# 42 (number)
**42** (forty-two; Roman numeral XLII) is the natural number following 41 and preceding 43, the product of the primes 2, 3, and 7, and the number that [Douglas Adams](https://www.edgepedia.org/douglas-adams)'s comic computer Deep Thought gives as the "Answer to the Ultimate Question of Life, the Universe, and Everything" in *The Hitchhiker's Guide to the Galaxy*.[1](https://www.theguardian.com/books/2011/feb/03/douglas-adams-42-hitchhiker) • [2](https://www.numberempire.com/42) That single joke has made 42 one of the most recognized numbers in popular culture.[1](https://www.theguardian.com/books/2011/feb/03/douglas-adams-42-hitchhiker)
| Key fact | Detail |
|---|---|
| Arithmetic | 42 = 2 × 3 × 7; divisors 1, 2, 3, 6, 7, 14, 21, 42 (8 in total); sum of divisors 96; Euler's totient φ(42) = 12[2](https://www.numberempire.com/42) |
| Representations | Binary 101010, octal 52, hexadecimal 2A, Roman XLII[2](https://www.numberempire.com/42) • [3](https://science.slc.edu/~jmarshall/courses/2007/spring/hello-world/notes/week06/number_systems.html) |
| Combinatorics | The 5th Catalan number, C₅ = 42[2](https://www.numberempire.com/42) • [4](https://www.samuelfhopkins.com/classes/UMN/4707/catalan_worksheet.pdf) |
| Fictional role | Deep Thought's answer, announced after 7.5 million years of computation in Adams's *The Hitchhiker's Guide to the Galaxy*[1](https://www.theguardian.com/books/2011/feb/03/douglas-adams-42-hitchhiker) |
| First broadcast | BBC Radio 4, "Fit the Fourth", 29 March 1978, with Geoffrey McGivern voicing Deep Thought[5](https://quotle.info/who-said/the-answer-to-the-great-question-of-life-the-universe-and/) |
| Chemistry | Atomic number of molybdenum (Mo), relative atomic mass 95.95[6](https://periodic-table.rsc.org/element/42/MolybdenumH) |
| Geometry | Largest possible r in a solution of 1/p + 1/q + 1/r = 1/2, namely (3, 7, 42)[7](https://math.ucr.edu/home/baez/42.html) |
## Origin in The Hitchhiker's Guide to the Galaxy
In the story, a giant computer called Deep Thought spends exactly 7.5 million years pondering Life, the Universe, and Everything, then solemnly announces that the Ultimate Answer is "Forty two".[1](https://www.theguardian.com/books/2011/feb/03/douglas-adams-42-hitchhiker) The gag first reached the public on radio: Deep Thought gives its answer in "Fit the Fourth", broadcast on [BBC Radio 4](https://www.edgepedia.org/bbc-radio-4) on 29 March 1978, with Geoffrey McGivern voicing the computer.[5](https://quotle.info/who-said/the-answer-to-the-great-question-of-life-the-universe-and/) The radio comedy was first broadcast in 1978 and the joke went on to develop, in the Guardian's words, "a life quite unlike any other joke, before or since".[1](https://www.theguardian.com/books/2011/feb/03/douglas-adams-42-hitchhiker)
**Adams's own explanation.** Adams repeatedly said the choice was arbitrary. In his own words: "It was a joke. It had to be a number, an ordinary, smallish number, and I chose that one. I sat at my desk, stared into the garden and thought '42 will do'. End of story."[8](https://www.sd43.bc.ca/school/riverside/ProgramsServices/ClubsActivities/Movie%20Club%20Documents/Classics%20of%20Comedy%20Review.pdf) The Guardian notes that Adams deflated several fan theories himself, including that he was paying tribute to [Lewis Carroll](https://www.edgepedia.org/lewis-carroll), that he was joking in a base 13 number system, or that he was making an obscure reference to the traditional number of rulers of Tibet; the same article reports that Adams never revealed the full story except, according to his friend [Stephen Fry](https://www.edgepedia.org/stephen-fry), to Fry, who claims he will take the secret to his grave.[1](https://www.theguardian.com/books/2011/feb/03/douglas-adams-42-hitchhiker)
## Cultural afterlife
The number is now used as a shared signal among readers of the book. The Guardian describes hearing it as "a gently conclusive 'Douglas Adams'", said whenever a spontaneous 42 appears as a seat number, a restaurant table, or a homework answer; fans post lists of favorite 42s, snap "42" photos, and throw 42nd birthday parties ([Pink Floyd](https://www.edgepedia.org/pink-floyd) performed on stage with Adams for his).[1](https://www.theguardian.com/books/2011/feb/03/douglas-adams-42-hitchhiker) Celebrated real and fictional instances include Gutenberg's 42-line bible, the 42-foot star-spangled banner of the US national anthem, the address 42 Wallaby Way on the diving mask in Pixar's *Finding Nemo*, and [Ian Fleming](https://www.edgepedia.org/ian-fleming) making 42 the age of the Bond villain [Auric Goldfinger](https://www.edgepedia.org/auric-goldfinger).[1](https://www.theguardian.com/books/2011/feb/03/douglas-adams-42-hitchhiker) Both Google's headquarters and CERN's Large Hadron Collider have office complexes named for the number.[1](https://www.theguardian.com/books/2011/feb/03/douglas-adams-42-hitchhiker)
The joke also shaped how some technologists talk about artificial intelligence. [Elon Musk](https://www.edgepedia.org/elon-musk) told his biographer Ashlee Vance that *The Hitchhiker's Guide to the Galaxy* was the book that influenced him most, because "it taught [him] that the tough thing is figuring out what questions to ask", referring to the passage in which Deep Thought answers 42 and insists the question itself must be found.[9](https://www.lemonde.fr/en/summer-reads/article/2025/08/04/the-sci-fi-classic-that-shaped-elon-musk-s-worldview_6744061_183.html)
