# Paul Cohen (mathematician)

Not to be confused with Paul Cohn.
Paul J. Cohen
Born April 2, 1934
Long Branch, New Jersey
Died March 23, 2007 (aged 72)
Stanford, California, near Palo Alto
Fields Mathematics
Institutions Stanford University
Alma mater Stuyvesant High School
Brooklyn College
University of Chicago
Doctoral students Peter Sarnak
Known for Cohen forcing
Continuum hypothesis
Influences Georg Cantor, Kurt Gödel
Notable awards Bôcher Prize (1964)
Fields Medal (1966)
National Medal of Science (1967)

Paul Joseph Cohen (April 2, 1934 – March 23, 2007)[1] was an American mathematician best known for his proof of the independence of the continuum hypothesis and the axiom of choice from Zermelo–Fraenkel set theory, the most widely accepted axiomatization of set theory.

## Early years

Cohen was born in Long Branch, New Jersey, into a Jewish family that had immigrated to the United States from what is now Poland; he grew up in Brooklyn.[2][3] He graduated in 1950, at age 16, from Stuyvesant High School in New York City.[1][3]

Cohen next studied at the Brooklyn College from 1950 to 1953, but he left before earning his bachelor's degree when he learned that he could start his graduate studies at the University of Chicago with just two years of college. At Chicago, Cohen completed his master's degree in mathematics in 1954 and his Doctor of Philosophy degree in 1958, under supervision of the Professor of Mathematics, Antoni Zygmund. The subject of his doctoral thesis was Topics in the Theory of Uniqueness of Trigonometric Series.[4]

## Contributions to mathematics

Cohen is noted for developing a mathematical technique called forcing, which he used to prove that neither the continuum hypothesis (CH), nor the axiom of choice, can be proved from the standard Zermelo–Fraenkel axioms (ZF) of set theory. In conjunction with the earlier work of Gödel, this showed that both of these statements are logically independent of the ZF axioms: these statements can be neither proved nor disproved from these axioms. In this sense, the continuum hypothesis is undecidable, and it is the most widely known example of a natural statement that is independent from the standard ZF axioms of set theory.

For his result on the continuum hypothesis, Cohen won the Fields Medal in mathematics in 1966, and also the National Medal of Science in 1967.[5] The Fields Medal that Cohen won continues to be the only Fields Medal to be awarded for a work in mathematical logic, as of 2014.

Apart from his work in set theory, Cohen also made many valuable contributions to analysis. He was awarded the Bôcher Memorial Prize in mathematical analysis in 1964 for his paper "On a conjecture by Littlewood and idempotent measures", and lends his name to the Cohen-Hewitt factorization theorem.

Cohen was a full professor of mathematics at Stanford University, where he supervised Peter Sarnak's graduate research, among those of other students.

Angus MacIntyre of the University of London stated about Cohen: "He was dauntingly clever, and one would have had to be naive or exceptionally altruistic to put one's 'hardest problem' to the Paul I knew in the '60s." He went on to compare Cohen to Kurt Gödel, saying: "Nothing more dramatic than their work has happened in the history of the subject."[6] Gödel himself wrote a letter to Cohen in 1963, a draft of which stated, "Let me repeat that it is really a delight to read your proof of the ind[ependence] of the cont[inuum] hyp[othesis]. I think that in all essential respects you have given the best possible proof & this does not happen frequently. Reading your proof had a similarly pleasant effect on me as seeing a really good play."[7]

## On the continuum hypothesis

While studying the continuum hypothesis, Cohen is quoted as saying in 1985 that he "had the feeling that people thought the problem was hopeless, since there was no new way of constructing models of set theory. Indeed, they thought you had to be slightly crazy even to think about the problem."[8]

"A point of view which the author [Cohen] feels may eventually come to be accepted is that CH is obviously false. The main reason one accepts the axiom of infinity is probably that we feel it absurd to think that the process of adding only one set at a time can exhaust the entire universe. Similarly with the higher axioms of infinity. Now $\aleph_1$ is the cardinality of the set of countable ordinals, and this is merely a special and the simplest way of generating a higher cardinal. The set $C$ [the continuum] is, in contrast, generated by a totally new and more powerful principle, namely the power set axiom. It is unreasonable to expect that any description of a larger cardinal which attempts to build up that cardinal from ideas deriving from the replacement axiom can ever reach $C$.

Thus $C$ is greater than $\aleph_n, \aleph_\omega, \aleph_a$, where $a = \aleph_\omega$, etc. This point of view regards $C$ as an incredibly rich set given to us by one bold new axiom, which can never be approached by any piecemeal process of construction. Perhaps later generations will see the problem more clearly and express themselves more eloquently."[9]

An "enduring and powerful product" of Cohen's work on the continuum hypothesis, and one that has been used by "countless mathematicians"[8] is known as "forcing", and it is used to construct mathematical models to test a given hypothesis for truth or falsehood.

Shortly before his death, Cohen gave a lecture describing his solution to the problem of the continuum hypothesis at the Gödel centennial conference, in Vienna in 2006. A video of this lecture is now available online.[10]

## Selected publications

• Cohen J., Paul (December 1963). "The independence of the continuum hypothesis". Proceedings of the National Academy of Sciences of the United States of America 50 (6): 1143–1148.
• Cohen J., Paul (January 1964). "The independence of the continuum hypothesis, II". Proceedings of the National Academy of Sciences of the United States of America 51 (1): 105–110.

## References

1. ^ a b Levy, Dawn (2007-03-28). "Paul Cohen, winner of world’s top mathematics prize, dies at 72". Stanford Report. Retrieved 2007-10-31.
2. ^ Macintyre, A.J. "Paul Joseph Cohen", London Mathematical Society. Accessed March 3, 2011. "Cohen's origins were humble. He was born in Long Branch, New Jersey on 2 April 1934, into a Polish immigrant family."
3. ^ a b Albers, Donald J.; Alexanderson, Gerald L.; Reid, Constance, eds. (1990), "Paul Cohen", More Mathematical People, Harcourt Brace Jovanovich, pp. 42–58.
4. ^ Paul J. Cohen (1958), Topics in the theory of uniqueness of trigonometrical series , http://www.chronomaitre.org/cohen.pdf
5. ^ National Science Foundation - The President's National Medal of Science
6. ^ Davidson, Keay (2007-03-30). "Paul Cohen -- Stanford professor, acclaimed mathematician". San Francisco Chronicle. Retrieved 2007-10-31.
7. ^ Solomon Feferman, The Gödel Editorial Project: A synopsis [1] p. 11.
8. ^ a b Pearce, Jeremy (2007-04-02). "Paul J. Cohen, Mathematics Trailblazer, Dies at 72". New York Times. Retrieved 2007-10-31.
9. ^ Cohen, P. Set Theory and the continuum hypothesis. p. 151.
10. ^