You are at a party and overhear a conversation between Lucy and her friend.
In the conversation, Lucy mentions she has a secret number that is less than 100.
She also confesses the following information:
“The number is uniquely describable by the answers to the following four questions:”
Q1) Is the number divisible by two?
Q2) Is the number divisible by three?
Q3) Is the number divisible by five?
Q4) Is the number divisible by seven?
…
I loved this puzzle. I first solved it a brute-force way, with Excel. Then I found increasingly elegant and logical solutions. Then I shared it with some friends: I love it! Go read the whole thing.
Imagine I’m in a desert, and there are two wells where I can obtain water. If I want to go to the nearest well, which well do I visit? Clearly, it depends one where I am standing.
It’s possible to draw a line dividing the desert. To the ‘left’ of the line, it’s nearer to go to the well on the well on the ‘left’, to the ‘right’ of the line, it’s closer to go to
the well on the ‘right’…
So, I’ve not been well lately. And because a few days lying on my back with insufficient mental stimulation is a quick route to insanity for me, I’ve been trying to spend my
most-conscious moment doing things that keep my brain ticking over. And that’s how I ended up calculating pi.
When I say I’ve been unwell, that might be an understatement. But we’ll get to that another time.
Pi (or π) is, of course, the ratio of the circumference of a circle to its diameter, for every circle. You’ll probably have learned it in school as 3.14, 3.142, or 3.14159, unless you
were one of those creepy kids who tried to memorise a lot more digits. Over the years, we’ve been able to calculate it to increasing precision, and although there’s no practical or theoretical reason that we need to know it beyond the 32 digits worked out by
Ludolph van Ceulen in the 16th Century, it’s still a fascinating topic that attracts research and debate.
Our calculation of pi has rocketed since the development of the digital computer.
Most of the computer-based systems we use today are hard to explain, but there’s a really fun computer-based
experimental method that can be used to estimate the value of pi that I’m going to share with you. As I’ve been stuck in bed (and often asleep) for the last few days, I’ve not
been able to do much productive work, but I have found myself able to implement an example of how to calculate pi. Recovery like a nerd, am I right?
Pi goes on forever. Pie, sadly, comes to an end.
Remember in school, when you’ll have learned that the formula to describe a circle (of radius 1) on a cartesian coordinate system is x2 + y2 = 1? Well you can work
this backwards, too: if you have a point on a grid, (x,y), then you can tell whether it’s inside or outside that circle. If x2 + y2 < 1, it’s inside, and if
x2 + y2 > 1, it’s outside. Meanwhile, the difference between the area of a circle and the area of a square that exactly contains it is π/4.
Think back to your school days. Ever draw a circle like this? Do the words “Cartesian coordinates” ring any bells?
Take those two facts together and you can develop an experimental way to determine pi, called a Monte Carlo
method. Take a circle of radius 1 inside a square that exactly contains it. Then randomly choose points within the square. Statistically speaking, these random points have a
π/4 chance of occurring within the circle (rather than outside it). So if we take the number of points that lie within the circle, divide that by the total number of
points, and then multiply by 4, we should get something that approaches the value of pi. You could even do it by hand!
I wrote some software to do exactly that. Here’s what it looks like – the red points are inside the circle, and the black points are outside.
The software illustration I’ve written is raw JavaScript, HTML, and SVG, and should work in any modern web browser (though it can get a little slow once it’s drawn a few thousand
points!). Give it a go, here! When you go to that page, your browser will start drawing dots at random points, colouring them red if
the sum of the squares of their coordinates is less than 1, which is the radius of the circle (and the width of the square that encompasses it). As it goes along, it uses the formula I
described above to approximate the value of pi. You’ll probably get as far as 3.14 before you get bored, but there’s no reason that this method couldn’t be used to go as far as
you like: it’s not the best tool for the job, but it’s super-easy to understand and explain.
Oh, and it’s all completely open-source, so you’re welcome to take it and do with it what you wish. Turn off the graphical output
to make it run faster, and see if you can get an accurate approximation to 5 digits of pi! Or slow it down so you can see how the appearance of each and every point affects the
calculation. Or adapt it into a teaching tool and show your maths students one way that pi can be derived experimentally. It’s all yours: have fun.
And I’ll update you on my health at some other point.
When I learned to program, back when dinosaurs walked the earth and the internet had no cats, there was an idea: if you were good at math, you’d be good at programming. I was great at
math as a kid, but perhaps because I didn’t like it much, no one steered me towards programming. I…
Have you ever come across non-transitive dice? The classic set, that you can get in most magic shops, consists of three different-coloured six-sided dice:
A “Grime’s” style set of 3 non-transitive dice. Notice the unusual numbering.
There are several variants, but a common one, as discussed by
James Grime, involves one die with five “3” sides and one “6” side (described as red below), a second die with three “2” sides and three “5” sides (described as
green below), and a third die with one “1” side and five “four” sides (described as blue below).
