## Tau Day Leftovers

6 July 2011

One week after Tau Day 2011 has come and gone, I thought a few things that came up in comment threads deserved a little more attention.

### 1. τ-doku

I mentioned the possibility of tau-themed sudoku.  Thanks to juanmah, I know that at least one such exists, and here it is.  Kudos to the puzzle creator and thanks for the link.  I’ve solved that puzzle, so I know that it works.  It’s quite easy, so you could give it even to sudoku novices to promote Tau Day.  One quirk of the linked puzzle is that it uses two colors so that the repeated digits in 6.2831853 can count as distinct symbols.

I wondered whether a more interesting puzzle could be found by not distinguishing the 8s and 3s.  What follows is my take on that idea.  I offer for your consideration my first ever sudoku puzzle.

(Edit: I mistyped one of the givens when I first posted this.  This is corrected now.  The 1 in row six was incorrectly given as a dot.  My apologies for any frustration this may have caused anyone.)

taudoku 1 (fixed)

The central row emphasis is purely decorative.  Note that, unlike the sudoku linked to above, the 3 and 8 are not distinguished by color here; this is a sudoku puzzle with only seven symbols.  Each row, column, and block will have two 3s and two 8s (and one each of the other five symbols).

Near the end of the construction process, I was forced to sacrifice some of the difficulty level that I wanted in order to maintain symmetry of the givens.  I’m not happy about that, but I don’t think it turned out too easy to be interesting.  As I said, it was my first try at sudoku construction, but it won’t be my last.

### 2. τ as a natural period

We casually call tau the circle constant, but let’s not forget that its ubiquity extends well beyond geometry (would it really be ubiquity otherwise?).  Here’s a simple demonstration of that.  In the comment thread I formulated this in terms of differential equations, but here let me be more casual.

Imagine two particles (infinitesimal bugs, if you like) on the number line, call them Alice and Bob.  Their starting positions can be anywhere (but let’s avoid the trivial case where both bugs start at 0; having one but not the other start at 0 is fine).  Then the bugs start moving.  At any point in time, Alice’s velocity is given by Bob’s position, and Bob’s velocity is the negative of Alice’s position (imagine that they continuously look at each other to decide how to move).  Quite a simple dynamical system, but if you actually simulate it, you’ll see something potentially unexpected.  The bugs may wander far from where they started, but they always come back; indeed, they will always both come back to their starting position at exactly the same moment, and then the cycle will repeat.  How long will it take for this repetition to occur , that is, what is the period of the motion?  No matter where you start the bugs, the period is always exactly τ time units.

Why is this happening?  Because the problem setup encodes the differential equation $y''+y=0$, whose solutions are linear combinations of sine and cosine (in natural units), which are both periodic with period τ.

### 3. Is τ the right symbol?

Xamuel (whose blog is excellent, by the way) called into question the symbol choice, on the grounds that pi has two legs while tau has only one.  Since the circle constant is twice pi and not the other way around, what we should want is a symbol with four legs, the argument goes.  This made me smile, because it was the very same objection that occured to me when I first watched Vi Hart’s anti-pi video, and I even emailed her to that effect.  However, in the course of writing that email, I realized the natural counter.  Read the legs as Roman numerals.  Pi parses as “horizontal squiggle over two” while tau parses as “horizontal squiggle over one”.  Quite consistent with our usage.

(On a less whimsical note, τ is a good choice because it stands for turn; a right angle measures $\tau/4$ because it is a quarter-turn.)

## Happy Tau Day 2011!

28 June 2011

Dear readers, I hope you’ve all had a wonderful Tau Day this year.

My Tau Day started with a mass email from Michael Hartl (creator of the Tau Manifesto, newly updated today), which linked to What Tau Sounds Like, a terrifically timely video from Michael John Blake.  If you haven’t watched it yet, you should.  (This video made the internet rounds today, including this CNN article which follows up on their Pi Day Half Tau Day article.)

