Church, Alonzo (1903-1995)

US mathematician. In 1936 he published the first precise definition of a calculable function, and so contributed enormously to the systematic development of the theory of algorithms. The solving of algorithmic problems involves the construction of an algorithm capable of solving a given set with respect to some other set, and if such an algorithm cannot be constructed, it signifies that the problem is unsolvable. From English mathematician Alan Turing's thesis, Church proved that there were no algorithms for a class of quite elementary arithmetical questions.

Theorems establishing the unsolvability of such problems are among the most important in the theory of algorithms, and Church's theorem was the first of this kind. He also established the unsolvability of the solution problem for the set of all true propositions of the logic of prediction.

Church was educated at Princeton and remained there for 40 years, becoming professor of mathematics and philosophy. In 1967 he moved to the University of California in Los Angeles.