Around the turn of the 5th century, Hypatia led a renowned school of philosophy in Alexandria. A mathematician and astronomer, she wrote commentaries on major Greek texts, including works by Diophantus and Ptolemy, and drew students from afar. She is the first female mathematician whose life is well documented. In 415, she was murdered by a Christian mob amid violent political and religious conflict.
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.