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Introducing the Klein bottle

A Klein bottle can't hold any liquid because it doesn't have an inside. How do you construct this strange thing and why would you want to?
Collection

The 2014 Plus advent calendar

In this year's advent calendar we bring you some of our favourite books and other mathematical toys, so you can surprise your friends and family or, even better, yourself!

Article

So you think you can add up?

Why do we add fractions the way we do? A very close look at addition reveals the answer — and it also works for negative numbers.