## Is it divisible by 7? Death by regular expression

Here is a fun question at Quora:

How does one determine whether a number is evenly divisible by 7?

There are some remarkably geeky answers. Those of you who write code might enjoy the second answer, by Anders Kaseorg.

Enjoy!

## Carnival of Mathematics #99

Welcome to Carnival of Mathematics #99. Wikipedia provides some nice trivia about the number 99.

99 is the ninth repdigit, a palindromic number and a Kaprekar number. It is the sum of divisors of the first eleven positive integers.

99 is the sum of the cubes of three consecutive integers:

99 = 2^3 + 3^3 + 4^3

And, I personally like that 99 is the difference of two squares: 99=10^2-1^2.

Now, onto the carnival articles.

John Cook shares Recognizing numbers. For Python users, SymPy is a symbolic math package that "takes a floating point number and tries to simplify it: as a fraction with a small denominator, square root of a small integer, an expression involving famous constants, etc."

Mike Thayer, in Algebra and Geometry, asks this question: "I teach algebra 1, to 9th and 10th graders, mainly. I also teach geometry to the same age group. I'm wondering the following: Why is it that the conversations in geometry are so much more interesting, generally?"

Peter Rowlett takes a break from PhD preparation to explore Ox Block probabilities. "I'm not blogging much in the run up to my PhD thesis deadline, but my curiosity got the better of me with this one. Having seen (via Twitter) that it was being played at a Maths Jam, I bought an old game called Ox Blocks, which offers “Noughts and Crosses[/Tic Tac Toe] with a novel twist”. Here, I investigate the probabilities of rolling an unusual die."

Thomas Woolley writes Egg shells to turtle shells. "No matter how you initially orient the gömböc it will always wobble and rotate itself to finish standing upright. Importantly, the gömböc is made of only one material, so its density is uniform. Mathematically, the gömböc is known as a mono-monostatic body. This simply means that it has exactly one stable and one unstable equilibrium point."

Tony, a university maths professor in London, in My favorite equation considers whether there's a more interesting formula than Euler's formula. "So McKay's formula may not be as immediately beautiful as Euler's, but it has something of the same spirit (and perhaps even importance). It demonstrates a very deep connection between group theory and modular forms; it's mysterious and hard to understand, and it's inspiring important mathematics. And it says a lot about the serendipity which lies behind insights even in a subject as apparently logical and rigorous as mathematics."

Simon Gladman wonders what pendulum waves might sound like in The Sweet Sound of Pendulum Waves - in Glorious Stereo! "I had a little play with Pendulum Waves the other day and since then I've been wondering what sort of sound they would make if I played a tone as each pendulum reached its apex."

Have you ever wondered Why are determinants defined the weird way they are? If you've ever wondered why, whether or not you've studied linear algebra, you might enjoy this article. It'll give you some great material for your next party conversation!

Yao-Hong Kok is a Master's student studying control theory. Math, Control Theory and Two Issues invites interested parties into a discussion. "Control theory is one of those fields that requires a lot of mathematics. I have been in the field for roughly 2 years now and I have realized that they are 2 big issues within control theory, namely: (i) identity of a control engineer/theorist, and (ii) stagnation of fundamental theory advancements. In this post, I would like to relate mathematics to the above issues and perhaps generate some discussions."

Maria Droujkova shares Math dreams meeting May 20, 2013. "Curriculum developers' elephant in the room is a simple question: "Who wants that stuff, anyway?" We decided to ask parents what do they want for their kids, in math. Deep is the chasm between what parents want, and what existing curricula provide..."

Shecky Riemann, inspired by Martin Gardner's passing to start his blog, writes Remembering... Gardner three years after his death. "Not to take anything away from our Veterans, but this is a math blog, and I'll use the opportunity of Memorial Day to once again remember Martin Gardner, whose death just over 3 years ago inspired me to start this endeavor (with no idea it would still be up-and-running 3 years later!!)."

Herminio Lopez examines an interesting puzzle in A black (and red) hole. "Thanks to a prize consisting on the proceeds of a football match, we learn about some numbers that attract the others, which can't escape from them. Mathematical sequences which lead to mathematical black holes."

In Demystifying the Möbius, Burkard and Marty take readers on a nice journey through the many twists and turns that one can take with these paper treats.

Predicting Sums is a fun article at Grey Matters. It shows a nice math trick one can perform with a little knowledge of digital roots (aka nine's complements).

Math Munch is a great blog for children of all ages that describes itself as "A Weekly Digest of the Mathematical Internet." Their latest edition is Solitons, Contours, and Thinking Sdrawkcab. Check it out if you've not yet discovered this blog.

The Aperidical is another of my favorite blogs. They describe themselves as "a meeting-place for people who already know they like maths and would like to know more. It was begun by Katie Steckles, Christian Perfect and Peter Rowlett as a shared blogging outlet and grew out of our desire to have a place on the web where we could keep up to date with what’s going on elsewhere, and to share the mathematical things we do." You might also recognize Aperiodical as the stewards of this Math Carnival. Christian authored this fun piece, Integer sequence review: A000959.

If you've ever wondered what math and the meaning of life were related, check out 42 at Calculus Humor. This article deserves to go viral. Really.

Finally, I'll share one of my own favorite recent articles, Ken Fan: Inspired by Math #29. It's a podcast interview where Ken and I had a nice informal chat without much preparation before-hand.

The next carnival, #100, will be posted in July and hosted by Richard at Simple City. More information about the Carnival of Mathematics, and a submission link, is posted at the Aperiodical site.