Diophantus of Alexandria

Diophantus of Alexandria (living around 250 AD, though even that is a guess) is often called the "father of algebra". Working in the great Greek city of Alexandria, he wrote a sprawling series of books called the Arithmetica full of clever puzzles asking for whole-number or fractional solutions to equations — and he was among the first people ever to use shorthand symbols instead of writing everything out in words.

Written into the field

Any equation where we hunt specifically for whole-number answers is now called a Diophantine equation in his honour. The simplest kind, like ax + by = c, asks which integer pairs make the equation true — a question that turns out to be tied intimately to greatest common divisors. From these humble puzzles grew an entire branch of number theory that is still fiercely active today.

Centuries later, a French lawyer named Pierre de Fermat was reading his copy of the Arithmetica and scribbled in the margin that he had a "truly marvellous proof" of a certain claim — but that the margin was "too narrow to contain it". That claim became Fermat's Last Theorem, and mathematicians chased it for 358 years until Andrew Wiles finally cracked it in 1994. So Diophantus's little puzzle book didn't just found algebra — it lit a fuse that burned for three and a half centuries. His own age at death is itself a famous riddle, encoded as an equation.