Welcome to the 117th Carnival of Mathematics!

What treats are in this month's Carnival?

As is tradition we start with a fact about this Carnival's number: 117. It's divisible by 3. That might not be so exciting to everyone, but to someone such as myself, as a mathematician who is notoriously bad at numbers and arithmetic (my friends think it's hilarious to ask me to divide the bill at a restaurant), being able to tell this quickly about any integer gives me a feeling of vast power over these numbers. You can tell if a number is divisible by 3 if the sum of the number's digits is divisible by 3: in our case, 1+1+7=9. Neat!

This trick gives me a great sense of power over the integers as it allows me to rule out vast swathes of them (well, a third) as not being prime, at a glance. Combine that with our instant knowledge that any even numbers, or those ending in 5 or 0 aren't prime, and you've really got a good of chance of being able to tell if any random number someone tells you is prime. There's other neat division rules too, for 7 and 11 and so on. But the really interesting thing is why they work… Can you explain why the division rule for three works? Give us your best explanations in the comments below!

Now to the Carnival. We had many great submissions, thank you to everyone who sent one in. The one that had me hooked for the longest was the wonderful

*Parable of the Polygons* by

Vi Hart and

Nicky Case, submitted by Katie. This brilliant interactive post explores the maths of diversity based on the work of Nobel Prize-winning game theorist

Thomas Schelling.

I also really enjoyed Kevin Houston's fascinating and detailed article *The Beatles' magical mystery chord*, delving into the mathematics and music behind the opening chord to a hard days night. It's made me want to catch the next talk Kevin gives on the maths of autotune!

A very mathematical snowflake

For a real treat, why not spend a long lunch hour watching Terry Tao's talk *The Cosmic Distance Ladder*. This forms part of one of Mike Lawler's blog post about the lecture, where he discuses how the radius of the Earth and the radius of the Moon's orbit were measured, and how Tao's lecture was an incredible opportunity to show some fun and exciting maths to children. Lawler said in his submission: "Amazingly this lecture by Tao on youtube has fewer than 2,000 views right now. I wish that more people knew about it (kids especially) as it is such an amazing opportunity to learn from one of the greatest mathematicians living today." We heartily agree! (And you can find out more on Tao's work here.)

I, and any other people madly manufacturing paper snowflakes for decorating school halls and homes, must thank Mr Reid for his excellent post A public service announcement regarding paper snowflakes. Really, if you're in the middle of cutting a million and one of those suckers out you need to read this, it will make it all much better!

And here are some more treats to while away your paper-snowflake-cutting-time…

That's it for this Carnival, we hope you enjoyed the show! You can find the last Carnival at CavMaths and the details for all past and future Carnivals, as well as how to be involved at Aperiodical. Don't forget to submit ideas for January's Carnival, hosted by Andrew at AndrewT.net.

*(Hint for the division rule: A number with digits abcd=ax1000+bx100+cx10+d = ax(1+999)+bx(1+99)+c(1+9)+d…)*