### Mobile Numbers: Multiples of nine

In this series of posts, Katie investigates simple mathematical concepts using the Google Sheets spreadsheet app on her phone. If you have a simple maths trick, pattern or concept you’d like to see illustrated in this series, please get in touch.

We’re all (hopefully) aware that a pleasing property of numbers that are divisible by nine is that the sum of their digits is also divisible by nine.

It’s actually more well known that this works with multiples of three, and an even more pleasing fact is that the reason three and nine work is because nine is one less than the number base (10), and anything that’s a factor of this will also work – so, in base 13, this should work for multiples of 12, 6, 4, 3 and 2. Proving this is a bit of fun.

### The maths of the Grime Cube

Not content with already having five cubes named after him, internet maths phenomenon James Grime has now developed a new Rubik’s cube-style puzzle for internet maths joy merchants Maths Gear. I’ve been slightly involved in the development process, so I thought I’d share some of the interesting maths behind it.

Another name for a Rubik’s cube is ‘the Magic Cube’ – and Dr James Grime wondered if you could make a Magic Cube which incorporates its 2D friend, the Magic Square.

### Maths Journals for an engaged Sixth Former

Maths legend Colin Wright posed this question on Twitter:

It led to a flurry of interesting replies, and here’s some of them.

### Maths at the Manchester Science Festival 2016

Here’s our annual round-up of what’s happening in sums/thinking at this year’s Manchester Science Festival. If you’re local, or will be in the area around 20th-30th October, here’s our picks of the finest number-based shows, talks and events.

### Binary/Nail Varnish Puzzle – solution

Previously, we posted about Katie’s Binary Nail varnish tutorial video, and how you can use glitter (1) and no glitter (0) to encode binary messages in your nail varnish. We also posted an accompanying puzzle, stated as:

Suppose I want to paint my nails on one hand differently every day for a month – so I need to use all 31 combinations involving glitter. Assuming that a nail painted with plain varnish can have glitter added, but a nail with glitter needs to be nail-varnish-removed before it can become a plain nail again, what order do I apply the different combinations so that you minimise the amount of nail varnish remover I’ll need to use?

Here’s our take on the solution.

### Just how big is a big proof?

With news that a recent proof of the Boolean Pythagorean Triples Theorem is the ‘largest proof ever’, we collect and run-down some of the biggest, baddest, proofiest chunks of monster maths.

### The new Eurovision scoring system: the maths

It’s Eurovision time again! A chance for everyone to enjoy musical performances that are either good or so bad they’re good, ridiculous staging, and hilarious costumes, all sprinkled with a gently sarcastic Irish voiceover (if you’re lucky enough to be watching in the UK).

BUT WHAT’S THIS? They’ve changed the voting system? Don’t worry – some mathematicians are here to straighten it out for you.