TITLE: Joint distribution of non homogeneous Poisson event times?
QUESTION [5 upvotes]: I am trying to calculate the density of $(T_1,T_2)$ where $T_1$ is the time of the first event and $T_2$ is the time of the second event. I have been looking at the Wiki article on Poisson process and while it has been helpful, I haven't been able to figure out how to apply it to the non homogeneous case.

REPLY [5 votes]: If the density of $T_1$ is $\lambda e^{-\lambda t_1}I[0\le t_1]$ and the density of $T_2$ given $T_1=t_1$ is $\lambda e^{-\lambda (t_2-t_1)}I[t_1 \le t_2]$ then the density of $(T_1,T_2)$ is $\lambda^2 e^{-\lambda t_2} I[0\le t_1 \le t_2]$.
If the rate $\lambda$ varies over time then you have to adjust these, but the principle of multiplying the two together still applies.