Ah, you might be thinking of https://en.wikipedia.org/wiki/Diophantine_equation https://en.wikipedia.org/wiki/Diophantine_equation. Sure enough, https://en.wiki

Ah, you might be thinking of https://en.wikipedia.org/wiki/Diophantine_equation https://en.wikipedia.org/wiki/Diophantine_equation. Sure enough, https://en.wikipedia.org/wiki/Diophantus https://en.wikipedia.org/wiki/Diophantus says: "It should be mentioned here that Diophantus never used general methods in his solutions. Hermann Hankel, renowned German mathematician made the following remark regarding Diophantus. “Our author (Diophantos) not the slightest trace of a general, comprehensive method is discernible; each problem calls for some special method which refuses to work even for the most closely related problems. For this reason it is difficult for the modern scholar to solve the 101st problem even after having studied 100 of Diophantos’s solutions”." I didn't know this, thanks.