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Mathematicians and psychologists don't cross paths that often and when they do you wouldn't expect it to involve an (apparently) unassuming puzzle like the Tower of Hanoi. Yet, the puzzle holds fascination in both fields. 
In the 1930s the logician Kurt Gödel showed that if you set out proper rules for mathematics, you lose the ability to decide whether certain statements are true or false. This is rather shocking and you may wonder why Gödel's result hasn't wiped out mathematics once and for all. The answer is that, initially at least, the unprovable statements logicians came up with were quite contrived. But are they about to enter mainstream mathematics? 
The human brain faces a
difficult tradeoff. On the one hand it needs to be complex to ensure high performance, and on the other it needs to minimise "wiring cost" — the sum of the length of all the connections —
because communication over distance takes a lot of energy. It's a problem wellknown to computer scientists. And it seems that market driven human invention and natural selection have come up with similar solutions.

The human genome is represented by a sequence of 3 billion As, Cs, Gs, and Ts. With such large numbers, sequencing the entire genome of a complex organism isn't just a challenge in biochemistry. It's a logistical nightmare, which can only be solved with clever algorithms. 
In last issue's Graphical methods I Phil Wilson used maths to predict the outcome of a cold war in slug world. In this selfcontained article he looks at slug world after the disaster: with only a few survivors and all infrastructure destroyed, which species will take root and how will they develop? Graphs can tell it all.

To arm or to disarm? This is the question in Phil Wilson's article, which explores the maths behind a cold war in slug world.
