Let $a,\lambda,\delta > 0$ . Compute the expected number of arrivals in a Poisson process with intensity function $\alpha(t)=ae^{-\lambda t}$, which are not followed by another arrival with time interval of length $\delta$.
Suppose the number of such arrivals is $N(a)$. Then $E(N(a))=1+E_T(E(N(ae^{-\lambda (T+\delta)})))$ where $T$ is the time of first such arrival. I cant understand how can I find out the next expectation in the recursion.