# Dave's Free Press: Journal

## violence, pornography, and rude words for the web generation

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Fri, 14 Jun 2013

### MathML browser extensions

It's really annoying that there still isn't universal support for MathML. Things have got better recently though, with Safari finally supporting it. Of the major browsers, though, there are still two that don't.

First, IE. It doesn't matter. It is only used for three things: first, to download a browser that doesn't suck; second, if it is chosen by someone who is stupid, ignorant or malicious; third, by people who need to compare browsers.

Second, Chrome. Despite its lack of MathML, Chrome is IMO the best browser out there. It works on multiple platforms, has lots of extensions available, and has very good support for synchronising things like bookmarks and open pages across devices.

And now there's a plugin for it to support MathML! And of course Chrome supports it without a plugin on iOS, because it uses the Safari rendering engine there.

update: Safari on the desktop still doesn't render it particularly well, despite it working just fine on my phone.

Huzzah!

Posted at 11:04 by David Cantrell
keywords: geeky | maths | meta
Wed, 16 Jan 2013

### Lewis Carroll in Numberland, by Robin Wilson

, or for a student of the mathemagical art.

To most people, Lewis Carroll is nothing but an author of childrens' books and a photographer of ill repute. Nothing could be further from the truth. He was also Charles Lutwidge Dodgson, an eminent mathematician at Oxford, with most of his work being in geometry and matrix algebra. He spent much of his time teaching undergraduates, but also children in various schools around Oxford and private pupils, and published at least as much serious work as he did fiction and nonsense. It is said that after Queen Victoria read "Alice's Adventures in Wonderland", she was so charmed that she demanded that she be sent a copy of his next book. And duly, she received "An Elementary Treatise on Determinants, With Their Application to Simultaneous Linear Equations and Algebraic Equations" and was not amused.

Much of his mathematical work, being pedagogical, is now out of date and sometimes even wrong. For example, much of his energy was spent on teaching Euclid to undergraduates, material that is these days covered at A-level in a completely different way, and he was so opposed to the study of non-Euclidean geometries as to waste time writing against them. But that doesn't detract from the fact that he was a great teacher of mathematics, to students at all levels from young children to the most learnèd.

Of his works that are still relevant, his two-part "Symbolic Logic" appears (from its description in this short biography) to be worth looking at, for his method of figuring out syllogisms through diagrams, and how he expanded his diagrams to deal with showing the interactions of any number of sets, where Venn diagrams break down at six sets. In 1884 he published what is now an obscure work on psephology, "The Principles of Parliamentary Representation" in which he considers what it means for an election to be fair, and methods to achieve this in reality. I'm very pleased to say that he comes to the same conclusion that I did: large multi-member constituencies with some form of proportional representation within each constituency. And related to this, and driven by his interest in tennis, he analysed whether the traditional knockout style of tournament was a good way of ranking players by ability (it isn't: consider what happens if the best and second-best players meet in the first round) and published under his Carroll pen-name a pamphlet with the delightful title of "Lawn Tennis Tournaments: The True Method of Assigning Prizes with a Proof of the Fallacy of the Present Method" whose recommendations have not, unfortunately, been taken up. It is an interesting "what-if" to imagine whether they would have been if he had published it under the name of Dodgson and the effect this would have on knock-out tournaments in all manner of sports.

I would have liked to see a bit more space given in this biography to all three of those subjects, even bearing in mind that a biography is a book about the man, not about the details of his work. After all, if the author could fit in so much material about puns and mathematical games, he could surely fit in a bit more about those works of Dodgson's that are the most relevant today. I wonder if, perhaps, Wilson was concerned that that would make the book "too technical". If that's the reason, then Shame, Shame!

For those with an interest in mathematics (at any level, from schoolchild to professional), this book is very much worth reading and worth buying. For general readers I hesitate to recommend purchasing it except to Carroll's most ardent fans.

Posted at 00:36 by David Cantrell
keywords: biography | books | maths | psephology
Wed, 6 Jul 2011

### Star ratings re-revisited

I did, very briefly, consider a completely different rating system for my reviews, instead of just awarding 0 to 5 shiny gold stars.

I considered rating books out of ten on several axes - for example, entertainment, literary merit, imagination, consistency. I would then combine them by treating those scores as the co-ordinates of a point in an N-dimensional space, the overall rating being the distance of that point from the origin, or equivalently, they are components of a velocity vector in an N-dimensional space. Let me give a couple of examples:

The Quantum Thief might score 8/10 for entertainment, 10/10 for literary merit, 9/10 for imagination, and 10/10 for consistency. The score, then, is sqrt(82+102+92+102) = 18.6. A perfect score on those axes would be sqrt(4*102) = 20. So to normalise to a score out of ten we divide by 2, giving 9.3/10. I actually gave it 5/5.

A Mighty Fortress, on the other hand, might get 5/10 for entertainment, 2/10 for literary merit, 2/10 for imagination, and 8/10 for consistency, for a score of 9.8, which normalises to 4.9/10. I actually gave it 2/5.

There are at least three obvious reasons why I didn't go with this.

