# The Lumber Room

"Consign them to dust and damp by way of preserving them"

## Negative probabilities

Was planning to write, but just dumping open tabs for now.

Negative Probabilities by Dan Piponi (sigfpe)’s blog A Neighborhood of Infinity: as usual, the clearest exposition.

Article by Feynman

Abramsky and Brandenburger, An Operational Interpretation of Negative Probabilities and No-Signalling Models. Gives an interpretation. I read this (section 2) and thought I understood, but when I tried to explain in my own words on a new example, faltered. Should read this again.

Gábor J. Székely, Half of a Coin: Negative Probabilities. Simple example, and an interpretation as “difference”.

Mark Burgin, Interpretations of Negative Probabilities. [Haven’t read]

Post by John Baez. Less than his usual standard. :-(

Written by S

Thu, 2014-08-28 at 11:21:06 +05:30

Posted in mathematics, unfinished

## Colliding balls approximate pi

Found via G+, a new physical experiment that approximates $\pi$, like Buffon’s needle problem: The Pi Machine.

Roughly, the amazing discovery of Gregory Galperin is this: When a ball of mass $M$ collides with one of ball $m$, propelling it towards a wall, the number of collisions (assuming standard physics idealisms) is $\pi \lfloor\sqrt{M/m}\rfloor$, so by taking $M/m = 10^{2n}$, we can get the first $n+1$ digits of $\pi$. Note that this number of collisions is an entirely determinstic quantity; there’s no probability is involved!

Here’s a video demonstrating the fact for $M/m = 100$ (the blue ball is the heavier one):

The NYT post says how this discovery came about:

Dr. Galperin’s approach was also geometric but very different (using an unfolding geodesic), building on prior related insights. Dr. Galperin, who studied under well-known Russian mathematician Andrei Kolmogorov, had recently written (with Yakov Sinai) extensively on ball collisions, realized just before a talk in 1995 that a plot of the ball positions of a pair of colliding balls could be used to determine pi. (When he mentioned this insight in the talk, no one in the audience believed him.) This finding was ultimately published as “Playing Pool With Pi” in a 2003 issue of Regular and Chaotic Dynamics.

The paper, Playing Pool With π (The number π from a billiard point of view) is very readable. The post has, despite a “solution” section, essentially no explanation, but the two comments by Dave in the comments section explain it clearly. And a reader sent in a cleaned-up version of that too: here, by Benjamin Wearn who teaches physics at Fieldston School.

Now someone needs to make a simulation / animation graphing the two balls in phase space of momentum. :-)

I’d done something a while ago, to illustrate The Orbit of the Moon around the Sun is Convex!, here. Probably need to re-learn all that JavaScript stuff, to make one for this. Leaving this post here as a placeholder.

Or maybe someone has done it already?

Written by S

Mon, 2014-06-23 at 23:03:18 +05:30

Posted in mathematics, unfinished

## Prefatory apprehension

Robert Recorde’s 1557 book is noted for being the first to introduce the equals sign =, and is titled:

The Whetstone of Witte: whiche is the seconde parte of Arithmeteke: containing the extraction of rootes; the cossike practise, with the rule of equation; and the workes of Surde Nombers.

Its title page (see http://www.maa.org/publications/periodicals/convergence/mathematical-treasures-robert-recordes-whetstone-of-witte, see also the full book at https://archive.org/stream/TheWhetstoneOfWitte#page/n0/mode/2up) contains this verse:

