CAS CS 131 – Fall 2017 – Combinatoric Structures

Instructors: Prof. John Byers, Prof. Babis Tsourakakis
Piazza: Piazza Web Page CS 131
Semester: Fall 2017

When and Where
Lecture A1: Tues and Thurs 3:30 – 4:45 PM, CAS B12.
Lecture B1: Tues and Thurs 2 – 3:15 PM, CAS B12.

Official Course description

Representation, analysis, techniques, and principles for manipulation of basic combinatoric data structures used in computer science. Rigorous reasoning is emphasized. (Counts as a Background Course for the CS concentration.)
What this means in reality: we will train you in the mathematical foundations of CS so that you can make convincing logical arguments that programs you write and algorithms you design are correct and run efficiently. The term “combinatorics” is centered on combinations of objects belonging to a finite set – we will see applications in counting and probability theory. For example, we will teach you things like how to compute the probability of randomly guessing an alphanumeric password, or (similarly) how many Bitcoins you can expect to mine with a given amount of computation.

Office Hours

Instructor: Prof. John Byers
Email: byers @ cs . bu . edu (preferred)
When: Tues 11-12 AM (open), Wed 10-11 AM (open), and Wed 11-12 AM (by prior appointment)
Where: MCS 295B

Instructor: Prof. Babis Tsourakakis
Email: tsourolampis @ gmail . com (preferred)
When: Tues 11-12 AM (open), Wed 10-11 AM (open), and Wed 11-12 AM (by prior appointment)
Where: MCS 292

Teaching Fellow (TF): Wonyl Choi
When: TBD
Where: TBD

Teaching Fellow (TF): Konstantinos Sotiropoulos
When: TBD
Where: TBD

The class will be co-taught by Professors Byers and Tsourakakis. On any given lecture date, one of the two instructors will deliver the lecture for both the A1 and B1 sections. The TFs will lead the discussion sessions. The objective is to reinforce the concepts covered in the lectures through problem-solving, and to provide some clarifications and guidance on the homework assignments. The purpose of the office hours of the Instructors and Teaching Fellows is to answer specific questions or clarify specific issues. Your fastest route to get an answer to most questions is via Piazza. Office hours are not to be used to fill you in on a class you skipped or to re-explain entire topics. Office hours are scheduled at times to provide the most help to students who start the homework before the last minute

Discussion Labs

  1. Lab A2: Wed 9:05 – 9:55AM, MCS B25 (TBD)

  2. Lab A3: Wed 3:35 – 4:25PM MCS B25 (TBD)

  3. Lab A4: Wed 4:40 – 5:30PM EPC 206 (TBD)

  4. Lab B2: Wed 12:20 – 1:10PM, MCS B33 (TBD)

  5. Lab B3: Wed 1:25 – 2:15PM, MCS B23 (TBD)

  6. Lab B4: Wed 2:30 – 3:20PM, MCS B23 (TBD)

Labs will be an invaluable part of the course involving interactive problem-solving sessions, tips on homework questions, and supplemental material not covered in lecture. Attendance is mandatory and will be taken.

Homework Assignments

Assignments and all other handouts can be found on our Piazza page, under the Resources tab.


  1. First day of class is Tuesday 5/9. See you all there!


Basic (high school level) calculus and algebra.

Academic Conduct

Academic standards and the code of academic conduct are taken very seriously by our university, by the College of Arts and Sciences, and by the Department of Computer Science. Course participants must adhere to the CAS Academic Conduct Code – please take the time to review this document if you are unfamiliar with its contents.

Collaboration Policy

The collaboration policy for this class is as follows.
You are encouraged to collaborate with one another in studying the textbook and lecture material.
As long as it satisfies the following conditions, collaboration on the homework assignments is permitted and will not reduce your grade:
Before discussing each homework problem with anyone else, you must give it an honest half-hour of serious thought.
You may discuss ideas and approaches with other students in the class, but not share any written solutions. In other words, the writeups you submit must be entirely your own work. You must also acknowledge clearly in the appropriate portion of your solutions (e.g., at the top of your writeups) people with whom you discussed ideas for that portion.
You may get help from TFs and undergrad assistants for the class for specific problems. Don’t expect them to do it for you, however.
You may not work with people outside this class (but come and talk to us if you have a tutor), seek on-line solutions, get someone else to do it for you, etc.
You are not permitted to collaborate on exams.
The last point is particularly important: if you don’t make an honest effort on the homework but always get ideas from others, your exam scores (accounting for the majority of your grade) will reflect it.


