

Why pi? - pibefision
http://stanford-online.stanford.edu/seminars/knuth/101206-knuth-500.asx

======
fredoliveira
Since the article links straight to the video stream (OP: please add a note to
the title), here's Knuth's introduction to this particular Christmas Tree
Lecture (on Pi):

"Mathematicians have known for almost 300 years that the value of 'n
factorial' (the product of the first n positive integers)is approximately
equal to n/e to the nth power times the square root of 2(Pi)n, where Pi is the
ratio of the circumference of a circle to its diameter. That's 'Stirling's
approximation'. But it hasn't been clear why there is any connection whatever
between circles and factorials; the appearance of Pi has seemed to be purely
coincidental, one of the amazing accidents of nature. Recently however Johan
Wästlund has found a nice explanation of this phenomenon, and the reason turns
out to be connected with the theory of trees."

~~~
JoeAltmaier
The formulae are similar; are they really related? Any more than the dance of
bees are related to quantum mechanical states (another recent article)?

I'm sorry, I couldn't sit through the whole lecture (2 minutes actually); the
digressions and glacial pace reminded me viscerally why I had such trouble in
graduate school.

~~~
ximeng
It gets going after the first 2 minutes, not many digressions, pretty much
pure maths lecture. Summary below of the first hour or so if you want it,
might be hard to follow, but it shows the depth of the lecture pretty well.

He will establish a link between Pi and factorials, as well as trees, using
generally simple maths.

\---

The number of (ordered) forests with n nodes:

n=2 - 2 forests [o, o; o (o)], where , separates trees within forests, ;
separates forests, () indicates child node

n=3 - 5 forests [o, o, o; o, o (o); o (o), o; o (o) (o); o (o (o))]

In general the number of forests with n nodes is Cn where Cn is the nth
catalan number. For n=0..4, Cn=[1, 1, 2, 5, 14].

The nth catalan number is C(n):=((2n)!/n!(n+1)!). E.g. for n=3, 6!/(3! _4!)=5.

It turns out that:

C(n) approx= Exp(4,n)/(Sqrt(n_pi) _(n+1))

For n=1000, the first 4 significant figures match in a number with 598 digits.

Johan Wästlund published a result explaining the link between Pi and trees in
Dec 2007.

\---

Problem inspiring JW's investigation is biasing two dice (changing their
probability) so that 2..12 are all equally likely.

To do this set the probability of two 1s to x. This is the probability of
rolling a total of 2.

Now the probability of a total of 3 is the probability of a 1 and a 2, times
two. So we want the chance of rolling a 2 to be half the probability of
rolling a 1.

Similarly the probability of rolling a total of 4 is the probability of a 1
plus a 3 times two, plus the chance of two 2s.

If we set p1 = probability of a 1, p2 = probability of a 2, ...

(1) p1p1 = p1p2 + p2p1 (since a total of 2 is as likely as a total of 3) => p2
= p1/2

(2) p1p1 = p1p3 + p2p2 + p3p1 = p1p3 + p1p1/4 + p3p1 => p3 = 3p1/8

(3) ... p4 = 15p1/16

(4) ... p5 = 35p1/128

(5) ... p6 = 63p1/256

This set of probabilities satisfies the original problem up until a total of
6.

( ^ ) Note the constants are: 1/2, 1/2 _ 3/4, 1/2 * 3/4 * 5/6, 1/2 * 3/4 * 5/6
* 7/8.

( ^^ ) Proof of this: Let pj=a(j-1)p1. We have a0=1, a1=1/2.

Form an infinite series, A(z)=a0+a1z+a2Exp(z,2)+a3Exp(z,3)+... Square it:
Exp(A(z),2)=a0a0+(a0a1+a1a0)z+(a0a2+a1a1+a2a0)Exp(z,2)+... This equals
1+z+Exp(z,2)+Exp(z,3)+... (by hypothesis, ajp1=p1 for all aj), which is just a
geometric series with total 1/(1-z).

(+) So A(z) = 1/(sqrt(1-z)) = Exp(1-z,-1/2). Using binomial theorem, this
equals

Sum(n=0..infinity) of (-1/2 choose n [binomial coefficient])Exp(-z,n).

an=(-1/2 choose n) _Exp(-1,n)

E.g. a3= (-1/2 - 3/2 - 5/2)/(3 _ 2 * 1) * Exp(-1,n) = (1+3+5) / (2 * 3!),
which is the result we had above at ( ^ ).

( ^^^ ) We know p1+p2+p3+p4+p5+p6=1 as the dice are six-sided. This gives p1p1
_(a0+a1+...+a5) = 1 (see ( ^^ )). So what's a0+a1+...+a5?

Take A(z)(1+z+Exp(z,2)+...) = a0+(a0+a1)z+(a0+a1+a2)Exp(z,2)+... . Let's call
this S1 + S2z + S3z + ... . Then we want to find S6 = a0 + ... + a5. Remember
that p1p1s6=1 from ( ^^^ ).

