Euclid’s algorithm [1] for finding the greatest common divisor of two numbers probably belongs on the list. As the name implies it was first described by Euclid

Euclid’s algorithm [1] for finding the greatest common divisor of two numbers probably belongs on the list. As the name implies it was first described by Euclid, in 300 BC(!). It’s still widely used today. 1. https://en.m.wikipedia.org/wiki/Euclidean_algorithm https://en.m.wikipedia.org/wiki/Euclidean_algorithm