### Klein: outside the bottle

If you’ve heard of Felix Klein, it’s probably due to the Klein bottle, that strange four-dimensional object that is the subject of a new video here on The Aperiodical starring Katie Steckles and Matt Parker. Who is Klein and, apart from the bottle, what did he do?

Klein’s Times obituary records that he would point out “with a smile” that his date of birth comprised three squares of primes. So then, I will refer to his birth as taking place on the $( 5^2 )^{\textrm{th}}$ day of the $(2^2 )^{\textrm{th}}$ month in the year $43^2$ in Prussia. You might like to notice that today is the $( 5^2)^{\textrm{th}}$ day of the $( 2^2 )^{\textrm{th}}$ month as well, so it is the 163rd anniversary of Klein’s birth.

### Electoral reforms and non-transitive dice

Guest post by Andrew, of Manchester MathsJam. Andrew can be found on Twitter as @andrew_taylor and blogs occasionally about maths, among other things, at andrewt.net.

Grime Dice” are a set of five coloured dice with unusual combinations of numbers on them. The red die, for example, has five fours and a nine. The blue one has three twos and three sevens, so it loses to the red die about 58% of the time. The green die has five fives and a zero, and will lose to the blue one in 58% of rolls. What makes them interesting is that the green die will beat the red one in 69% of rolls. These three dice behave rather like rock-paper-scissors — in mathematical terms, they are ‘non-transitive’. The full set of Grime Dice also has a purple and a yellow die, so a better analogy would be rock-paper-scissors-lizard-Spock.

### Words to Fill Space

For the April 2012 issue of Puzzlebomb, I devised Hilbert’s Space-Filling Crossword:

The five clues lead to four four-letter words along the rows of the grid, and one sixteen-letter word snaking round the shape given by the thick lines. The puzzle gets its name from the shape traced out by the long word, which is the second iteration of Hilbert’s space-filling curve.

### Another black and white hats puzzle

A classic maths puzzle involves a line of one hundred prisoners, who have each been given a black or white hat by their nefarious captor, and must each correctly shout out the colour of their hat to win freedom. The twist is that the prisoners don’t know the colour of their own hat, and though they can see the colours of the hats in front of them, they don’t know many of each colour there are overall. They can confer on a strategy beforehand, and the aim is to get as many of them to correctly identify their hat colour as possible. You can find a full explanation here (and in many other places!)

There are several ‘sequels’ to this puzzle, some involving an infinite number of prisoners and requiring the axiom of choice to solve. This post is about a nice variation on the theme that I heard about at a recent MathsJam. It can (just about) be solved without knowledge of higher mathematics, and though it seems impossible at first glance, the prisoners in this situation can in fact save themselves with 100% certainty.