How to teach someone how to prove something
In a couple of my posts (most recently here), I’ve talked about the need for a course early on in undergraduate math classes on proof techniques.
The goals of the class are two-fold: first, teach the students basic skills, and second demystify the concept of proof. The students should come away from the class thinking, no it’s not magic, and I’ve learned how to do this stuff, and there are a few basic techniques which seem to come in handy.
Today I want to go further into what a curriculum for such a course might look like.
And I will, in a moment, but first I want to explain something. It’s actually a really important and dangerous question, how to teach such a course, because it could go wildly wrong, and sometimes does. From my commenter Jordan:
… “Numbers, Equations, and Proofs,” which I started at Princeton in 2002 and which is still going as well. Though here’s an interview with a dude who was an ace math competition dude and found the course so hard as to drive him out of the math major! So maybe it’s no longer as “for everyone” as I designed it to be….
This struck me, how perverted Jordan’s class became. For that matter, Math 55 at Harvard could have started out as a good idea as well, but by the time I got to Harvard as a grad student it was the reason so few math majors ever stuck at Harvard and why there were especially few women.
I remember Noam Elkies taught it while I was there and was famous for asking questions in class and getting students to compete to answer them quickly. It makes sense that he’d run a class like this, because he’s so fast and clever, and he’s naturally wondering, am I the fastest and clevererest of them all? But rather than a place where proof is demystified and people feel safe asking dumb questions, he’d created the polar opposite, a live quiz show of clever competition. Ew!
In order to combat this downfall and decay, I think the class needs to have a clearly stated mission as well as built-in curriculum requirements that works against ostentatious displays of cleverness, which indeed only serve to further the “I got it but you don’t” stereotype of math skills (but which mathematicians themselves are incentivized to further since that magical aura comes in handy).
For example, when I taught it, I let the students hand in homework again and again until they got a score they liked. Of course, this depending on me having an awesome grader (and a relatively small class), which luckily I had.
Also, I asked each student to give a presentation to the class on some proof they particularly enjoyed, and I sat through a preview of their presentation and gave them extensive advice on board work and eye contact, which took a lot of work but really helped them prepare and also boosted their egos while at the same time increased their sympathy with each other and with me.
But of course the most important thing was that I clearly stated at the beginning of each class in the first two weeks that proving things in math was a skill like any other that you get good at through practice. And when I left Barnard Dusa McDuff took over the class and still teaches it, so I know it’s in good hands.
If I hadn’t had Dusa, I’d probably have written a manifesto to be given to each person who would teach the class after me. Of course anyone could have just thrown that away but it’s an idea.
As for content, I taught them really basic proof techniques, so induction, proof by contradiction, the pigeon-hole principle, and some epsilon-delta practice. We covered some basic logic, graph theory, group theory, ordinals, and basic analysis. We constructed the reals two ways and the complex numbers once and talked for a long time about whether “i” is real and what that even means. We used A Transition to Higher Mathematics, which I recommend with a few reservations (please tell me if you’ve found a better text for something like this!).
Everything was done super explicitly and carefully, no rushing. I said things three times in three different ways. I wasn’t expecting people to be fast or clever, because I know intelligence works in different ways and that this stuff was completely new to most of the students. And at least one student in the class, who had been an artist, is now a grad student in math at Berkeley.
Looking over my post I realize I spent way more time talking about the tone of the class than the content, but that’s totally appropriate, since I think of this class as an introduction to the culture of mathematics (or rather the culture I wish we had) just as much as mathematics itself.
After all, there really is no time limit on good ideas, and you do get to do it over if you make a mistake, and going over things slowly gives you more time to ask good questions and find mistakes.