1. Maximum marks on one axis gets you half way to perfection with four axes, even closer with fewer. I don't want to give undue weight to good marks in any one axis. We could perhaps solve this by making it harder to attain maximum velocity in any direction the closer you get to the maximum. The physicists in the audience may now run away screaming;
2. different type of book require different axes. eg fiction vs textbook vs biography;
3. it over-complicates things, and is just a poor attempt to hide how subjective reviews are. Note that in the numbers above, I fudged the individual axis scores for both books so they'd mostly agree with the scores I actually gave :-)
Posted at 00:55 by David Cantrell
keywords: geeky | maths | meta
Fri, 10 Sep 2010

### Webkit and ASCIIMathML

As you will no doubt know by now, I occasionally perpetrate mathematics. But it sucks to have to say in something like this "take the product of p(i)int(n/p(i)) for i=1 to i=Φ(n)". It would be much better if I could embed a proper formula.

There's a standard way of doing this, called MathML, and it's fucking horrible. And in any case, browser support for MathML is piss-poor. However, it's getting better - Firefox now supports it fairly well, and Webkit does too, although not quite as well as Firefox. There are also a few useful tools for making MathML suck less, in particular ASCIIMathML, which I am now using.

Using that, I type the above equation thus:

\prod_(i=1)^(\phi(n))p(i)^(\lfloorn/(p(i))\rfloor)

and it renders thus in your browser:

\prod_(i=1)^(\phi(n))p(i)^(\lfloorn/(p(i))\rfloor)

it should render something like this:

Note that at the time of writing, Webkit doesn't properly render the floor(n/p(i)) as a superscript to p(i).

Even with the flaws in common browsers (and the complete lack of support in many, including Safari) I'm going to start using it, because it's just so damned useful. Webkit and Firefox support it, which is good enough for me, and because Webkit is really just the nightly builds of Safari, we can expect a near-future release of Safari to support it too.

Incidentally, this journal entry exposes a bug in ASCIIMathML as well as in the current Webkit - can you spot it?

Update: it took the author of ASCIIMathML mere hours to respond to my bug report with a fix. Makes me rather ashamed of some of the bugs that have been mouldering in my RT queue for over a year :-)

Posted at 21:23 by David Cantrell
keywords: geeky | maths | meta
Sat, 3 Oct 2009

### Graphing tool

I made a shiny thing! It can plot arbitrary functions of the form x=f(y) or y=f(x). Under the skin, it just massages its arguments and passes them through to Gnuplot. Here's the source code.

Update: now 48.3% even shinier - see on the right

Posted at 21:35 by David Cantrell
keywords: geeky | hacking | maths | perl
Wed, 30 Sep 2009

### Optimising n! - revisited again

For the background, see this post and this post.

I have had An Branewave! When I said that

100! = 250 * 333 * 520 * 714 * 119 * ...

was wrong (which it is) I stupidly didn't realise that there is a pattern to the missing terms. The 250 comes from observing that there a 50 multiples of 2 less than or equal to 100. There are also 25 multiples of 4 (also known as 22), 12 multiples of 8 (aka 23), 6 multiples of 24, 3 multiples of 25, and 1 of 26. the 25 multiples of 4 contribute another 225. The 12 multiples of 8 (which of course are also multiples of 4) contribute another 212, and so on.

So we see that 100! is divisible by 250 + 25 + 12 + 6 + 3 + 1, or 297.

There is, of course, a similar pattern for all the other prime factors.

Posted at 01:44 by David Cantrell
keywords: geeky | maths
Fri, 25 Sep 2009

### Combining functions

There was a great programme on Radio 4 yesterday about Newton and Leibnitz. I'm not sure why, but it inspired me to consider the curve that is constructed from y = x2 when x <= 0, and y = x when x > 0 . I want to reduce that to a single function.

It can obviously be done.

Consider that to fit a curve to any* three points, you can do it with a curve y = a + bx + cx2. Figuring out the three constants a, b and c is trivial, you just solve three simultaneous equations. Similarly, for four points, you fit y = a + bx + cx2 + dx3, and so on. To accurately fit the curve I described at the beginning, you need an infinite number of points, thus an infinite number of constants.

I'm sure that this can be done - obviously you can't actually calculate an infinite number of constants, but I'm sure that with a bit of integration it could be done. And it can be done for any such pair of functions which meet at a point. However, on further reflection I'm not entirely convinced that it can be done in the general case - you have that pesky discontinuous yes/no conditional in the middle: "is x <= n?".

* not strictly true - consider (0,0), (0,1), (1,0).

Posted at 00:54 by David Cantrell
keywords: geeky | maths
Tue, 28 Apr 2009

### Factoring Factorials

Take a factorial. Any factorial. Factor it*. Notice a Pattern.

That pattern is that if you list all of the prime factors in order and the number of times they appear, at no point does any larger factor appear more often than any smaller factor. This would appear to be obvious, and it "obviously" applies to all factorials** (I've verified it by hand up to 28!, at which point I got bored). But I'm finding it hard to put a proof into words - or more concisely but equivalently - into symbols.

