In 1742, Christian Goldbach wrote to Leonhard Euler: every even number greater than 2 is the sum of two primes, like 10, which is 3 plus 7 or 5 plus 5. Euler said he was convinced, but couldn't prove it. Nearly three centuries later, the claim has been checked by computer up to 4 quintillion, yet no one has proven it.
How many different structures can you build with six classic 2-by-4 Lego bricks of the same colour? The company used to quote just over 102 million. In 2005, Danish mathematician Søren Eilers recounted everything by computer: 915,103,765 combinations. The official estimate had been too low by a factor of about nine.
Fold an A4 sheet in half and you get an A5 with the same proportions. That only works for one ratio between length and width: about 1.414, the square root of 2. The A0 format measures exactly one square metre, and A4 is a sixteenth of it. With standard 80-gram-per-square-metre paper, an A4 sheet therefore weighs exactly 5 grams.