After 2000 years of searching we only know of 51 perfect numbers and they're all even. What else do and don't we know about these rare and precious jewels of maths?
Nobody has ever found an odd perfect number. So do they exist at all and, if yes, what do they look like?
Perfect numbers have foxed mathematicians for over 2000 years. Here's a quick look at the long struggle to find them.
A new prime number with nearly 25 million digits was discovered just in time for Christmas.