# Maths in a minute

Want facts and want them fast? Our Maths in a minute series explores key mathematical concepts in just a few words. From symmetry to Euclid's axioms, and from binary numbers to the prosecutor's fallacy, learn some maths without too much effort.

Maths in a minute: Perfect numbers

Of numbers, beauty and bizarre animals.

Maths in a minute: Osculating the curve…

We are very pleased with our new toy demonstrating the curvature of any smooth function – now you can try it out too!

Maths in a minute: Counting numbers

Are there more irrational numbers than rational numbers, or more rational numbers than irrational numbers? Well, there are infinitely many of both, so the question doesn't make sense. It turns out, however, that the set of rational numbers is infinite in a very different way from the set of irrational numbers.

Maths in a minute: Is greed good?

Not necessarily, when it comes to money.

Maths in a minute: The bridges of Königsberg

How to make a hard problem easy by changing the way you look at it.

Maths in a minute: The Riemann sphere

What happens when you shrink infinity to a point? You get a sphere!

Maths in a minute: Number mysteries

Number theory is famous for problems that everyone can understand and that are easy to express, but that are fiendishly difficult to prove. Here are some of our favourites.

Maths in a minute: Not always 180

Did you learn at school that the angles in a triangle always add up to 180 degrees? If yes then your teacher was wrong. Find out why here.

Maths in a minute: Shake to solve

A quick and easy way of adding using handshakes.

Maths in a minute: Arrow's theorem

Is there a perfect voting system? In the 1950s the economist Kenneth Arrow asked himself this question and found that the answer is no, at least in the setting he imagined.

Maths in a minute: The missing pound

Here's a well-known conundrum: suppose I need to buy a book from a shop that costs £7. I haven't got any money, so I borrow £5 from my brother and £5 from my sister. I buy the book and get £3 change. I give £1 back to each my brother and sister and I keep the remaining £1. I now owe each of them £4 and I have £1, giving £9 in total. But I borrowed £10. Where's the missing pound?

• Want facts and want them fast? Our Maths in a minute series explores key mathematical concepts in just a few words.