DOVER, N.H. – Kenneth Appel, a mathematician who was the first to use a computer to prove a major mathematical theorem, has died at age 80 in Dover, N.H.

The Tasker Funeral Home confirmed that Appel, who was diagnosed with esophageal cancer, died April 19 in hospice care.

Appel was a longtime educator who chaired the University of New Hampshire mathematics department, retiring in 2003. Before that, he was a faculty member at the University of Illinois at Urbana. In 1976, he and Wolfgang Haken used 1,200 hours of calculations from an IBM computer to prove that a flat map can be colored with just four colors, so that contiguous countries have different colors.

Proving the 100-year-old “four-color conjecture” was considered a major achievement at the time, though highly unpopular with some mathematicians who did not trust the performance of computers.

“The proof of the four-color conjecture is unlikely to be of applied significance,” a New York Times article said at the time. “Nevertheless, what has been accomplished is a major intellectual feat. It gives us an important new insight into the nature of two-dimensional space and of the ways in which such space can be broken into discrete portions.”

While Appel’s achievements were well known at UNH, and probably most math departments around the country, UNH math professor Kevin Short said Appel himself was “an incredibly humble man.”

“He felt that it was really something that came out of a great collaboration with Professor Wolfgang Haken, and that their interests, skills and background problems they had worked on dovetailed very well,” said Short, who was hired by UNH in the early 1990s when Appel was chairman of the math department.

He considered Appel a mentor.

“One of the things he prided himself on was trying to help other faculty members in general, but in particular, young faculty members, to get them started in a research career,” Short said.

Appel and Haken received the American Mathematical Society and the Mathematical Programming Society’s Delbert Ray Fulkerson prize in 1979.

Since retirement, Appel counseled students at Dover High School, helping to set up an Internet-based homework system developed at the University of Rochester. He also served on the Dover Board of Education.