They’re all fair dice, and – like a normal six-sided dice – they all have an average score of 3.5. But they’ve got an interesting property, which you can use for all kinds of magic
tricks and gambling games. Typically: the red die will beat the green die, the green die will beat the blue die, and the blue die will beat the red die! (think Rock, Paper,
Scissors…)
Seemingly paradoxically, the dice will generally beat one another in a circular pattern.
If you want to beat your opponent, have them pick a die first. If they pick green, you take red. If they take red, you take blue. If they take blue, you take green. You now have about a
60% chance of getting the highest roll (normally you’d have about a 33% chance of winning, and a 17% chance of a draw, so a 60% chance is significantly better). To make sure that you’ve
got the best odds, play “best of 10” or similar: the more times you play, the less-likely you are to be caught out by an unfortunate unlucky streak.
But if that doesn’t bake your noodle enough, try grabbing two sets of nontransitive dice and try again. Now you’ll see that the pattern reverses: the green pair tends to beat the red
pair, the red pair tends to beat the blue pair, and the blue pair tends to beat the green pair! (this makes for a great second act to your efforts to fleece somebody of their money in a
gambling game: once they’ve worked out how you keep winning, give them the chance to go “double or nothing”, using two dice, and you’ll even offer to choose first!)
When you pair up the dice, the cycle reverses! While red beats green, double-green beats double-red!
The properties of these dice – and of the more-exotic forms, like Oskar van Deventer’s seven-dice set (suitable for playing a game with three players and beating both of your
opponents) and like the polyhedral
varieties discussed on Wikipedia – intrigue the game theorist and board games designer in me. Could there be the potential for this mechanic to exist in a board game? I’m thinking
something with Risk-like combat (dice ‘knock out’ one another from highest to lowest) but with a “dice acquisition” mechanic (so players perform actions, perhaps in
an auction format, to acquire dice of particular colours – each with their own strengths and weaknesses among other dice – to support their “hand” of dice). There’s a discussion going on in /r/tabletopgamedesign…
I’ve even written a program (which you’re welcome to download, adapt, and use) to calulate the
odds of any combination of any variety of non-transitive dice against one another, or even to help you develop your own non-transitive dice sets.
Penney’s game
Heads or tails? Image courtesy David M. Diaz.
Here’s another non-transitive game for you, but this time: I’ve made it into a real, playable game that you can try out right now. In this game, you and I will each, in turn, predict
three consecutive flips of a fair coin – so you might predict “tails, heads, heads”. Then we’ll start flipping a coin, again and again, until one of our sequences comes up. And more
often than not, I’ll win.
If you win 10 times (or you lose 20 times, which is more likely!), then I’ll explain how the game works, so you know how I “cheated”. I’ll remind you: the coin flips are fair, and it’s
nothing to do with a computer – if we played this game face-to-face, with a real coin, I’d still win. Now go play!
The other Three Ringers and I are working hard to wrap up Milestone:
Jethrik, the latest version of the software. I was optimising some of the older volunteer availability-management code when, by coincidence, I noticed this new bug:
Well, at least she’s being rational about it.
I suppose it’s true: Lucy (who’s an imaginary piece of test data) will celebrate her birthday in 13/1 days. Or 13.0 days, if you prefer. But most humans seem to be happier
with their periods of time not expressed as top-heavy fractions, for some reason, so I suppose we’d better fix that one.
They’re busy days for Three Rings, right now, as we’re also making arrangements for our 10th
Birthday Conference, next month. Between my Three Rings work, a busy stretch at my day job, voluntary work at Oxford Friend, yet-more-executor-stuff, and three different courses, I don’t have much time for anything else!
But I’m still alive, and I’m sure I’ll have more to say about all of the things I’ve been getting up to sometime. Maybe at half term. Or Christmas!
Like the TARDIS, your time machine has a fault. The fault isn’t a failure of its chameleon circuit, but a quirk in its ability to jump to particular dates. Picture courtesy
aussiegall (Flickr), licensed Creative Commons.
You own a time machine with an unusual property: it can only travel to 29th February. It can jump to any 29th February, anywhere at all, in any year (even back
before we invented the Gregorian Calendar, and far into the future after we’ve stopped using it), but it can only
finish its journey on a 29th of February, in a Gregorian leap year (for this reason, it can only jump to years which are leap years).
One day, you decide to take it for a spin. So you get into your time machine and press the “random” button. Moments later, you have arrived: it is now 29th February in a
random year!
Without knowing what year it is: what is the probability that it is a Monday? (hint: the answer is not1/7 – half of your challenge is to work
out why!).
Like the TARDIS, your time machine has a fault. The fault isn’t a failure of its chameleon circuit, but a quirk in its ability to jump to particular dates. Picture courtesy aussiegall
(Flickr), licensed Creative Commons.
You own a time machine with an unusual property: it can only travel to 29th February. It can jump to any 29th February, anywhere at all, in any year (even back before we
invented the Gregorian Calendar, and far into the future after we’ve stopped using it), but it can only finish its
journey on a 29th of February, in a Gregorian leap year (for this reason, it can only jump to years which are leap years).