Well by (+) A(z)(1+z+Exp(z,2)+...)=Exp(1-z,-1/2) (1+z+Exp(z,2)=Exp(1-z,-1/2)
Exp(1-z,-1). This is just Exp(1-z,-3/2).

Expanding that, we have Sum(n=0..infinity) of (-3/2 choose n)_Exp(-z,n). The -
signs cancel out again.

Let's work out the values of Sn: S1=1, S2=a0+a1=3/2, S3=a0+a1+a2=15/8=3/2 *
5/4 ... and S6=3/2 * 5/4 * 7/6 * 9/8 * 11/10.

Picture here:

    
    
      6zzzzzz
      5""""""zzzzz
      4oooooo"""""zzzz
       xxxxxxooooo""""zz
      3xxxxxxooooo""""zz
       ######xxxxxoooo""zz
      2######xxxxxoooo""zz
       ######xxxxxoooo""zz
       ......#####xxxxoo""z
       ......#####xxxxoo""z
      1......#####xxxxoo""z
       ......#####xxxxoo""z
         1      2   3  4 56 (each number j has width / height a(j-1)), total width = S6)
    

This is meant to be symmetric around x=y. Each different character represents
a different total. All these areas are the ones we've been trying to make the
same: "." is the probability of getting a total of two on two dice. "#" is the
probability of a 3 total, and so on. It gets closer and closer to a circle.

So this is the link to the circle. If n is the area shown in the diagram
above, we have n approx= (pi/4) Exp(Sn,2), since this is a quarter of a circle
with radius Sn. We want to prove the circle goes through the rectangles just
outside the edge, if so we can show that n - 1 < (pi/4) Exp(Sn,2) < n + 1 (!)
. This is because we are setting the area of each total (i.e 2..7) to be 1, so
adding an extra total (which looks more and more like a circle as n gets
bigger) will just add one to the total, and we are going to prove an ideal
circle would go through the outer rim (e.g. the "z"s in the diagram above).

Let's look at some examples.

Exp(S6,2) = Exp(3/2,2) * Exp(5/4,2) * Exp(7/6,2) * Exp(9/8,2) * Exp(11/10,2)
from above.

Exp(S9,2) = Exp(s6,2) * Exp(13/12,2) * Exp(15/14,2) * Exp(17,16,2)

Note Exp(1+1/2k,2)=1+ 1/k + 1/(4 _Exp(k,2)) > 1 + 1/k

So Exp(S9,2) > Exp(S6,2) (1+1/6)(1+1/7)(1+1/8) =
Exp(S6,2)(7/6)(8/7)(9/8)=Exp(S6,2)(9/6) by cancelling. He also proves that
Exp(S9,2) < Exp(S6,2)_(8/5).

In general Exp(Sa,2) _(a+b)/a < Exp(s(a+b),2) < Exp(Sa,2)_((a+b-1)/(a-1)).
From this inequality (ineq), he can prove his result on the approximate value
of Sn, (!) above.

He wants the circle of radius S6 to go outside the inner most notches in the
diagram (between " and z in the diagram above). Note that these coordinates
(S5,S1), (S4,S2), ... . By Pythagoras we want Exp(Sk,2) + Exp(S(n-k),2) <
Exp(Sn,2) < Exp(Sk, 2) + Exp(S(n+1-k),2), this follows from his inequality
(ineq) above.

(Incidentally you can do the same construction with 3 dice forming a ball,
there is a picture at 56:00 which I won't try to reproduce in ascii.)

\---

So we now have a relationship between Sn and Pi, Sn approx= 4n/Pi. Also Sn is
a sum of values of an, which are just (1/2) (3/4) ... (2n-1/2n). Finally Sn=2n
_an (not quite sure why??). From all this we can get an=1/(Sqrt(Pi_ n).

The number an is famous in connection with a random walk of length n. The
probability of flipping a coin 2n times and getting equal number of heads and
tails is an. This probability is (2n choose n)/Exp(2,2n). But (2n choose n) is
just ((2n) (2n-1) ... 1) / (n! n!), which is just an an above.

Rather than a coin, he takes the binary digits of pi/4 as heads and tails:

    
    
       .11001001...
    

This looks like this (- marks 0)

    
    
         x
        x x x
       x---x-x-x- etc.
              x x
                 x
                  x
    

We know Sum(ak, a(n-k))=1.

This is equivalent to (apparently!) Sum((2k choose k) * (2n-2k choose
n-k)=Exp(n,4)

(A digression here on "balanced random walks" and the links to 2d barcodes
like QR-codes. You decompose a finite random sequence of bits like the bits of
pi, into two parts. The first part is the sequence up to the last point the
sequence crosses the origin, this part is balanced. The second part diverges
from zero and needs to be balanced by flipping some bits according to a
"Christmas tree pattern".)

