Zhang's proof, which is related to the gaps between pairs of consecutive prime numbers, is perhaps something only another mathematician can fully appreciate. But the story of his perseverance in the face of adversity was so dramatic that he became that rarity, a mathematician whose story breaks out into the mainstream press and social media. One article, in Wired Magazine online, garnered nearly 1,400 comments on Reddit, ranging from mathematical shoptalk to debates about the value of pure math to jokes about Subway "sandwich artists" with Ph.D.s.

Zhang has defied conventional wisdom in a number of ways. Previously unknown to many other scholars in his field, he's never held a tenure-track job. (He's been an instructor with a renewable contract at UNH since 1999.) And at 57, he's helped refute mathematician G.H. Hardy's famous assertion that math, "more than any other art or science, is a young man's game.

There's more. Although he dwells on a higher mathematical plane—and has a noticeable accent—Zhang (pronounced Jong) is a beloved calculus instructor. At the start of each semester, he writes his name on the board and says, "My name is Yitang Zhang, but in China, you would call me Zhang Yitang." Then he erases his name and says, "Just call me Tom." Students writing on RateMyProfessors.com rave about his humor, his enthusiasm, his straightforward approach, and his concern for students. He explains math with more math, they say. And you get used to the accent.

For Zhang, the path to mathematical celebrity started in China, where, as a teenager, he was exiled to the countryside during Mao's Cultural Revolution and forced to perform hard labor along with his parents, his sister, and other educated urban Chinese. Unable to attend school, he taught himself as much as he could from any book he could get his hands on. As the Cultural Revolution came to an end, he enrolled at Peking University in 1978 and earned both a bachelor's degree and a master's degree, a top student in a class of illustrious mathematicians.

Having always wanted to move to the United States, Zhang applied to Purdue. He completed his doctorate there in 1991 but couldn't get a university job after graduation. He worked for some time as an accountant for a company in Kentucky that owned several Subway sandwich shops. In a pinch, he would help out behind the counter, a fact that has been exaggerated in the press and has inspired online banter about a mathematical genius making sandwiches for a living.

After about seven years, Zhang was offered a position at UNH, thanks to the efforts of a couple of professors, including Kenneth Appel, then chair of the department and a renowned mathematician in his own right.

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