One day, you decide to take it for a spin. So you get into your time machine and press the “random” button. Moments later, you have arrived: it is now 29th February in a
random year!
Without knowing what year it is: what is the probability that it is a Monday? (hint: the answer is not1/7 – half of your challenge is to work out
why!).
After our attempt at a relaxing day off, which resulted in us
getting pretty-much soaked and exhausted, we returned on day five of our holiday to the comedy scene for more fun and laughter.
Ruth, JTA, Matt and Hannah-Mae outside the Canons’ Gait. Do I win a prize for being the first Abnibber to publish a photo of Matt’s new girlfriend?
After failing to get into Richard Wiseman‘s Psychobabble, which attracted a huge queue
long before we got to the venue, Ruth, JTA and I
instead went to RomComCon: a two-woman show telling the story of how they road-tested all of the top romantic
comedy “boy meets girl” cliché situations, to see if they actually worked in real life. It was sweet, even where it wasn’t funny, and it was confidently-performed, even where
it wasn’t perfectly-scripted. The mixture of media (slides, video, audience participation, and good old-fashioned storytelling) was refreshing enough to help me overlook the
sometimes-stilted jumps in dialogue. I’ll admit: I cried a little, but then I sometimes do that during actual RomComs, too. Although I did have to say “Well d’uh!” when
the conclusion of the presentation was that to get into a great relationship, you have to be open and honest and willing to experiment and not to give up hope that you’ll find one. You
know: the kinds of things I’ve been saying for years.
Ruth & JTA in the Voodoo Rooms, waiting for Owen Niblock’s “Codemaker” to start.Ruth & JTA in the Voodoo Rooms, waiting for Owen Niblock’s “Codemaker” to start.
We met up with Matt and his new girlfriend, Hannah-Mae, who turns out to be a lovely, friendly, and
dryly-sarcastic young woman who makes a wonderful match for our Matt. Then, after a drink together, parted ways to see different shows; promising to meet up again later in the day.
“Codemaker” Owen Niblock presents Google Image Search pictures that come up when the search engine is presented with a picture of his wife. The audience member who’s half-standing
didn’t laugh throughout the entire performance: this might not have been the right show for him.
We watched Owen Niblock‘s Codemaker, and were pleased to discover that it was everything
that Computer Programmer Extraordinaire (which we saw on day two)
failed to be. Codemaker was genuinely geeky (Owen would put up code segments and then explain why they were interesting), funny (everything from the
five-months-a-year beard story to his relationship Service Level Agreement with his wife was fabulously-crafted), and moving. In some ways I’m sad that he isn’t attracting a larger
audience – we three represented about a quarter to a fifth of those in attendance, at the end – but on the other hand, his computer-centric humour (full of graphs and pictures of old
computers) is rather niche and perhaps wouldn’t appeal to the mainstream. Highly recommended to the geeks among you, though!
Ruth discovers a police box on the way back to the flat and is inordinately excited. Apparently she’d somehow managed to never see one before.
Back at the flat, we drank gin and played Ca$h ‘n’ Gun$ with Matt and
Hannah-Mae. JTA won three consecutive games, the jammy sod, despite the efforts of the rest of us (Matt or I with a hand grenade, Ruth or I as The Kid, or even Hannah-Mae once she had a
gun in each hand), and all the way along every single time insisted that he was losing. Sneaky bugger.
Hannah-Mae, Matt, JTA and I with Richard Wiseman. JTA was aware that a photo was going to be taken at some point, but was distracted by talking to another comedian, off-camera.
We all reconvened at the afternoon repeat of Richard Wiseman’s show, where he demonstrated (in a very fun and engaging way) a series of psychological, mathematical, and slight-of-hand
tricks behind the “mind-reading” and illusion effects used by various professional entertainers. I’ve clearly studied this stuff far too much, because I didn’t end up learning anything
new, but I did enjoy his patter and the way he makes his material interesting, and it’s well-worth a look. Later, Ruth and I would try to develop a mathematical formula for the smallest
possible sum totals possible for integer magic squares of a given order (Wiseman’s
final trick involved the high-speed construction of a perfect magic square to a sum total provided by a member of the audience: a simple problem: if anybody wants me to demonstrate how
it’s done, it’s quite fun).
Thom Tuck wants to be where the people are. He wants to see… wants to see them, dancing. Walking around on those… what’s what word again? Seriously: what’s that word again?
Finally, we all went to see Thom Tuck again. Matt, JTA and I had seen him earlier in the week, but we’d insisted that Hannah-Mae and Ruth get the chance to see his fantastic
show, too (as well as giving ourselves an excuse to see it again ourselves, of course). He wasn’t quite so impressive the second time around, but it was great to see that his knowledge
of straight-to-DVD Disney movies really is just-about as encyclopaedic as he claims, when we gave us new material we hadn’t heard on his previous show (and omitted some that we had), as
well as adapting to suggestions of films shouted out by the audience. Straight-To-DVD remains for me a chilling and hilarious show and perhaps the most-enjoyable thing
I’ve ever seen on the Fringe.
