Paul Erdos

Paul Erdos, who died in 1996, was one of the most prolific mathematicians who ever lived, publishing with hundreds of collaborators and travelling between them with almost no possessions. He is remembered as much for the problems he posed as for the ones he solved, and many carried small cash prizes he paid himself.

Those open problems are now catalogued online, each one precisely stated and independently checkable. That makes them close to an ideal benchmark for AI systems, which is why every few months another Erdos problem falls to a model and makes the news.