# 20 (number)

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Cardinaltwenty
Ordinal20th
(twentieth)
Numeral systemvigesimal
Factorization22 × 5
Divisors1, 2, 4, 5, 10, 20
Greek numeralΚ´
Roman numeralXX
Binary101002
Ternary2023
Senary326
Octal248
Duodecimal1812
ArmenianԻ
Hebrewכ / ך
Babylonian numeral
Egyptian hieroglyph𓎏

20 (twenty; Roman numeral XX) is the natural number following 19 and preceding 21. A group of twenty units may also be referred to as a score.[1][2]

## In mathematics

Twenty is a pronic number, as it is the product of consecutive integers, namely 4 and 5.[3] It is the third composite number to be the product of a squared prime and a prime, and also the second member of the 22 × q family in this form.

20 has an aliquot sum of 22; a semiprime, within an aliquot sequence of four composite numbers (20, 22, 14, 10, 8) that belong to the prime 7-aliquot tree.

• 20 is the smallest primitive abundant number.[4]
• 20 is the third tetrahedral number.[5]
• 20 is the basis for vigesimal number systems.[6]
• 20 is the number of parallelogram polyominoes with 5 cells.[7]
• 20 is the number of moves (quarter or half turns) required to optimally solve a Rubik's Cube in the worst case.[8][9]
• 20 is the length of a side of the fifth smallest right triangle that forms a primitive Pythagorean triple, (20,21,29).[10] This is the second Pythagorean triple ${\displaystyle (a,b,c)}$ that can be formed using Pell numbers where ${\displaystyle a}$ and ${\displaystyle b}$ are one unit apart.[a]
• 20 is the smallest non-trivial decimal neon number equal to the sum of its digits when raised to the thirteenth power (2013 = 8192 × 1013).[citation needed]

20 is the first number to have an abundance of 2, followed by 104.[11]

Gelfond's constant and pi very nearly have a difference equal to twenty:

${\displaystyle e^{\pi }-\pi \;\approx \;20}$

differing only by about ${\displaystyle -0.000900020811\ldots }$ from an integer value.[12][13]

There are twenty edge-to-edge 2-uniform tilings by convex regular polygons, which are uniform tessellations of the plane containing 2 orbits of vertices.[14][15]

The largest number of faces a Platonic solid can have is twenty faces, which make up a regular icosahedron.[16] A dodecahedron, on the other hand, has twenty vertices, likewise the most a regular polyhedron can have.[17] There are a total of 20 regular and semiregular polyhedra, aside from the infinite family of semiregular prisms and antiprisms that exists in the third dimension: the 5 Platonic solids, and 15 Archimedean solids (including chiral forms of the snub cube and snub dodecahedron). There are also four uniform compound polyhedra that contain twenty polyhedra (UC13, UC14, UC19, UC33), which is the most any such solids can have; while another twenty uniform compounds contain five polyhedra (that are not part of classes of infinite families, where there exist three more). The compound of twenty octahedra can be obtained by orienting two pairs of compounds of ten octahedra, which can also coincide to yield a regular compound of five octahedra.

In total, there are 20 semiregular polytopes that only exist up through the 8th dimension, which include 13 Archimedean solids and 7 Gosset polytopes (without counting enantiomorphs, or semiregular prisms and antiprisms).

Bring's curve is a Riemann surface of genus four, whose fundamental polygon is a regular hyperbolic twenty-sided icosagon, with an area equal to ${\displaystyle 12\pi }$ by the Gauss-Bonnet theorem.[18]

The Happy Family of sporadic groups is made up of twenty finite simple groups that are all subquotients of the friendly giant, the largest of twenty-six sporadic groups. The largest supersingular prime factor that divides the order of the friendly giant is 71, which is the 20th indexed prime number, where 26 also represents the number of partitions of 20 into prime parts.[19] Both 71 and 20 represent self-convolved Fibonacci numbers, respectively the seventh and fifth members ${\displaystyle j}$ in this sequence ${\displaystyle F_{j}^{2}}$.[20][21]