As the CNN article reminds us, Tau Day is currently a holiday without any particular traditions or practices.  I’ve already done my part by proposing the official dessert.   Tau Day is a holiday with delicious potential.

blackberry / apple taue baked for Tau Eve 2011

I made that one yesterday (lesson learned: blackberry filling is runny, should have been the bottom layer).  Today I made this one.

blueberry-cherry taue, baked for Tau Day 2011

We haven’t cut into that one yet.  The next one I plan to make will have a blueberry spice layer and an apricot layer, but it may be a while before I feel the need for more pie, taue, or the like.

As Michael Hartl put it in the email, this is “the best Tau Day yet. (OK, it’s only the second one, but still!)”.  This is a very young holiday.  There is a lot of room for the creative here.  And we should all want Tau Day 2012 to be better than Tau Day 2011.  Poems whose word lengths come from the digits of pi have been around for quite some time.  What about tau-digit poetry?  On Pi Day Half Tau Day, I saw a sudoku variant based on the digits and shape of pi (though I can’t seem to find it now).  If there can be pidoku, there can and should be taudoku.  And that can be just the beginning.

We have 366 days until Taue Day 2012.  What are you going to do?

## 2 Days to Tau Day: A Problem Deliciously Solved

26 June 2011

Just two more days until Tau Day 2011!

If the Tau movement and its central tenet, that it should be tau and not pi which is foregrounded in our math curriculum, our discourse, our literature, our mathophile culture, are at all new to you, then you may  be filled with doubts and objections.

Is this really worth caring about?  Isn’t changing such a well-established notation totally infeasible? Won’t this just be more confusing for the children? And so on.  Every objection and problem I can think of is addressed in Michael Hartl’s Tau Manifesto.

Every problem except one.

The real unsolved problem is one of dessert.

Vi Hart lays the problem out with mouthwatering clarity in this video.  It seems like 1/2 pi radians should cover half a pie, but it doesn’t.  It only covers a quarter of a pie.  This captures the nonintuitive nature of pi very well (and a fortiori a reason why tau should be in the classroom).

But there’s a problem.  No one is suggesting that we change pi to mean 6.28and-so-on.  That would be horribly confusing.  And I doubt we can change the meaning of the word “pie”.  One solution might be to change units to double radians, or dradians.  Then a pie would contain pi dradians.  But remember, we have already seen that radians are the “right” units to use.

It’s a tricky problem.  Here is my solution.

I propose the taue, a double-decker pie.  (Just remember that a taue is like 2 pie, or if you’re really into this that a pie is like half a taue.)  Pi radians of a taue is the rough equivalent of a whole ordinary pie (on the basis of filling, of course; valuing the crust is subtler and ignored here).  Pi/4 radians (one eighth of a circle) contains a quarter of a pie in yummy fruit goodness.

It’s not obvious a priori that this will actually work.  Will the bottom layer of filling vent properly?  What will you actually bake the thing in, if you don’t have some kind of double-deep pie pan (which should be called a taue pan if it exists).

Well, I did my first official test of my proposed solutions to those problems yesterday (Saturday), and I’m here to tell you that progress tastes delicious.

• Solution to Problem #1: Use lattice crusts for the top and middle.
• Solution to Problem #2: I baked my concoction in a 10″ springform pan, because they have nice high walls.
• For the test run I used canned fillings.  One can of cherry on top, one can of apple on the bottom.  (Each layer had the amount of filling I’d ordinarily use in an 8″ or 9″ pie.)
• My crust was made from 3 3/4 cups of flour (I’m partial to King Arthur whole wheat), 3/4 C of ice water, 3/4 tsp salt, and 1 1/2 cups of organic salted butter, in the “usual” way.  Mix the dry, dice and cut in the butter, and add the water a little at a time
• I divided the dough into three balls (proportioned roughly 2:1:1 or maybe 5:2:2) and let chill a little less than 4 hours.
• I rolled out the  large ball of dough and used that to cover the bottom and sides of the pan.  Then the apple layer.  Then rolled out one of the small balls and made a lattice (three strips each way and two diagonals).  Then the cherry layer.  Then rolled out the other ball for the top lattice (five strips each way).