* this is easy. While factoring 1124000727777607680000 might be quite hard, if you know that it's 22! it becomes trivial because we know that it is divisible by 2, 3, 4, 5, ..., 21, 22, each of which is trivially factorable. Given that each number is the product of a unique set*** of factors, the factors of 22! can only be the set of all the factors of all the numbers we multipled to get 22!. Easy!

** it becomes obvious when you consider Eratosthenes' method for testing primality.

*** yes, I know, it's not really a set.

Posted at 17:53 by David Cantrell
keywords: geeky | maths
Mon, 10 Nov 2008

### Umbrellas

What's the point of umbrellas? To be effective, their diameter needs to be slightly more than h * sin(ϑ) (where h is your height and ϑ is the angle of the rain from the vertical). So, if rain is falling vertically you need an umbrella with diameter slightly more than* the width of your shoulders. If rain is going horizontally, you need an umbrella slightly wider than you are tall. If rain is coming down at 20° from the vertical, you need an umbrella diameter just over 0.34 * your height, and so on.

Given that rain can subtend any angle from 0 to 90° from the vertical, then unless you wish to be your umbrella supplier's very best friend in the whole world, you need a Very Large umbrella. And yet, no-one carries such a thing. Indeed, I don't think anyone makes such a thing suitable for anyone other than midgets. So we see that every umbrella user has, from the point of view of keeping themselves dry, made the wrong decision. Their partial solutions are no better than what they would achieve by wearing a good coat and a hat.

Unfortunately, their partial solutions come at the cost to everyone else of getting poked in the face by the metal spikes at the edge of the umbrella.

* the value of "slightly more" is a function of ϑ and the shape of the human body. A good approximation would be to assume that a person's width is h/3 and their depth is h/4.

Posted at 22:21 by David Cantrell
keywords: etiquette | maths | rant
Wed, 27 Aug 2008

### Optimising n! - revisited

For the background, see this post.

Late one night I thought that you might be able to simplify n-factorial thus:

\prod_(i=1)^(\phi(n))p(i)^(\lfloorn/(p(i))\rfloor)

Where:

• n is the number whose factorial we wish to calculate;
• p(i) is the ith prime number;
• Φ(n) is the number of primes less than or equal to n

Now, without using a lookup table, p() and Φ() are hard to calculate, but at least you'd avoid a lot of the problems that come from using the stupendously big numbers that come as intermediate results in calculating factorials.

Unfortunately, that formula is wrong anyway. It's a restatement of this:

100! = 250 * 333 * 520 * 714 * 119 * ...

which came from noticing that 100! is the product of 50 numbers which have 2 as a prime factor, 33 numbers which have 3 as a prime factor, 20 numbers that have 5 as a prime factor, and so on. Unfortunately, it doesn't take account of numbers like 4 and 18 which have a repeated prime factor - 4, for example, is 2 * 2 and 18 is 2 * 3 * 3. Bother.

Posted at 17:13 by David Cantrell
keywords: geeky | maths
Tue, 22 Jul 2008

### Wikipedia: WRONG AGAIN

Wikipedia has a reputation for being of rather low quality, particularly in the humanities. In the sciences and technology it is supposed to be pretty good. But today, I found an error in their article about ... the triangle. One of the most basic mathematical concepts. And they got it wrong. Joy.

OK, to be fair, it's only subtlely wrong, is correct for the majority of cases, and one could argue that the context in which the inaccurate information is presented makes it OK. But I disagree.

In a section called "Basic facts" they first mention that triangles were described thoroughly by Euclid. Good. In the next paragraph, they say that the internal angles add up to 180°. Not so good. They only do so in some spaces, the most commonly known of which is the Euclidean space. Anyway [click click click type click] I've fixed it now. I wonder how long it'll stay fixed, or whether some wanker will revert my pedantry.

Posted at 00:31 by David Cantrell
keywords: geeky | maths | web
Wed, 11 Jun 2008

### Optimising n!

If you want to add up all the numbers from 1 to n, you can do it two ways. The slow, inefficient way is to calculate:

1 + 2 + 3 + ... + n

The quick way is to remember what you were taught in school, that:

1 + 2 + 3 + 4 + 5 + 6 = (6 + 1) + (2 + 5) + (3 + 4) = 7 + 7 + 7

and that in general:

1 + 2 + ... + n = n(n+1)/2

This optimisation might not matter for small values of n, but for n = 1000000 it really makes a big difference. This got me thinking whether there was a similar trick for calculating the product of all numbers from 1 to n - that is, n-factorial, or n!.

This matters even more for n! because it gets so large so quickly that computers are unable to accurately represent the intermediate values. In fact, 20! is the largest that can be respresented on a 64 bit machine. 21! can only be approximated, as you need to use a floating point number. This means that not only is calculating really big factorials time-consuming, it's not possible at all to do it accurately with native datatypes.

I don't even know if there is a simple re-statement of n! like there is for the sum above, but I'm gonna spend a few idle minutes working on it.

Posted at 20:10 by David Cantrell
keywords: geeky | maths