The course grade will break down as follows:

  1. Problem sets: 30%

  2. Midterms: 35%

  3. Final exam: 30%

  4. Lab attendance and participation in {lab, lecture, Piazza}: 5%

Last day to drop without a W: Oct 10. With a W: November 10. Our midterms are scheduled with these dates in mind.
Incompletes for this class will not be granted.

Exams: There will be two in-class midterms held during the middle of the semester, tentatively Thursday, October 5 and Tuesday November 14. The cumulative final will be held during the normal two-hour final exam slot: Saturday, Dec 16, 3-5PM for students in the 2PM lecture and Tuesday, Dec 19, 3-5PM for students in the 3:30PM lecture. All exams will be in MCS B12. Please make your end-of-semester travel plans accordingly!

Homework Assignments, Submission, and Late Policy: Assignments will typically be due Fridays at 5PM. You must submit a hardcopy no later than 5PM in the drop box on the first floor of the MCS building, near the CS department office. From the CS office, walk toward the shorter end of the hallway, and turn right. Drop box is immediately on your right. Assignments must go in the box, not on the shelves above, which is where we used to return assignments. Plan on assignments being due every week, except right after a midterm, tentatively Sep 14, Sep 21, Sep 28, Oct 12, Oct 19, Oct 26, Nov 2, Nov 9, Nov 30, Dec 7.

We will collect assignments and post solutions at 5PM sharp, so homework assignments will not be accepted late. Therefore, do NOT cut it close!!
To compute your homework grade, we will automatically drop the one lowest score from the assignments, so one bad homework grade is not the end of the world. However, we strongly recommend putting forth your best effort on all assignments, as they provide the best preparation for the exams. As you likely already know, assignments requiring substantial creativity can take more time than you expect, so plan to finish a day early.

Regrading Procedure: If, after reviewing the posted solutions, you still believe a portion of your homework was graded in error, you may request a regrade. Please write, on a PostIt, the problem number and a brief description of the incorrect deduction, stick it on your homework, and give it to one of the Professors in lecture. Note that when we regrade a problem, your score may go up or down.

Attendance: It is expected that you will attend lecture and the laboratory section for this course. Attendance will be taken in labs. On occasion, some material covered in lecture and lab will not be covered by our textbook. We ask that you please arrive in class on time, since it is disruptive to have students flowing in throughout the class period. Moreover, when students are at a borderline between grades, I will factor in attendance before making a final determination.


Discrete Mathematics and Its Applications, Kenneth H. Rosen, 7th Edition.


We will be using Piazza for class discussion. The system is highly catered to getting you answers to your questions fast and efficiently from classmates, the TF, and the instructors. Please do not email questions to the teaching staff – post your questions on Piazza instead. We also encourage you to post answers to student questions there (but obviously, not answers to problems on the problem sets!). Our class page is located at: https:piazza.combuspring2015cs131home. Please go there to sign up today. We will also use Piazza to post announcements and all handouts, including homework assignments and solutions.

(Tentative) Schedule

Lecture Topic Readings Homework Due Dates
9/5 The Foundations: Logic and Proofs Chapter 1
9/7 The Foundations: Logic and Proofs Chapter 1
9/12 The Foundations: Logic and Proofs Chapter 1
9/14 The Foundations: Logic and Proofs Chapter 1 HW 1
9/19 The Foundations: Logic and Proofs Chapter 1
9/21 The Foundations: Logic and Proofs Chapter 1 HW 2
9/26 Basic Structures: Sets, Functions, Sequences, Sums Chapter 2
9/28 Induction Chapter 5 HW 3
10/3 Induction Chapter 5
10/5 Midterm
10/10 No class (Columbus day)
10/12 Number Theory and Applications Chapter 4 HW 4
10/17 Number Theory and Applications Chapter 4
10/19 Number Theory and Applications Chapter 4 HW 5
10/24 Recursive Algorithms and Recurrences Chapter 5
10/26 Recursive Algorithms and Recurrences Chapter 5 HW 6
10/31 Recursive Algorithms and Recurrences Chapter 5
11/2 Recursive Algorithms and Recurrences Chapter 5 HW 7
11/7 Recursive Algorithms and Recurrences Chapter 7
11/9 Midterm 2
11/14 Counting Chapter 6
11/16 Counting Chapter 6 HW 8
11/21 Counting Chapter 6
11/23 No Class
(Happy Thanksgiving!)
11/28 Counting Chapter 6
11/30 Counting Chapter 6 HW 9
12/5 Probability Chapter 7
12/7 Probability Chapter 7 HW 10
12/12 Probability Chapter 7
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