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Showing posts from July, 2013

### Solving Sir Arthur Eddington's Zoo Puzzle using Dirac Matrices

Sir Arthur Eddington posed this difficult logic puzzle to the readers of Caliban's (Hubert Phillips) newspaper puzzle columns, and famously (or infamously) provided a solution using Dirac matrices.

Sir Eddington's zoo puzzle:

I took some nephews and nieces to the Zoo, and we halted at a cage marked
Tovus Slithius, male and female. Beregovus Mimsius, male and female. Rathus Momus, male and female. Jabberwockius Vulgaris, male and female.  The eight animals were asleep in a row, and the children began to guess which was which. "That one at the end is Mr Tove." "No, no! It's Mrs Jabberwock," and so on. I suggested that they should each write down the names in order from left to right, and offered a prize to the one who got most names right.

As the four species were easily distinguished, no mistake would arise in pairing the animals; naturally a child who identified one animal as Mr Tove identified the other animal of the same species as Mrs Tove.

The keepe…

### Solving the TopSpin Puzzle using GAP

TopSpin is an oval-track permutation puzzle that was made by Binary Arts; similar puzzles are made and sold by other manufacturers. Here's the Binary Arts TopSpin.

It's not a difficult puzzle to solve if you play around with it for a few hours and figure out how to generate various permutations. It's more interesting (and difficult) if you observe that the turntable has a distinguishable top and bottom. This suggests an interesting question - can you invert the turntable while keeping the numbers in the track in the same order?

The answer is, perhaps surprisingly, yes. Here's one way to find a sequence of operations that produces precisely this outcome.

GAP (Groups, Algorithms and Programming) is a freely available programming language that specializes in computational group theory, and it's perfect for solving permutation puzzles. Here's my GAP code for TopSpin. Label the top of the turntable with 21 and the bottom with 22. Flipping the turntable generates the …

### A Slightly Less Pointless Solution to Le Monde puzzle #824

Here's Le Monde puzzle #824:

Show that, for any integer $$y$$, $(\sqrt{3}-1)^{2y}+(\sqrt{3}+1)^{2y}$ is an integer multiple of a power of two.

Solution:

Consider $f(n) = (-1+\sqrt{3})^{n}+(-1-\sqrt{3})^{n}$ and observe that the two bases are the roots of the quadratic $$x^2 + 2x - 2$$, hence $$f(n)$$ obeys the recursion $$x_{n+2} = -2 x_{n+1} + 2 x_n$$ with $$x_0=2$$ and $$x_1=-2$$. It follows that $$f(n)$$ is always an even integer.