DOVER -- Kenneth I. Appel, 80, of Dover, a member of the city’s school board since 2010, died Friday, April 19, after a short hospitalization. He had been diagnosed with esophageal cancer in October.

He chaired the mathematics department of the University of New Hampshire from 1993 to 2002, retiring as professor emeritus in 2003. From 1961 to 1993 he was a mathematics professor at the University of Illinois, where he and his colleague Wolfgang Haken solved one of the most famous problems in mathematics: the Four Color Conjecture. They answered this question: could it be proved that any map can be colored with just four colors so that contiguous countries have different colors? Their proof that four colors were sufficient, announced in 1976, was the first major mathematical theorem to be proved with the aid of a computer.

Kenneth Ira Appel was born in Brooklyn, NY, on October 8, 1932, and grew up in Queens, NY, the son of Irwin Appel and Lillian Sender Appel. He received a bachelor’s degree in mathematics from Queens College in 1953. After working briefly as an actuary, he served two years in the U.S. Army, at Fort Benning, Georgia, and in Baumholder, Germany.

He received his Ph.D. in mathematics from the University of Michigan in 1959. In Philadelphia that year he married Carole S. Stein. They moved to Princeton, NJ, where Ken worked for the Institute for Defense Analyses, doing research in cryptography. After joining the University of Illinois at Urbana he taught mathematics, did research on group theory and computability theory. He served on the Urbana city council and later on the city’s zoning board.

The New York Times wrote in 1976, "Now the four-color conjecture has been proved by two University of Illinois mathematicians, Kenneth Appel and Wolfgang Haken. They had an invaluable tool that earlier mathematicians lacked--modern computers. Their present proof rests in part on 1,200 hours of computer calculation during which about ten billion logical decisions had to be made. The proof of the four-color conjecture is unlikely to be of applied significance. 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."

In 1979 the American Mathematical Society and the Mathematical Programming Society awarded the Delbert Ray Fulkerson prize to Appel and Haken. In 2012 he was elected a Fellow of the American Mathematical Society.

Ken Appel was passionate about mathematics education, at all levels from elementary school to graduate school. At every stage of his life, he sought opportunities to teach – as a graduate student in Michigan, as a volunteer lecturer at Princeton University, in more than fifty years as a professor, and in retirement as a volunteer in mathematics enrichment programs in Dover and southern Maine public schools. He particularly believed that students should be afforded the opportunity to study mathematics at the level of their ability, even if it is well above their grade level. To that end, he enjoyed working with gifted high school students on the kind of math problems he would give to his graduate students.

He was treasurer of the Strafford County Democratic Committee. He was an avid tennis player until his fifties, a lifelong stamp collector, a skillful player of the game of Go, a baker of bread. He was a loving husband, father, and grandfather.

Kenneth Appel is survived by his wife Carole S. Appel, son Andrew W. Appel and his wife Maia Ginsburg, of Princeton, NJ; son Peter H. Appel and his wife Bárbara Zamora-Appel, of Alexandria, VA; sister Lois Green of Henderson, NV, and her husband David Green; nephew Douglas Green and niece Lisa Bouley; by five grandchildren, Avi and Joseph Appel, Rebecca and Nathaniel Weir, and Carmen Appel, and by his brother-in-law Howard Stein and his wife Lea.

Carole and Ken’s daughter Laurel F. Appel, a biologist at Wesleyan University, died on March 4 this year at the age of 50 and is survived by her husband, Michael P. Weir.

Contributions in Kenneth Appel’s memory can be made to the Laurel F. Appel Fund for Student Research, Wesleyan University, c/o M. Herlihy, 318 High St., Middletown, CT 06459, or at give.Wesleyan.edu, specifying that the gift is for the Laurel Appel fund.

Funeral services will be on Monday, April 22, at 10 a.m., at Tasker Funeral Home in Dover with visiting hours at 9:30 a.m. and burial at Pine Hill Cemetery.