On a Saturday morning in 1984, when Ken Ono was in high school, he opened his family's mailbox in Baltimore and found an envelope as thin as rice paper covered in brilliantly colored stamps. It was addressed to his father, a reserved Japanese mathematician. When Ono handed over the mail, the elder Ono looked up from the yellow legal pad on which he was always scribbling equations and set down his ballpoint pen. Gently, he pried open the seal and unfolded the letter inside.