 Original spelling Though many ſtones doe beare greate price, The whetſtone is for exerſice As neadefull, and in woorke as ſtraunge: Dulle thinges and harde it will ſo chaunge, And make them ſharpe, to right good vſe: All arteſmen knowe, thei can not chuſe, But uſe his helpe: yet as men ſee, Noe ſharpeneſſe ſemeth in it to bee. The grounde of artes did brede this ſtone: His vſe is greate, and moare then one. Here if you lift your wittes to whette, Moche ſharpeneſſe thereby ſhall you gette. Dulle wittes hereby doe greately mende, Sharpe wittes are fined to their fulle ende. Now proue, and praiſe, as you doe finde, And to your ſelf be not vnkinde. Modern spelling Though many stones do bear great price, The whetstone is for exercise As needful, and in work as strange: Dull things and hard it will so change And make them sharp, to right good use: All artsmen know they cannot choose But use his help; yet as men see, No sharpness seemeth in it to be. The ground of arts did breed this stone; His use is great, and more than one. Here if you lift your wits to whet, Much sharpness thereby shall you get. Dull wits hereby do greatly mend, Sharp wits are fined to their full end. Now prove and praise as you do find, And to yourself be not unkind.

Apparently the full title contains a pun (see http://www.pballew.net/arithm17.html): “the cossike practise” in the title refers to algebra, as the Latin cosa apparently meaning “a thing” was used to stand for an unknown, abbreviated to cos — but the Latin word cos itself means a grindstone.

The author again reminds readers not to blame his book, at the end of his preface:

 To the curiouſe ſcanner. If you ought finde, as ſome men maie, That you can mende, I ſhall you praie, To take ſome paine ſo grace maie ſende, This worke to growe to perfecte ende. But if you mende not that you blame, I winne the praiſe, and you the ſhame. Therfore be wiſe, and learne before, Sith ſlaunder hurtes it ſelf moſte ſore.

Authors are either anxious about how their book is received, or make sure to be pointedly uncaring.

Sir Arthur Conan Doyle, in a mostly forgettable volume of poetry (Songs of the Road, 1911), begins:

If it were not for the hillocks
You’d think little of the hills;
The rivers would seem tiny
If it were not for the rills.
If you never saw the brushwood
You would under-rate the trees;
And so you see the purpose
Of such little rhymes as these.

Kālidāsa of course begins his Raghuvaṃśa with a grand disclaimer:

kva sūryaprabhavo vaṃśaḥ kva cālpaviṣayā matiḥ /
titīrṣur dustaram mohād uḍupenāsmi sāgaram // Ragh_1.2 //

mandaḥ kaviyaśaḥ prārthī gamiṣyāmy upahāsyatām /
prāṃśulabhye phale lobhād udbāhur iva vāmanaḥ // Ragh_1.3 //

atha vā kṛtavāgdvāre vaṃśe ‘smin pūrvasūribhiḥ /
maṇau vajrasamutkīrṇe sūtrasyevāsti me gatiḥ // Ragh_1.4 //

But the most nonchalant I’ve seen, thanks to Dr. Ganesh, is this gīti by Śrīkṛṣṇa Brahmatantra Yatīndra of the Parakāla Maṭha, Mysore:

nindatu vā nandatu vā
mandamanīṣā niśamya kṛtim etām
harṣaṃ vā marṣaṃ vā
sarṣapamātram api naiva vindema

Screw you guys. :-)

Written by S

Wed, 2014-05-28 at 23:56:11 +05:30

Posted in history, mathematics

## Big O() notation: a couple of sources

with one comment

This post contains, just for future reference, a couple of primary sources relevant to the $O$ (“Big O”) notation:

1. Some introductory words from Asymptotic Methods in Analysis by de Bruijn
2. An letter from Donald Knuth on an approach to teaching calculus using this notation.

Written by S

Thu, 2014-03-13 at 16:33:20 +05:30

## Visualizing product of permutations

A simple pedagogical trick that may come in handy: represent a permutation $\sigma$ using arrows (curved lines) from $k$ to $\sigma(k)$ for each $k$. Then, the product of two permutations can be represented by just putting the two corresponding figures (sets of arrows) one below the other, and following the arrows.

Representing permutations and products of permutations.

The figure is from an article called Symmetries by Alain Connes, found via the Wikipedia article on Morley’s trisector theorem (something entirely unrelated to permutations, but the article covers both of them and more).