## Mathematical properties
As an integer, 42 is composite with the prime factorization 2 × 3 × 7, eight divisors, a divisor sum of 96, and Euler's totient φ(42) = 12; it is a pronic number, 6 × 7, and the 5th [Catalan number](https://www.edgepedia.org/catalan-number).[2](https://www.numberempire.com/42) Because it is the product of three distinct primes, it is a sphenic number, and it is also the third primary pseudoperfect number and the product of the first three members of Sylvester's sequence.[10](https://planetmath.org/fortytwo) In positional notation it is 101010 in binary, 52 in octal, and 2A in hexadecimal.[2](https://www.numberempire.com/42) • [3](https://science.slc.edu/~jmarshall/courses/2007/spring/hello-world/notes/week06/number_systems.html)
**Egyptian fractions and geometry.** The mathematician John Baez, a mathematical physicist at the [University of California, Riverside](https://www.edgepedia.org/university-of-california-riverside), highlights two extremal properties. Among solutions of 1/p + 1/q + 1/r = 1/2 in positive integers with p ≤ q ≤ r, the largest possible value of r is 42, from the solution (3, 7, 42), which corresponds to an equilateral triangle, a regular heptagon, and a regular 42-gon meeting snugly at a point.[7](https://math.ucr.edu/home/baez/42.html) Equivalently, the sum 1/2 + 1/3 + 1/7 is the largest value of 1/a + 1/b + 1/c that stays below 1, falling short by exactly 1/42.[7](https://math.ucr.edu/home/baez/42.html) This matters in geometry: tiling the hyperbolic plane with triangles whose angles are π/2, π/3, and π/7 gives Riemann surfaces of genus g ≥ 2 with the maximum number of conformal transformations, 84(g − 1), the bound known as Hurwitz's automorphism theorem; the 84 is twice 42 because 1 − 1/2 − 1/3 − 1/7 = 1/42.[7](https://math.ucr.edu/home/baez/42.html) The first surface to reach the maximum, Klein's quartic curve with three holes and 168 symmetries, was studied by [Felix Klein](https://www.edgepedia.org/felix-klein) starting in 1878.[7](https://math.ucr.edu/home/baez/42.html)
## 42 in science
In chemistry, 42 is the atomic number of molybdenum, a silvery metal in group six of the periodic table between chromium and tungsten, with a relative atomic mass of 95.95.[6](https://periodic-table.rsc.org/element/42/MolybdenumH) The Royal Society of Chemistry's podcast on the element opens by noting the coincidence: "The answer to the ultimate question... is, as every Douglas Adams fan knows, 42. And 42, as every Mendeleev fan knows, is the atomic number of molybdenum."[6](https://periodic-table.rsc.org/element/42/MolybdenumH)
Physicist Paul Davies, on the [Australian Broadcasting Corporation](https://www.edgepedia.org/australian-broadcasting-corporation)'s Science Show, recounted that he had met Adams and looked for deeper meanings, noting with amusement that under the simple code a = 1, b = 2, c = 3, and so on, the value of "the Big Bang" works out to 42; he concluded that nobody really knows what Adams intended.[11](https://www.abc.net.au/listen/programs/scienceshow/42%21/8843302)
## References
1. [Douglas Adams and the cult of 42, The Guardian (3 February 2011)](https://www.theguardian.com/books/2011/feb/03/douglas-adams-42-hitchhiker)
2. [Properties of the number 42, Number Empire](https://www.numberempire.com/42)
3. [Number Systems, course notes, Sarah Lawrence College](https://science.slc.edu/~jmarshall/courses/2007/spring/hello-world/notes/week06/number_systems.html)
4. [Catalan numbers worksheet, Math 4707, University of Minnesota (Samuel F. Hopkins)](https://www.samuelfhopkins.com/classes/UMN/4707/catalan_worksheet.pdf)
5. [Who really said "The Answer to the Great Question of Life, the Universe and Everything is Forty-two"?, Quotle.info](https://quotle.info/who-said/the-answer-to-the-great-question-of-life-the-universe-and/)
6. [Molybdenum, element 42, Royal Society of Chemistry periodic table](https://periodic-table.rsc.org/element/42/MolybdenumH)
7. [John Baez (2015). 42. University of California, Riverside.](https://math.ucr.edu/home/baez/42.html)
8. [Douglas Adams on 42, quoted in a Riverside school district comedy review document](https://www.sd43.bc.ca/school/riverside/ProgramsServices/ClubsActivities/Movie%20Club%20Documents/Classics%20of%20Comedy%20Review.pdf)
9. [The sci-fi classic that shaped Elon Musk's worldview, Le Monde (4 August 2025)](https://www.lemonde.fr/en/summer-reads/article/2025/08/04/the-sci-fi-classic-that-shaped-elon-musk-s-worldview_6744061_183.html)
10. [forty-two, PlanetMath](https://planetmath.org/fortytwo)
11. [42!, The Science Show, ABC Radio National](https://www.abc.net.au/listen/programs/scienceshow/42%21/8843302)
---
*Topic: Encyclopedia › Physical world and mathematics › Mathematics and statistics › Numbers and algebra › Arithmetic and number systems › Integer sequences and partitions › Special and named integers › Named individual integers*
*Initially written Sep 17, 2026 · Reviewed: Oct 15, 2026 · Edited: Oct 15, 2026 · Last review: Oct 15, 2026*
*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*
License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license