## In science

### Biology

• The number of proteinogenic amino acids that are encoded by the standard genetic code
• In some countries, the number 20 is used as an index in measuring visual acuity. 20/20 indicates normal vision at 20 feet, although it is commonly used to mean "perfect vision" in countries using the Imperial system. (The metric equivalent is 6/6.) When someone is able to see only after an event how things turned out, that person is often said to have had "20/20 hindsight"[22]

## As an indefinite number

• A 'score' is a group of 20 (often used in combination with a cardinal number, e.g. fourscore to mean 80),[23] but also often used as an indefinite number[24] (e.g. the newspaper headline "Scores of Typhoon Survivors Flown to Manila").[25]

## Music

Albums
20 Related Twenty Related
Harry Connick, Jr.[34] 20+ Baron Rojo[35] Bob Baldwin[36] Twenty+ All-4-One[37]
Orbital[38] 20% Maria Bamford[39] Boyz II Men[40] 20 [Twenty] FT Island[41]
TLC[42] Chicane[43]
Jars of Clay[44] Robert Cray[45]
Africa Unite[46] Jebediah[47]
Kate Rusby[48] Lynyrd Skynyrd[49]
Hyukoh[50] Taking Back Sunday[51]
Pastora Soler[52]
Sofa Surfers[54]
Capo Plaza[55]
John Dahlbäck[56]
Songs
20 Related Twenty
Ebbo Kraan[57] #20 - Fat Tuesday[58] The Rippingtons[59]
Seventeen[60] #20 - Aphex Twin[61]
Janet Jackson[62] #20 - Tony Wilson Sextet[63]
Gorky's Zygotic Mynci[64] #20 - David Paul Mesler[65]
Covenant[66] $20 - Low (band)[67] Car 44[68]$20 - Qwiksand[69]
Karma to Burn[70] $20 - Norman Palm[71] Ike Yard[72] 20% - Furyus[73] Noah Stone[74] 20% - Banfi[75] Alec Empire[76] -20 - Brendon Moeller[77] Beans[78] -20 - Mr. 76ix[79] Damián Schwartz[80] 20 (intro) - Janet Jackson[81] Anarchestra[82] 20 (Palbarrio) - El Barrio[83] ## In other fields ## See also ## References 1. ^ The first such triple is the smallest Pythagorean triple, (4,3,5). These can be formed using Pell numbers that yield a Pythagorean triple of the form ${\displaystyle \left(2P_{n}P_{n+1},P_{n+1}^{2}-P_{n}^{2},P_{n+1}^{2}+P_{n}^{2}=P_{2n+1}\right)}$. 1. ^ John H. Conway and Richard K. Guy, The Book of Numbers. New York: Copernicus (1996): 11. ""Score" is related to "share" and comes from the Old Norse "skor" meaning a "notch" or "tally" on a stick used for counting. ... Often people counted in 20s; every 20th notch was larger, so "score" also came to mean 20." 2. ^ "score | Origin and meaning of score by Online Etymology Dictionary". www.etymonline.com. Retrieved 2020-08-16. 3. ^ "Sloane's A002378: Pronic numbers". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2020-11-30. 4. ^ "Sloane's A071395 : Primitive abundant numbers". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2016-05-31. 5. ^ "Sloane's A000292 : Tetrahedral numbers". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2016-05-31. 6. ^ Weisstein, Eric W. "Vigesimal". mathworld.wolfram.com. Retrieved 2020-08-16. 7. ^ 8. ^ "God's Number is 20". Cube20.org 9. ^ Jonathan Fildes (August 11, 2010). "Rubik's Cube quest for speedy solution comes to an end". BBC News. 10. ^ 11. ^ Sloane, N. J. A. (ed.). "Sequence A088831 (Numbers k whose abundance is 2: sigma(k) - 2k is 2.)". