Leon Albert Henkin (19 April 1921–1 November 2006) was a logician at the University of California, Berkeley. He was principally known for the "Henkin completeness proof": his version of the proof of the semantic completeness of standard systems of firstorder logic.
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Henkin's result was not novel; it had first been proved by Kurt Gödel in his doctoral dissertation which was completed in 1929. (See Gödel's completeness theorem. Gödel published a version of the proof in 1930.) Henkin's 1949 proof is much easier to survey than Gödel's and has thus become the standard choice of completeness proof for presentation in introductory classes and texts.
The proof is nonconstructive (a pure existence proof): while it guarantees that if a sentence α follows (semantically) from a set of sentences Σ, then there is a proof of α from Σ, it gives no indication of the nature of that proof.
Henkin originally proved the completeness of Church's higherorder logic, and then observed that the same methods of proof could be applied to firstorder logic. Henkin's proof for higherorder logic uses a variant of the standard semantics. This variant uses general models (also called Henkin models): the higher types need not be interpreted by the full space of functions; a subset of the function space may be used instead.
He was born in Brooklyn, into a Russian Jewish immigrant family. His father expressed his high expectations for him by giving him the middle name "Albert"; at the time, the New York Times ran a series of articles on Albert Einstein's theory of relativity. However, Henkin did not use his middle name in his mathematical publications.
His first degree was in mathematics and philosophy from Columbia College, in 1941. He took a master's degree there in 1942.
He then worked in the Signal Corps Radar Laboratory, Belmar, New Jersey. As participant in the Manhattan project, he worked on isotope diffusion, in New York, and Oak Ridge, Tennessee.
He was a doctoral student of Alonzo Church at Princeton University, receiving his Ph.D. in 1947. He became Professor of Mathematics at the University of California, Berkeley, where he had a position from 1953. He received the 1964 Chauvenet Prize for exposition.[3] He was a collaborator of Alfred Tarski, and an ally in promoting logic.^{[1]}
Henkin was also a social activist who since the 1960s worked to increase higher education opportunities for women and minorities. In 1964 he spearheaded the formation of the Special Scholarships Committee at UC Berkeley, which resulted in setting up Special Opportunity Scholarships and other outreach programs at Berkeley. The Berkeley program served as a model for the federal Upward Bound Program that was founded several years later and for many outreach and special opportunities programs at other U.S. universities.^{[2]}^{[3]}^{[4]}
