# Emil Leon Post

Emil Leon Post | |
---|---|

Scientific career | |

Fields | Mathematics, logic |

Institutions | Princeton University |

Thesis | Introduction to a General Theory of Elementary Propositions (1920) |

Doctoral advisor | Cassius Jackson Keyser |

**Emil Leon Post** (

## Life

Post was born in

^{[2]}

Post had been interested in astronomy, but at the age of twelve lost his left arm in a car accident. This loss was a significant obstacle to being a professional astronomer, leading to his decision to pursue mathematics rather than astronomy.^{}[3]

Post attended the Townsend Harris High School and continued on to graduate from City College of New York in 1917 with a B.S. in mathematics.^{[1]}

After completing his

Post married Gertrude Singer in 1929, with whom he had a daughter, Phyllis Post Goodman (1932–1995).^{}[4] Post spent at most three hours a day on research on the advice of his doctor in order to avoid manic attacks, which he had been experiencing since his year at Princeton.^{[5]}

In 1936, he was appointed to the mathematics department at the City College of New York. He died in April 1954 of a heart attack following electroshock treatment for depression, ^{[5]}^{[6]} less than two months before the death of Alan Turing. He was 57.

## Early work

In his doctoral thesis, later shortened and published as "Introduction to a General Theory of Elementary Propositions" (1921), Post proved, among other things, that the propositional calculus of *Principia Mathematica* was complete: all tautologies are theorems, given the *Principia* axioms and the rules of substitution and modus ponens. Post also devised truth tables independently of C. S. Peirce and Ludwig Wittgenstein and put them to good mathematical use. Jean van Heijenoort's well-known source book on mathematical logic (1966) reprinted Post's classic 1921 article setting out these results.

While at Princeton, Post came very close to discovering the incompleteness of *Principia Mathematica*, which Kurt Gödel proved in 1931. Post initially failed to publish his ideas as he believed he needed a 'complete analysis' for them to be accepted.^{[2]} As Post said in a postcard to Gödel in 1938:

- I would have discovered Gödel's theorem in 1921—if I had been Gödel.
^{[7]}

## Recursion theory

In 1936, Post developed, independently of

Correspondence systems were introduced by Post in 1946 to give simple examples of

In an influential address to the

## Polyadic groups

Post made a fundamental and still-influential contribution to the theory of polyadic, or *n*-ary, groups in a long paper published in 1940. His major theorem showed that a polyadic group is the iterated multiplication of elements of a normal subgroup of a group, such that the quotient group is cyclic of order *n* − 1. He also demonstrated that a polyadic group operation on a set can be expressed in terms of a group operation on the same set. The paper contains many other important results.

## Selected papers

- Post, Emil Leon (1919). "The Generalized Gamma Functions". JSTOR 1967871.
- Post, Emil Leon (1921). "Introduction to a General Theory of Elementary Propositions".
*.* - Post, Emil Leon (1936). "Finite Combinatory Processes – Formulation 1".
*.* - Post, Emil Leon (1940). "Polyadic groups". JSTOR 1990085.
- Post, Emil Leon (1943). "Formal Reductions of the General Combinatorial Decision Problem".
*American Journal of Mathematics*.**65**(2): 197–215. . - Post, Emil Leon (1944). "Recursively enumerable sets of positive integers and their decision problems".
*Bulletin of the American Mathematical Society*.**50**(5): 284–316.doi:10.1090/s0002-9904-1944-08111-1. Introduces the important concept of many-one reduction.

*
*## See also

- Arithmetical hierarchy
- Functional completeness
- List of multiple discoveries
- List of pioneers in computer science

## Notes

- ^
^{a}^{b}Urquhart (2008) - ^
^{a}^{b}^{c}O'Connor, John J.; Robertson, Edmund F., "Emil Leon Post",*MacTutor History of Mathematics Archive*, University of St Andrews **^**Urquhart (2008), p. 429.**^**"Phyllis Post Goodman Park".*NYC Parks*.- ^
^{a}^{b}Urquhart (2008), p. 430. - ISBN 9781139498432.
**.****.**

**
**## References

- Urquhart, Alasdair (2008). "Emil Post" (PDF). In Gabbay, Dov M.; Woods, John Woods (eds.).
*Logic from Russell to Church*. Handbook of the History of Logic. Vol. 5. Elsevier BV. - Neary, Turlough (2015), "Undecidability in binary tag systems and the post correspondence problem for five pairs of words", International Symposium on Theoretical Aspects of Computer Science, Leibniz International Proceedings in Informatics (LIPIcs), pages 649–661, 2015.

## Further reading

- Anshel, Iris Lee; Anshel, Michael (November 1993). "From the Post–Markov Theorem Through Decision Problems to Public-Key Cryptography".
*The American Mathematical Monthly*.**100**(9). Mathematical Association of America: 835–844.JSTOR 2324657.- Dedicated to Emil Post and contains special material on Post. This includes "Post's Relation to the Cryptology and Cryptographists of his Era: ... Steven Brams, the noted game theorist and political scientist, has remarked to us that the life and legacy of Emil Post represents one aspect of New York intellectual life during the first half of the twentieth century that is very much in need of deeper exploration. The authors hope that this paper serves to further this pursuit". (pp. 842–843)

- Davis, Martin, ed. (1993).
*The Undecidable*. Dover. pp. 288–406.ISBN 0-486-43228-9.- Reprints several papers by Post.

- Davis, Martin (1994). "Emil L. Post: His Life and Work".
*Solvability, Provability, Definability: The Collected Works of Emil L. Post*. Birkhäuser. pp. xi–xxviii.- A biographical essay.

- Jackson, Allyn (May 2008). "An interview with Martin Davis".
*Notices of the AMS*.**55**(5): 560–571.- Much material on Emil Post from his first-hand recollections.

- Jackson, Allyn (October 2018). "Emil Post: Psychological Fidelity".
*Inference: International Review of Science*.S2CID 240012225.- A biographical article.

## External links

- Emil Leon Post Papers 1927-1991, American Philosophical Society, Philadelphia, Pennsylvania.
- "Celebrating Emil Post & His "Intractable Problem" of Tag: 100 Years Later".
*YouTube*. Wolfram. May 19, 2021. Archived from the original on 2021-12-21.