ECS 120 – Theory of Computation – Spring 2023 – Phillip Rogaway

You are responsible for all information on this webpage.

Where and when we meet


See the online doc for current office hours


The course webpage is a click from my personal hompage. It is not on Canvas.


The anticipated breakdown is I consider class attendance to be mandatory, but will not be monitoring it in determining or your grade.

In assigning grades I may deviate from the stated numerical percentages if I see a good reason to.

The class is effectively curved. In the past, median grades on exams were low compared to what contemporary students expect, often just over 50%.

No classroom electronics

Violating these norms will be treated like any other form of academic misconduct, as described below. Your phone addiction probably isn’t worth that F.

While the above might sound draconian to some, it is supported by a body of research that demonstrates how extreme is the distraction from that stupid phone of yours, even when it is on your person and not in use. It is also supported by my personal experience, which is that that classes evolve a far better dynamic when the classroom is as tech-free as possible.

No homeworks?!

For decades I have given weekly, challenging homeworks designed to develop students’ creative problem-solving skills. The difficulty was dialed in so that the top few students might get everything on a problem set, while average students would be able to solve some problems but not others, even if they work quite hard. While this type of homework used to be routine, many contemporary students find this type of homework unreasonable and stressful (mostly because they are obsessed with grades, and imagine that I find this absurd, but what can you do? For the first time ever, I have decided to eliminate the homework.

Well, not quite. I will still put out homework assignments each week. While homeworks will not count for your grade, I encourage you to try to solve all the homework problems I give out. The TAs will discuss the homeworks in office hours. Expect to see related questions on exams. I will even set things up so that you can turn in your solutions. Please turn in only clear, correct solutions. Homeworks that get turned in will not get grades, but may get comments.

I know quite well that students are more willing to spend long hours hacking in front of a stupid machine than calmly thinking about a theory problem beneath an oak tree. Maybe try to give the other mindset a try?

Given the sort-of elimination of homeworks, I don’t plan to make up any new homework assignments: I’m just going to recycle homeworks from a prior year.


The recommended text is by Michael Sipser, Introduction to the Theory of Computation. We will cover most of what’s in Chapters 0–5 and 7. It’s a good book that used to be unconscionably expensive, but now seems fine.


Be aware that you might be inadvertently captured on camera.

I expect our class to be recorded, with recordings put online. Many students think this means that there’s no need to come to class. Experience tells me that this belief is wrong: students who miss classes usually do terribly.


ECS 20 is a serious prerequisite in the sense that you will not do well in this course if you do not have a level of mathematical maturity consistent with having taken, and understood, ECS 20. In particular, you need to be able to understand and create proofs. If you are a CS major who has trouble with math, consider taking some or all of your math electives before taking this class.

Academic misconduct

For exams you may not sit next to a friend. “Friend” means anyone you’ve discussed course-material with and whose name you know. “Next to” means that the person is the person on your left, right, front, front-left, or front-right.

See “Classroom electronics” for rules about phones, laptops, and the like.

If you turn in a homework, you may not have consulted any old solutions. Acknowledge any source from which you got important ideas.

I refer all academic misconduct matters to Judicial Affairs (OSSJA), and some terms there have been many referrals. While I personally believe that those who cheat should be dismissed from the university, I have no control over that. All I can do is give you an F in the class, which is what I do.

If you are guilty of academic misconduct, either by your own admission or by a finding from OSSJA, I will assign you an F grade in the course. This is in addition to whatever OSSJA does. I call this the misconduct-implies-F policy. I started this policy years ago, which has become the default policy across the CS Department.

If you are having personal or academic problems motivating you towards academic misconduct, please come and talk to me instead. I am not such an ogre. (Well, is that not what all ogres claim?)


I’ve heard some students claim this to be the hardest class they take at UCD. So let us give you a few hints explaining what I am after and how to do well.

First, I really want you to think. Don’t try to solve the problems by doing some sort of “pattern matching.” It might sound like the quickest way to get there, but it just doesn’t work. This course is about learning a certain sort of problem-solving skill more than about learning some specific material. Keeping this in mind may help put things into better perspective.

Even more than with other courses, it’s important not to fall behind. Maybe the Friday quizzes will keep that from happening.

I would suggest that you take no notes. The material is easily found in the recommended book or in other books or online resources, and I think it works best to sit back, listen, and follow. I myself will distribute my notes, and maybe some students who just loves to take beautiful notes will share theirs online.

Parting thoughts

This is my favorite technical class in the CS/CSE curricula. We get to forget about those messy physical machines that have lead humankind to ever greater distraction and mischief and, instead, address the question of what is computation. A subject that would be interesting even without any physical computers. What could be more interesting or fun?

I expect that this will be my last term at UCD. It is a bittersweet time. There have been many good things about spending half a lifetime at UCD, but many bad things, too. Perhaps we will pause every now and then to discuss such matters. Perhaps you too are aware that university teaching is too much concerned with conveying facts and techniques, and too little concerned with providing perspective or encouraging genuine introspection.