I wish I’d used less flour when rolling out the dough, and I could have been more careful making my lattice strips prettier, but I brought the taue to my parents’ house for tasting by myself, my wife, my parents, and my children.  Success.

With the test out of the way and successful, I’ll be making at least two “official” taues this Tau Day season: one on Tau Eve and another on Tau Day proper.  I’ll post pictures and recipes of those here.

P.S.

Do you have an objection or doubt not addressed in the Tau manifesto?  By all means raise it here.  I’ll address it if I can and forward it if I cannot.

## 4 Days to Tau Day: Tau and Its Digits

24 June 2011

Like so many mathematicians, I have lots of pi stuff, which is a little embarrassing now that I’m so overtly on Team Tau.  (Eesh.. how nerdy do you have to be to not want to wear a Super-Pi t-shirt in public … because it’s the wrong convention for naming the circle constant?)

My favorite, though, is that shirt … you’ve probably seen one … with the digits of pi formatted in the shape of pi.  (I’ve also seen the same for phi, the golden ratio, but I don’t own that.)  I wish I had the analogous thing for tau, but I don’t think anyone’s made that yet.

As a first step… here’s this, which I whipped up with Mathematica.

That was dashed off pretty quick, over lunch; I plan to make some prettier versions, including a mathematica notebook to automatically do the same thing for an image of your choice.

## 5 Days to Tau Day 2011: why use radians?

23 June 2011

Ah, friends, Tau Day draws ever closer.

Last time I talked a little about why some conventions are better .. righter .. than others, in a sense more meaningful than mere personal preference.

Circle constants are not just found in geometry.  They are ubiquitous in mathematics.  Everywhere you find yourself tripping over 3.1415somethings and 6.2831and-so-ons, in applied and theoretical stuff.  You probably call those numbers $\pi$ and $2\pi$, and I’d rather call them $\tau/2$ and $\tau$. The Tau Day claim is that the latter choice is better than the former.

There are lots of ways to make the case for the primacy of $\tau$ (and I’ll mention more as it gets closer), but the most concise is this: There are $\tau$ radians in a full circle.

This begs a question, of course.  How many angle-units are in a full circle depends on what units we use.  There are 360 degrees, or 4 right-angleses (much early Euclid-style geometric writing measured angles relative to the right angle), or 400 gradians, or 4 million myriogrades, if that’s your thing.  So we’ve traded “Why $\tau$?” for “Why radians?”.

But that’s a relatively easy question, and certainly a well-known one.  There are lots of reasons why radians are the most natural/desirable, and lots of places to read about them.  Even the wikipedia article does a decent job.  So let me just give you a few concise points.

1. The radian measure of an angle can be defined intrinsically as follows.  For any angle, draw a circle centered at the vertex, and compute the ratio of the arc length inside the angle to the radius of the circle.  (Hence the term radian ; the angle measure in radians tells you how many multiples of the radius are in the arc length contained in the angle.)  It seems like this might depend on how big you draw the circle, but a moment’s thought shows that it doesn’t, and you end up with a nonarbitrary angle measure.
2. The calculus-related behavior of the trigonometric functions is best when you work in radians.  For example, if you work in radians, the derivative of $\cos x$ is $\sin x$ and the derivative of $\sin x$ is $-\cos x$.  If you don’t work in radians, you get ugly correction factors.
3. (This is actually very closely related to the previous.)  If we work in radians, sine and cosine have the simplest possible power series, and it is easy to see (at least formally) the celebrated Euler’s formula $e^{ix} = \cos x + i\sin x$, where that e is the base of the exponential function, the one we met yesterday.
4. If we use radians, then the best first-order approximation to $\sin x$ is, if $x$ is small enough, simply $x$ itself.  (That is, $\sin x\approx x$ if $x\approx 0$.)  For any other choice of units, there’s a correction factor.

