The letter from Dedekind to Cantor, dated November 30, 1873, that went missing for more than a century. In it, Dedekind provides a proof that the set of algebraic numbers is the same size as the set of whole numbers — a result that Cantor later plagiarized.

According to the mathematician Ferdinand von Lindemann, who proved that π isn’t algebraic — you can never pose an ordinary algebra problem where π is the answer — Kronecker once told him his work was worthless, since such “transcendental” numbers didn’t exist.

Dedekind quickly replied that neither could he — but that he’d worked out a proof that the algebraic numbers (the numbers you get as solutions to algebra problems) could be counted.