Kenneth Ira Appel (October 8, 1932 – April 19, 2013) 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.

He worked as an actuary for a brief time and then served in the U.S. Army for two years at Fort Benning, Georgia, and in Baumholder, Germany.

After working for the Institute for Defense Analyses, in 1961 Appel joined the Mathematics Department faculty at the University of Illinois as an Assistant Professor.

During his retirement he volunteered in mathematics enrichment programs in Dover and in southern Maine public schools.

"[3] Kenneth Appel is known for his work in topology, the branch of mathematics that explores certain properties of geometric figures.

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.

Their present proof rests in part on 1,200 hours of computer calculation during which about ten billion logical decisions had to be made.

"[7] The actual proof was described in an article as long as a typical book titled Every Planar Map is Four Colorable, Contemporary Mathematics, vol.

Appel and Haken agreed in a 1977 interview that it was not "elegant, concise, and completely comprehensible by a human mathematical mind".

[8] Nevertheless, the proof was the start of a change in mathematicians' attitudes toward computers—which they had largely disdained as a tool for engineers rather than for theoreticians—leading to the creation of what is sometimes called experimental mathematics.