A parting math-Zen koan, especially for those of you who haven’t thought about this much.  I have asserted (and do deeply believe) that there is a canonical, nonarbitrary right unit for measuring angles.  What is the analogous best unit for measuring length?

## 6 days to Tau Day

22 June 2011

Even now, a select group of mathematicians and mathophiles are counting down to what should (in my opinion) be one of the principal holidays on the mathematician’s calendar.  You’ve all heard of Pi Day, some of you even celebrate it in one way or another (the number of my facebook friends signed up for “The Only Pi Day of Our Lives” surprised me), and it grieves me to say so, but it’s overrated.  At best it should be a minor math holiday.  If you celebrate only one circle-constant-related holiday in a year, it should be Tau Day.  And if you use only one Greek letter as shorthand for a particular constant which is related to circles and mathematically ubiquitous, better that it be $\tau$, the circumference of a circle with unit radius (approximately 6.28318531, which is twice that other number).

Tau Day is 6/28 (by analogy with Pi Day Half Tau Day, 3/14), so our countdown stands at 6 days ($\lfloor \tau\rfloor$ days, if you will).

I won’t get into the details here, but the key contention is that τ, not π, is the circle constant we should focus on.  Instead of $C = 2 \pi r$ (or $C=\pi d$), it would be better to write $C = \tau r$.  For a quick and cute introduction, check out the Pi is Still Wrong video from everybody’s favorite mathemusician, Vi Hart.  For the whole manifesto, check out Michael Hartl’s wonderful site (updated a few months ago for Pi Day Half Tau Day).

Especially for the non-mathematicians here, “should” and “better” might seem strange word choices.  Today I want to focus on what exactly they might mean in this context.

(Remark: If you’re more science-minded, this is a little like the oft-heard-in-some-circles assertion that the convention for positive and negative charges is “backwards”, the opposite of what it “ought to be”.  Apropos xkcd comic!)

## Counting on Monsters

28 September 2010

While we’re on the subject of books, here’s a book for the smaller mathematicians in your life.  The ones who can’t necessarily spell “mathematician”.

The book is You Can Count on Monsters by Richard Evan Schwartz (ISBN 1568815786), and it’s a picture book about prime number decompositions.  It’s much more colorful than you’re picturing, I promise.

One thing that makes this book so nice is that it doesn’t beat you over the head with anything.  There are some very basic remarks about primes and multiplication at the beginning  and some slightly deeper remarks at the very end, but the vast bulk of the pages have no text at all.

Each number from 1 to 100 gets a double page.  On the left is the the number and a configuration of that many dots (usually clustered into spirals or some such in an interesting way).  On the right is a whimsical (and strangely compelling) drawing of a monster.  For a prime number, the monster is some simple monster which smoehow embodies the nature of the number (the right number of teeth, or legs, or whatever).  For a composite number, the monster is in some way a conglomeration of the corresponding prime monsters.  (So the 70-monster incorporates the natures of the 2-, 5-, and 7-monsters.)

There’s plenty to stare at, plenty of patterns sitting right near the surface, and plenty more lurking underneath to be discovered over time.  An artistically-inclined child might try to invent prime monsters larger than 100, or to draw some composite monster.  (I’ll confess that when I bought the book, I entertained myself for quite some time trying to draw the 1001 monster.)  It’s just fun to look at, and it provides lots of interesting avenues for mathematical conversation with “grown-up” mathematicians.

Speaking as a number theorist, I love the way this book conveys the essential predictable/chaotic dual nature of prime numbers.  There are always more monsters, but you’re never quite sure when you’ll meet the next one.