I just started reading “The Unfinished Game” by Keith Devlin which documents a correspondence between Fermat and Pascal that literally changed the course of mathematics and even, some would argue, of history in general. The correspondence laid the foundations for probability theory.
What I just learned was something I’d never heard before:
“The modern, so-called Hindu-Arabic number system, developed in India between 200 and 700 A.D., was the first truly efficient way to record and compute with numbers, though the system was not available in the West until Leonardo of Lisa (whom later historians dubbed ‘Fibonacci’) learned it from North African traders and described it in his book Liber abaci (The Book of Calculating), which first appeared in 1202. ”
Fibonacci! What a guy!