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2024-01-23. 12. ^ Maze, Gérard; Minder, Lorenz (2007). "A new family of almost identities". Elemente der Mathematik. 62 (3). Hensinki: European Mathematical Society: 90. arXiv:math/0409014. doi:10.4171/EM/61. MR 2350250. S2CID 56024534. Zbl 1213.40002. 13. ^ Sloane, N. J. A. (ed.). "Sequence A018938 (Decimal expansion of e^Pi - Pi.)". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2023-12-09. 14. ^ Grünbaum, Branko; Shephard, Geoffrey C. (1977). "Tilings by regular polygons" (PDF). Mathematics Magazine. 50 (5): 235. doi:10.2307/2689529. JSTOR 2689529. S2CID 123776612. Zbl 0385.51006. 15. ^ Sloane, N. J. A. (ed.). "Sequence A068599 (Number of n-uniform tilings.)". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2023-01-07. 16. ^ Weisstein, Eric W. "Icosahedron". mathworld.wolfram.com. Retrieved 2020-08-16. 17. ^ Weisstein, Eric W. "Dodecahedron". mathworld.wolfram.com. Retrieved 2020-08-16. 18. ^ Weber, Matthias (2005). "Kepler's small stellated dodecahedron as a Riemann surface" (PDF). Pacific Journal of Mathematics. 220 (1): 172. doi:10.2140/pjm.2005.220.167. MR 2195068. S2CID 54518859. Zbl 1100.30036. 19. ^ Sloane, N. J. A. (ed.). "Sequence A000607 (Number of partitions of n into prime parts.)". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2024-03-24. 20. ^ Sloane, N. J. A. (ed.). "Sequence A001629 (Self-convolution of Fibonacci numbers.)". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2023-07-16. 21. ^ Moree, Pieter (2004). "Convoluted Convolved Fibonacci Numbers" (PDF). Journal of Integer Sequences. 7 (2). Waterloo, Ont., CA: University of Waterloo David R. Cheriton School of Computer Science: 13 (Article 04.2.2). arXiv:math.CO/0311205. Bibcode:2004JIntS...7...22M. MR 2084694. S2CID 14126332. Zbl 1069.11004. 22. ^ "Definition of 20/20". www.merriam-webster.com. Retrieved 2020-08-16. 23. ^ "Definition of SCORE". www.merriam-webster.com. Retrieved 2020-08-16. 24. ^ "Biblical Criticism", The Classical Journal 36:71:83ff (March 1827) full text 25. ^ "CBS News", Scores of Typhoon Survivors Flown to Manila (November 2013) 26. ^ Cleaver, Dylan (2010-11-03). Brendon McCullum: Inside Twenty20. Hachette New Zealand. ISBN 978-1-86971-238-9. 27. ^ Ziemba, William T. (2017-08-23). The Adventures Of A Modern Renaissance Academic In Investing And Gambling. World Scientific. p. 352. ISBN 978-981-314-853-6. It has always been for a long time a maximum of 20 horses ... 28. ^ Jordan, Bill. Opening Moves Made Easy: A new way to learn how to play Chess openings. Bill Jordan. there are 20 legal moves for White and 20 legal replies for Black 29. ^ Draper, Nick (2014-12-05). Exercise Physiology: For Health and Sports Performance. Routledge. p. 404. ISBN 978-1-317-90260-7. played over four 20 min quarters 30. ^ Harari, P. J.; Ominsky, Dave (2002). Basketball Made Simple: A Spectator's Guide. First Base Sports, Inc. p. 58. ISBN 978-1-884309-13-7. Duration two 20-min. halves 31. ^ "International Ice Hockey Federation - Olympic Sport". International Olympic Committee. 9 November 2020. Retrieved 22 January 2021. 