STAT3021 Stochastic Processes
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FACULTY OF SCIENCE
SCHOOL OF MATHEMATICS AND STATISTICS
STAT3021 Stochastic Processes —- Main
June, 2022 (Semester 1)
INSTRUCTIONS
Exam Duration: 2 hrs + 10 mins reading time; 15 mins will be allocated to upload handwritten elements into Canvas (add link to assignment dropbox).
Exam Conditions: Open book.
Detailed Instructions:
1. This exam has 4 questions, together with one student declaration statement. You should attempt all questions and follow the instructions for each question carefully.
2. Please review the Student Charter and respond. Responding YES to the dec- laration means that you undertake the exam in the prescribed exam conditions with no assistance from a third party or the use of prohibited resources.
3. This exam must be taken on a computer or laptop with satisfactory internet connectivity. It should NOT be taken on a mobile device.
4. All electronic devices and reference material besides those permitted must be removed from the exam environment.
5. Compatible web browsers include updated versions of Mozilla Firefox or Google Chrome. Any other browser may not display questions correctly.
6. Please be mindful we may access logs of your Canvas activity in the event of any discrepancy or concerns regarding breaches of integrity.
7. The content of this exam is not to be shared or distributed in any form.
1. (12 marks. Upload your handwritten answers and short justifications.
Short justifications are worth half of the marks.)
Consider a Markov chain {Xn }n>0 having the following transition diagram:
diagram-01.pdf
For this chain, there are two recurrent classes R1 = {6, 7} and R2 = {1, 2, 5}, and one transient class R3 = {3, 4}.
(a) Find the period of state 3.
(b) Find f33 and f22 .
(c) Starting at state 3, find the probability that the chain is absorbed into R1 .
(d) Starting at state 3, find the mean absorbation time, i.e., the expected number of steps that the chain is absorbed into R1 or R2 .
2. (12 marks. Upload your handwritten answers and short justifications.
Short justifications are worth half of the marks.)
A population begins with 2 individuals having the capacity to produce offspring. In each generation, each individual in the population dies with probability 0 < α < 1 or the individual is doubled with probability 1 − α . Let Xn denote the number of individuals in then-th generation and suppose that all individuals act independently
of each other.
(a) Explain why {Xn }n>0 is a branching process with X0 = 2 and find the offspring distribution.
(b) Calculate the probability of the event that all individuals die at the second generation.
(c) Assume that α = 1/3. Find the expected number of individuals in n-th gener-ation and the probability that the population eventually dies out.
3. (20 marks. Upload your handwritten answers and short justifications.
Short justifications are worth half of the marks.)
A Cafe near Uni opens during 10am to 4pm on weekdays. From 10am until 12 noon, customers seem to arrive, on the average, at a steadily increasing rate that starts with an initial rate of 4 customers per hour at 10am and reaches a maximum of 20 customers per hour at 12 noon. From 12 noon until 2pm the rate seems to remain constant at 20 customers per hour. However, the arrival rate then drops steadily from 2pm until closing time at 4pm at which time it has the rate of 6 customers per hour. It is observed that the customer arriving at any time is female
with probability 1/3.
Denote by Nt the number of customers arriving at the Cafe by time t (hours) from 10am on a given weekday and suppose that {Nt }t>0 is a nonhomogeneous Possion
process with rate function λ(t).
(a) Find λ(t), 0 ≤ t ≤ 6.
(b) Suppose that each customer spends A$10 on average. Find the average income of the Cafe on a given weekday.
(c) Find the probability that no female customers arrive between 10am and 11am.
(d) Find the probability that there are two female customers only that arrive be-tween 10am and 11am.
(e) Suppose that there are 30 customers arriving from 1pm to 2pm. Find the probabilty that there are 12 customers during 1pm to 1:30pm.
4. (16 marks. Upload your handwritten answers and short justifications.
Short justifications are worth half of the marks.)
In an immigration department, only selected clerks are allowed to locate files in certain boxes. When the department officers want to access a file, they must queue until a clerk become available. Suppose interarrival times between the officers re- questing for files are iid exponential with mean 10 mins, the time for a clerk to
locate a file is also an exponential random variable with mean 15 mins.
Suppose that there are 2 clerks avaiable to locate the files in certain boxes.
(a) Find the probability that all clerks are busy when an officer comes to find a file.
(b) Find the expected number of the officers waiting to be served.
(c) How much time, on the average, does an officer spend in finding a file?
(d) If the immigration department opens from 9am to 3pm on weekdays, how many hours does a clerk expect to be idle on Monday?
2023-06-07