Marshall [44] shows that for an M/G/1 queueing system the Laplace-Stieltjes transform of the interdeparture time is given by
$$
D^*[\theta]=\frac{(\theta+\mu) \rho W_s^*[\theta]}{\theta+\lambda} .
$$
Using the above result, show that the interdeparture time distribution is exponential if and only if the service time is exponential. (Disney et al. [15] showed that the only $\mathrm{M} / \mathrm{G} / 1$ queueing system having independent, identically distributed, interdeparture times is the $M / M / 1$ system. Such a stream is called a renewal process. Laslett [39] showed that the only GI/M/1 queueing system having renewal output was the $\mathrm{M} / \mathrm{M} / 1$ system.)