\--- c. 1h15m in at this point.

It gets a bit sketchy from here, but he goes to the Stirling formula from
here, which uses the Catalan numbers.

------
wallflower
> Now I will go on with my own experience as a youngster in mathematics.
> Another thing that my father told me--and I can't quite explain it, because
> it was more an emotion than a telling--was that the ratio of the
> circumference to the diameter of all circles was always the same, no matter
> what the size. That didn't seem to me too unobvious, but the ratio had some
> marvelous property. That was a wonderful number, a deep number, pi. There
> was a mystery about this number that I didn't quite understand as a youth,
> but this was a great thing, and the result was that I looked for pi
> everywhere.

When I was learning later in school how to make the decimals for fractions,
and how to make 3 1/8, 1 wrote 3.125 and, thinking I recognized a friend,
wrote that it equals pi, the ratio of circumference to diameter of a circle.
The teacher corrected it to 3.1416.

I illustrate these things to show an influence. The idea that there is a
mystery, that there is a wonder about the number was important to me--not what
the number was. Very much later, when I was doing experiments in the
laboratory--I mean my own home laboratory, fiddling around--no, excuse me, I
didn't do experiments, I never did; I just fiddled around. Gradually, through
books and manuals, I began to discover there were formulas applicable to
electricity in relating the current and resistance, and so on. One day,
looking at the formulas in some book or other, I discovered a formula for the
frequency of a resonant circuit, which was f = 1/2 pi LC, where L is the
inductance and C the capacitance of the... circle? You laugh, but I was very
serious then. Pi was a thing with circles, and here is pi coming out of an
electric circuit. Where was the circle? Do those of you who laughed know how
that comes about?

I have to love the thing. I have to look for it. I have to think about it. And
then I realized, of course, that the coils are made in circles. About a half
year later, I found another book which gave the inductance of round coils and
square coils, and there were other pi's in those formulas. I began to think
about it again, and I realized that the pi did not come from the circular
coils. I understand it better now; but in my heart I still don't know where
that circle is, where that pi comes from.

-Richard Feynman

"What is Science?"

1966 Lecture

<http://www.fotuva.org/feynman/what_is_science.html>

~~~
crististm
Very nice indeed. From the begining I felt it sounded like Feynman - he was an
interesting guy.

------
njonsson
On a purely superficial note, here are Knuth's recent Google searches,
according to the Safari search box dropdown list that flashes by at 2:22:

• google calculator

• wolfram alpha

• williams cutlery

• craig gentry

• "the lost chord"

• acrobat scroll speed

• html bulleted list

• html subscript

• doerr "protocol partition number"

~~~
yycom
How do you know they are his?

~~~
chanux
The weird wallpaper can be a sign it's Knuth's Computer :)

------
aeurielesn
You should add "[video]" (or alike) to the title at least.

~~~
chanux
Just in case, the poster is not around, even the moderators can do that,
right?

------
michael_dorfman
Dupe: <http://news.ycombinator.com/item?id=2008182> and:
<http://news.ycombinator.com/item?id=1987994>

------
burgerbrain
`mplayer
"mmsh://proedvid.stanford.edu/knuth/musings/101206/101206-knuth-500.wmv?MSWMExt=.asf"`

For those of you who hate it when your browser decides to try to embed
videos...

~~~
orlandu63
Thanks!

Also,

> mplayer -dumpstream
> "mmsh://proedvid.stanford.edu/knuth/musings/101206/101206-knuth-500.wmv?MSWMExt=.asf"

for for those you who'd like to download the video for later viewing.

------
techiferous
I thought this was going to be pi's rebuttal to tau. :)

~~~
blahedo
Me too, although in that vein I'll note that the π in the formula is actually
a 2π, so the τists win again. ;)

------
Zolomon
What's the type of pen he uses while writing/drawing? I really would like to
have one of those.

~~~
jim_h
It looks like a permanent marker felt tipped pen. It's probably a Sharpie or
Pilot brand.

I highly recommend the new Sharpie 'Pen' if you're looking for a nice writing
pen. (This is not the one he's using.) <http://pen.sharpie.com/>

------
henning
Good to see that Knuth's mind is still as sharp as ever.

~~~
willing4it
so hard to listen to.

------
tibbon
Very odd. My browser (Chrome Mac 8.0.552.237) downloads the page
(101206-knuth.asx) instead of showing it. Is this a bug on my end or on the
hosting end?

EDIT: Nevermind, just realized I was expecting a web page and its a streaming
media file. I confused the extension with .asp

~~~
fredoliveira
It's not a bug, that's actually a video stream. Open VLC, and then use Open
Network to open the link. It'll stream the video lecture.

------
Lagged2Death
But what about Preset 3? Has it, at long last, been stored?