I’m thinking how one might write a program to actually draw these: if we decide that the “height” of the figure is some $h$, then each arrow needs to go from some $(k, 0)$ to $(\sigma(k), h)$ (using here the usual screen convention of $x$ coordinate increasing from left to right, and $y$ coordinate increasing from top to bottom). Further, each curve needs to have vertical slope at its two endpoints, so that successive curves can line up smoothly. The constraint on starting point, ending point, and directions at the endpoints defines almost a quadratic Bezier curve, except that here the two directions are parallel. So it’s somewhere between a quadratic and the (usual) cubic Bezier curve, which is given by the start point, end point, and derivatives at the start and end point. (Here we only care about the direction of the derivative; we can pick some arbitrary magnitude to fix the curve: the larger we pick, the more smooth it will look at the ends, at the cost of smoothness in the interior.)

Even knowing the curve, how do we generate an image?

Written by S

Thu, 2014-03-06 at 23:15:44 +05:30

## The idea of logarithms, and the first appearance of e

The notion of the number $e$, the exponential function $e^x$, and logarithms $\log x$ are often conceptual stumbling blocks even to someone who has an otherwise solid understanding of middle-school mathematics.

Just what is the number $e$? How was it first calculated / where did it first turn up? Premature exposure to its numerical value

$\displaystyle e \approx 2.718281828459045\dots$

only serves to deepen the mysteriousness and to make it seem arbitrary.

Here a historical perspective helps: as is often the case, here too, the first appearance is simpler and more well-motivated than the accounts in dry textbooks. This is from this account by Matthew P. Wiener (originally posted on USENET somewhere, as quoted by MJD). I’m just going to quote it directly for now, and edit it later:

Napier, who invented logarithms, more or less worked out a table of logarithms to base $\frac1e$, as follows:

     0  1  2  3   4   5   6    7    8    9    10 ...
1  2  4  8  16  32  64  128  256  512  1024 ...


The arithmetic progression in the first row is matched by a geometric progression in the second row. If, by any luck, you happen to wish to multiply 16 by 32, that just happen to be in the bottom row, you can look up their “logs” in the first row and add 4+5 to get 9 and then conclude 16·32=512.

For most practical purposes, this is useless. Napier realized that what one needs to multiply in general is $1+\epsilon$ for a base—the intermediate values will be much more extensive. For example, with base 1.01, we get:

       0 1.00   1 1.01   2 1.02   3 1.03   4 1.04   5 1.05
6 1.06   7 1.07   8 1.08   9 1.09  10 1.10  11 1.12
12 1.13  13 1.14  14 1.15  15 1.16  16 1.17  17 1.18
18 1.20  19 1.21  20 1.22  21 1.23  22 1.24  23 1.26
24 1.27  25 1.28  26 1.30  27 1.31  28 1.32  29 1.33
30 1.35  31 1.36  32 1.37  33 1.39  34 1.40  35 1.42
[...]
50 1.64  51 1.66  52 1.68  53 1.69  54 1.71  55 1.73
[...]
94 2.55  95 2.57  96 2.60  97 2.63  98 2.65  99 2.68
100 2.70 101 2.73 102 2.76 103 2.79 104 2.81 105 2.84
[...]


So if you need to multiply 1.27 by 1.33, say, just look up their logs, in this case, 24 and 29, add them, and get 53, so 1.27·1.33=1.69. For two/three digit arithmetic, the table only needs entries up to 9.99.

Note that $e$ is almost there, as the antilogarithm of 100. The natural logarithm of a number can be read off from the above table, as just [approximately] $\frac1{100}$ the corresponding exponent.

What Napier actually did was work with base .9999999. He spent 20 years computing powers of .9999999 by hand, producing a grand version of the above. That’s it. No deep understanding of anything, no calculus, and $e$ pops up anyway—in Napier’s case, $\frac1e$ was the 10 millionth entry. (To be pedantic, Napier did not actually use decimal points, that being a new fangled notion at the time.)

