Kenneth Appel

Kenneth Ira Appel was an American mathematician who in 1976, with colleague Wolfgang Haken at the University of Illinois at Urbana–Champaign, solved the four-color theorem, one of the most famous problems in mathematics. They proved that any two-dimensional map, with certain limitations, can be filled in with four colors without any adjacent "countries" sharing the same color. The proof was controversial because it depended on thousands of computer calculations that could not be double-checked by hand, the first prominent example of such a process.