Jacob Tsimerman won the biggest prize in math. Now he’s working on the most important problem of his career.When he won the Fields Medal last week, Jacob Tsimerman accepted the most prestigious honor in math wearing a powder-blue tuxedo with satin lapels.Then he made an announcement as striking as his tux. After the ceremony, the four winning mathematicians were asked what they planned to do next. One said he would keep working on partial differential equations, one said he wanted to experiment with artificial intelligence and one said she had absolutely no idea. That left Tsimerman.“I’ll be starting a position...