNt+s] = lamda* t (1 + lamda* (t + s))

问题2：
During a four-hour working session at a factory, errors occur according to a
Poisson process with time-dependent rate lamda(t) = 0.2 t, 0 <= t <= 4. Specify the
distribution of the number of errors made in a working-session and evaluate the
probability that more than one error is made in the session.

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