Matankic · CC BY-SA 4.0

Jacques Hadamard proved the prime number theorem in 1896, settling a question about how prime numbers are distributed that had stood since Gauss, and went on to leave his name on a string of results still taught today, including the Hadamard matrix and the Cauchy-Hadamard theorem. A staunch defender of Alfred Dreyfus, his second cousin's husband, he fled occupied France for Columbia University during the Second World War, and trained doctoral students including André Weil and Paul Lévy.