Introducing “Thirdsday”

I’m pleased to announce that the greatest holiday in mathematics is almost upon us: the jubilant festival known as THIRDSDAY!


Thirdsday is that magical day on which we celebrate the wonder and mystery of the fraction 1/3.

How come I haven’t heard of it before?

Don’t feel bad; I didn’t know about it either until it was discovered a couple of months ago. Or was it invented? Math can be so ambiguous that way. Continue reading

Stance and distance in popular writing about math

Here’s something you’ll never see in popular writing about musicians:

“Music. For most of us, the mere word conjures up memories of metronomes and endless scales, the student’s never-ending fear of playing a wrong note (or the right note at the wrong time), and the frowns of teachers from whom a curt ‘Good’ was the highest expression of praise. And yet there are some people who just can’t get enough of making music; they practice hour after hour, honing their skills and punishing their bodies, long after the stage of life when there are parents and teachers forcing them to do it. What strange quirk of character compels this behavior?”

As I said, nobody writes about musicians that way. And yet, when the subject is a mathematician, writers sometimes come up with passages like this:

“You and math – one of the greatest love/hate relationships of all time. What is it about the subject that excites us yet sends a chilling tingle down our spine at the same time? How can it be so precise, yet so fickle? We may never know the answers to these questions, but we do know that math is ubiquitous, though some of us may try to hide from it.”

This is from the introduction to a 2010 profile of mathematics editor Vickie Kearn, which I recently saw on the blog-site of Princeton University Press, where Ms. Kearn has worked as senior editor for many years. The intro continues in a similar vein. “While math may sometimes cause us to cry tears of despair, it has caused Vickie to cry tears of joy.” Continue reading

Between the World and the Mind

The wizard’s-cap graphic that appears at the top of my blog as part of the logo is a piece of an infinite mathematical surface called the pseudosphere.

I don’t study the pseudosphere in my research, and I can’t say I have a lot of intuition about it; in fact I don’t especially like the thing. So why did I choose it to visually represent what this blog is about? Continue reading

ChipChip: A new sort of sorting

A uniquely French way to express contempt for someone is to call them an “espèce d’espèce” (see Endnote #1); literally, “a sort of a sort”.  This month I’m going to tell you about a sort of a sort (or rather, a sort of sorting) that, from a practical standpoint, merits this degree of contempt: the procedure is ambiguous, is annoyingly slow, and doesn’t always sort things correctly. Yet there’s an unresolved mathematical mystery arising from the way that the procedure works better than it has any right to.

But first, a puzzle:

Continue reading

A New Game with Infinity

Aleph-null bottles of beer on the wall,
Aleph-null bottles of beer!
Take one down, pass it around,
Aleph-null bottles of beer on the wall!

— Math nerd drinking song

You may already know the standard story about infinite sets like {1,2,3,…} and {2,3,4,…}. Even though the second set seems to be smaller (it’s missing one of the elements in the first set), Cantor taught us that the two sets are the same size (in the sense that there’s a one-to-one correspondence between them). The two sets have the same “number” of elements (namely aleph-null), and aleph-null minus one equals aleph-null. For many students, that anomaly takes some getting used to.


Cartoon by Ben Orlin, now the author of (I urge you to buy a copy rather than steal one, since there are only finitely many copies.)

But there’s a perfectly respectable mathematical sense in which the two sets do not have the same number of elements. With a suitable notion of what it means to “count” the elements of an infinite set of numbers, different from Cantor’s, the size of {2,3,4,…} is smaller than the size of {1,2,3,…}; in fact, it has one fewer element. Likewise, in this alternative way of measuring how big sets of numbers are, the set {1,3,5,…} is slightly bigger than the set {2,4,6,…}. How much bigger? Half an element! (Though see Endnote #2.) Continue reading

Knots and Narnias

Say you’re walking north across a meadow surrounded by hills when you come across a solitary doorframe with no door inside it. Stranger still, through the doorway you see not the hills to the north of the field but a desert vista. Consumed by curiosity and heedless of danger, you cross the threshold into the desert. The sun beats down on your bare head; you see a vulture off in the distance. In sudden panic you spin around; fortunately the doorway is still there. You run through the doorway back into the field, grateful that the portal works both ways.

Now what?

You cross through the doorway into the desert again, and turn around, and once more you see the doorframe behind you, and through it, the southern hills surrounding the meadow. But now a question occurs to you: what would you see if, staying in the desert world, you went around the doorframe, and looked through the portal from the other side? What kind of new world would you see? Arctic tundra, maybe?
Continue reading

A pair of shorts

This month I wrote two short essays for The Aperiodical‘s Big Internet MathOff: “The Mystery of the Vanishing Rope Trick” and “Cantor’s Paradise Meets Skolem’s Paradox”. Whittling an essay down to a thousand words is hard but it’s good exercise!


Have you ever done something impossible?

About twenty-five years ago I invented an impossible rope trick by accident, and afterwards I could never figure out what I’d done or get the trick to work again. It wasn’t the rope that had vanished, but the trick itself. The incident took place at a math conference in Amherst, Massachusetts, and no, I hadn’t been drinking, though I admit that it was late at night and I was tired.
Continue reading