Later, in his historic meeting with Briggs, two changes were made. A switch to a base $> 1$ was made, so that logarithms would scale in the same direction as the numbers, and the spacing on the logarithm sides was chosen so that $\log(10)=1$. These two changes were, in effect, just division by $-\log_e(10)$.

In other words, $e$ made its first appearance rather implicitly.

(I had earlier read a book on Napier and come to the same information though a lot less clearly, here.)

I had started writing a series of posts leading up to an understanding of the exponential function $e^x$ (here, here, here), but it seems to have got abandoned. Consider this one a contribution to that series.

Written by S

Wed, 2013-11-27 at 10:52:51 +05:30

Posted in mathematics

## The functional equation f(x+y) = f(x)f(y)

Suppose $f: \mathbb{R} \to \mathbb{R}$ satisfies $f(x+y) = f(x) f(y)$. What can we say about $f$?

Putting $y = 0$ gives

$\displaystyle f(x) = f(x+0) = f(x)f(0),$

which can happen if either $f(x) = 0$ or $f(0) = 1$. Note that the function $f$ which is identically zero satisfies the functional equation. If $f$ is not this function, i.e., if $f(x) \neq 0$ for at least one value of $x$, then plugging that value of $x$ (say $x^*$) into the equation gives $f(0) = 1$. Also, for any $x$, the equation $f(x^*) = f(x +x^* - x) = f(x)f(x^* - x)$ forces $f(x) \neq 0$ as well. Further, $f(x) = f(x/2 + x/2) = f(x/2)^2$ so $f(x) > 0$ for all $x$.

Next, putting $y = x$ gives $f(2x) = f(x)^2$, and by induction $f(nx) = f(x)^n$. Putting $\frac{x}{n}$ in place of $x$ in this gives $f(n\frac{x}{n}) = f(\frac{x}{n})^n$ which means $f(\frac{x}{n}) = f(x)^{\frac1n}$ (note we’re using $f(x) > 0$ here). And again, $f(\frac{m}{n}x) = f(x)^{m/n}$. So $f(\frac{m}{n}) = f(1)^{m/n}$, which completely defines the function at rational points.

[As $f(1) > 0$, it can be written as $f(1) = e^k$ for some constant $k$, which gives $f(x) = e^{kx}$ for rational $x$.]

To extend this function to irrational numbers, we need some further assumptions on $f$, such as continuity. It turns out that being continuous at any point is enough (and implies the function is $f(x) = f(1)^x$ everywhere): note that $f(x + m/n) = f(x)f(m/n) = f(x)f(1)^{m/n}$. Even being Lebesgue-integrable/measurable will do.

Else, there are discontinuous functions satisfying the functional equation. (Basically, we can define the value of the function separately on each “independent” part. That is, define the equivalence class where $x$ and $y$ are related if $y = r_1x + r_2$ for rationals $r_1$ and $r_2$, pick a representative for each class using the axiom of choice (this is something like picking a basis for $\mathbb{R}/\mathbb{Q}$, which corresponds to the equivalence class defined by the relation $y = r_1x$), define the value of the function independently for each representative, and this fixes the value of $f$ on $\mathbb{R}$. See this article for more details.)

To step back a bit: what the functional equation says is that $f$ is a homorphism from $(\mathbb{R}, +)$, the additive group of real numbers, to $(\mathbb{R}, \times)$, the multiplicative monoid of real numbers. If $f$ is not the trivial identically-zero function, then (as we saw above) $f$ is in fact a homomorphism from $(\mathbb{R}, +)$, the additive group of real numbers, to $(\mathbb{R_+^*}, \times)$, the multiplicative group of positive real numbers. What we proved is that the exponential functions $e^{kx}$ are precisely all such functions that are nice (nice here meaning either measurable or continuous at least one point). (Note that this set includes the trivial homomorphism corresponding to $k = 0$: the function $f(x) = 1$ identically everywhere. If $f$ is not this trivial map, then it is in fact an isomorphism.)