34. Four workers share an office that contains four telephones. At any time, each worker is either “working” or “on the phone.” Each “working” period of worker i lasts for an exponentially distributed time with rate λi , and each “on the phone” period lasts for an exponentially distributed time with rate μi , i = 1, 2, 3, 4.

(a) What proportion of time are all workers “working”?

Let Xi(t) equal 1 if worker i is working at time t, and let it be 0 otherwise.

Let X(t) = (X1(t),X2(t),X3(t),X4(t)).

(b) Argue that {X(t), t  0} is a continuous-time Markov chain and give its infinitesimal rates.

(c) Is {X(t)} time reversible? Why or why not?

Suppose now that one of the phones has broken down. Suppose that a worker who is about to use a phone but finds them all being used begins a new “working”

period.

(d) What proportion of time are all workers “working”?