A pair of mathematicians has solved a problem that had tantalized number-theory researchers for more than 8 decades. It is the so-called final problem of the legendary Indian mathematical genius Srinivasa Ramanujan.

"The mock theta functions are like beautiful butterflies that Ramanujan happened to find," says Freeman Dyson, an emeritus professor at the Institute for Advanced Study in Princeton, N.J. "But if you're a scientist, you want more—you want a theory of evolution, a framework of ideas to fit the butterflies in."

"I didn't really hope to see someone actually do this," says George Andrews of Pennsylvania State University in University Park, who had called the description of the mock theta functions one of the hardest math problems for the new millennium. Ono and Bringmann's accomplishment is "absolutely stunning," he concludes.