# Alonzo Church

> American mathematician and logician (1903–1995)

**Wikidata**: [Q92741](https://www.wikidata.org/wiki/Q92741)  
**Wikipedia**: [English](https://en.wikipedia.org/wiki/Alonzo_Church)  
**Source**: https://4ort.xyz/entity/alonzo-church

## Summary
Alonzo Church was an American mathematician and logician (1903–1995) best known as the creator of lambda calculus, a foundational system in computer science and mathematical logic. His work established the Church–Turing thesis, which defines the limits of computability.

## Biography
- Born: June 14, 1903, Washington, D.C.
- Nationality: United States
- Education: Ph.D. in Mathematics from Princeton University (1927); also studied at Harvard University (1927), University of Göttingen (1928), and University of Amsterdam (1929)
- Known for: Lambda calculus, Church–Turing thesis, Church–Rosser theorem
- Employer(s): Princeton University (1929–1967), University of California, Los Angeles (1967–1990)
- Field(s): Mathematical logic, theoretical computer science, mathematics, logic

## Contributions
Alonzo Church developed lambda calculus in the 1930s, creating a formal system for expressing computation through function abstraction and application. This work became fundamental to the theory of programming languages and functional programming. In 1936, Church independently proved the unsolvability of the Entscheidungsproblem (decision problem), publishing his findings alongside Alan Turing's work on computability. His Church–Turing thesis established that any effectively calculable function can be computed by a Turing machine, defining the theoretical limits of computation. Church also formulated the Church–Rosser theorem, which proves the consistency of lambda calculus by showing that different reduction paths lead to the same result. His doctoral students included future computing pioneers like Stephen Cole Kleene, John George Kemeny, and Michael O. Rabin, who collectively shaped theoretical computer science.

## FAQs
### Q: What is lambda calculus and why is it important?
A: Lambda calculus is a formal system for expressing computation through function definition and application, developed by Alonzo Church in the 1930s. It became the foundation for functional programming languages and theoretical computer science.

### Q: What is the Church–Turing thesis?
A: The Church–Turing thesis states that any function that can be effectively computed by an algorithm can be computed by a Turing machine. This principle defines the theoretical limits of what computers can calculate.

### Q: Where did Alonzo Church work during his career?
A: Church spent most of his career at Princeton University (1929–1967) as a professor, then moved to the University of California, Los Angeles (1967–1990) where he continued his research and teaching.

## Why They Matter
Alonzo Church's work established the theoretical foundations of computer science before electronic computers existed. His lambda calculus provided the mathematical framework for functional programming, influencing languages like Lisp, Haskell, and modern functional features in mainstream languages. The Church–Turing thesis created the conceptual boundary between what is computable and what is not, guiding computer scientists for generations. His students went on to make fundamental contributions to logic, computability theory, and computer science, creating a lineage of theoretical computer science that continues today. Without Church's formal systems and theorems, modern computing would lack its rigorous mathematical underpinnings, and our understanding of computation's limits would be far less developed.

## Notable For
- Created lambda calculus, the foundation of functional programming
- Established the Church–Turing thesis defining computability
- Supervised numerous influential computer scientists as doctoral students
- Formulated the Church–Rosser theorem proving consistency of lambda calculus
- Published foundational papers on the Entscheidungsproblem in 1936

## Body
### Early Life and Education
Alonzo Church was born on June 14, 1903, in Washington, D.C. He earned his Ph.D. from Princeton University in 1927 under the supervision of Oswald Veblen, with a dissertation titled "Alternatives to Zermelo's Assumption." His early education included studies at Harvard University (1927), University of Göttingen (1928), and University of Amsterdam (1929).

### Academic Career
Church joined Princeton University as a faculty member in 1929, where he remained until 1967. He then moved to the University of California, Los Angeles, serving there until his retirement in 1990. Throughout his career, he supervised 10 doctoral students who became prominent figures in mathematics and computer science.

### Key Theoretical Contributions
In 1936, Church published his proof of the unsolvability of the Entscheidungsproblem, introducing lambda calculus as a formal system for expressing computation. This work paralleled Alan Turing's development of the Turing machine concept. Church's lambda calculus used anonymous functions and substitution to represent computation, a concept that would later influence programming language design.

### Major Theorems and Concepts
The Church–Rosser theorem, proved by Church, demonstrates that in lambda calculus, different reduction paths that terminate will always reach the same result. This property ensures the consistency of the system. The Church–Turing thesis, developed through Church's work with Turing, established that any effectively calculable function can be computed by a Turing machine, defining the theoretical limits of computation.

### Legacy and Influence
Church's doctoral students included Stephen Cole Kleene (who developed regular expressions and recursion theory), John George Kemeny (co-developer of the BASIC programming language), and Michael O. Rabin (Turing Award winner for randomized algorithms). His work continues to influence theoretical computer science, programming language theory, and mathematical logic.

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## References

1. Virtual International Authority File
2. BnF authorities
3. MacTutor History of Mathematics archive
4. Find a Grave
5. [Source](http://nassauchurch.org/about/princetoncemetery/interment-records/)
6. Mathematics Genealogy Project
7. MGP
8. International Standard Name Identifier
9. CiNii Research
10. NNDB
11. [Source](https://vls.hsa.ethz.ch/client/link/de/archiv/einheit/195ac2f6479044ef8ff1d609d697be08)
12. SNAC
13. Brockhaus Enzyklopädie
14. Internet Philosophy Ontology project
15. Croatian Encyclopedia
16. Proleksis Encyclopedia
17. Freebase Data Dumps. 2013
18. Integrated Authority File
19. CONOR.SI
20. Quora
21. LIBRIS. 2018
22. Bibliography of the History of the Czech Lands