32. ^ "Adulthood | Introduction to Psychology". lumenlearning.com. 33. ^ "Japan's Age of Majority Changed to 18 - Living the Japon.com". www.japan-experience.com. Retrieved 19 March 2018. 34. ^ 20 - Harry Connick, Jr. | Songs, Reviews, Credits | AllMusic, retrieved 2020-08-16 35. ^ 20+ - Baron Rojo | Songs, Reviews, Credits | AllMusic, retrieved 2020-08-16 36. ^ Twenty - Bob Baldwin | Songs, Reviews, Credits | AllMusic, retrieved 2020-08-16 37. ^ Twenty + - All-4-One | Songs, Reviews, Credits | AllMusic, retrieved 2020-08-17 38. ^ 20 - Orbital | Songs, Reviews, Credits | AllMusic, retrieved 2020-08-16 39. ^ 20 - Harry Connick, Jr. | Songs, Reviews, Credits | AllMusic, retrieved 2020-08-16 40. ^ Twenty - Boyz II Men | Songs, Reviews, Credits | AllMusic, retrieved 2020-08-16 41. ^ 20 (Twenty) - FT Island | User Reviews | AllMusic, retrieved 2020-08-17 42. ^ 20 - TLC | Songs, Reviews, Credits | AllMusic, retrieved 2020-08-16 43. ^ Twenty - Chicane | Songs, Reviews, Credits | AllMusic, retrieved 2020-08-17 44. ^ 20 - Jars of Clay | Songs, Reviews, Credits | AllMusic, retrieved 2020-08-16 45. ^ Twenty - Robert Cray | Songs, Reviews, Credits | AllMusic, retrieved 2020-08-16 46. ^ 20 - Africa Unite | Songs, Reviews, Credits | AllMusic, retrieved 2020-08-16 47. ^ Twenty - Jebediah | Songs, Reviews, Credits | AllMusic, retrieved 2020-08-17 48. ^ 20 - Kate Rusby | Songs, Reviews, Credits | AllMusic, retrieved 2020-08-16 49. ^ Twenty - Lynyrd Skynyrd | Songs, Reviews, Credits | AllMusic, retrieved 2020-08-16 50. ^ 20 - Hyukoh | User Reviews | AllMusic, retrieved 2020-08-16 51. ^ Twenty - Taking Back Sunday | Songs, Reviews, Credits | AllMusic, retrieved 2020-08-16 52. ^ 20 - Pastora Soler | Songs, Reviews, Credits | AllMusic, retrieved 2020-08-16 53. ^ 20 - The Gift | Songs, Reviews, Credits | AllMusic, retrieved 2020-08-16 54. ^ 20 - Sofa Surfers | Songs, Reviews, Credits | AllMusic, retrieved 2020-08-17 55. ^ 20 - Capo Plaza | Songs, Reviews, Credits | AllMusic, retrieved 2020-08-17 56. ^ 20 - John Dahlbäck | Songs, Reviews, Credits | AllMusic, retrieved 2020-08-17 57. ^ 20 - Ebbo Kraan | Song Info | AllMusic, retrieved 2020-08-17 58. ^ #20 - Fat Tuesday | Song Info | AllMusic, retrieved 2020-08-17 59. ^ Twenty - The Rippingtons | Song Info | AllMusic, retrieved 2020-08-17 60. ^ 20 - Seventeen | Song Info | AllMusic, retrieved 2020-08-17 61. ^ #20 - Aphex Twin | Song Info | AllMusic, retrieved 2020-08-17 62. ^ 20 - Janet Jackson | Song Info | AllMusic, retrieved 2020-08-17 63. ^ #20 - Tony Wilson Sextet | Song Info | AllMusic, retrieved 2020-08-17 64. ^ 20 - Gorky's Zygotic Mynci | Song Info | AllMusic, retrieved 2020-08-17 65. ^ # 20 - David Paul Mesler | Song Info | AllMusic, retrieved 2020-08-17 66. ^ 20 - Covenant | Song Info | AllMusic, retrieved 2020-08-17 67. ^$20 - Low | Song Info | AllMusic, retrieved 2020-08-17
68. ^ 20 - Car 44 | Song Info | AllMusic, retrieved 2020-08-17
69. ^ $20 - Qwiksand | Song Info | AllMusic, retrieved 2020-08-17 70. ^ 20 - Karma to Burn | Song Info | AllMusic, retrieved 2020-08-17 71. ^$20 - Norman Palm | Song Info | AllMusic, retrieved 2020-08-17
72. ^ 20 - Ike Yard | Song Info | AllMusic, retrieved 2020-08-17
73. ^ 20% - Furyus | Song Info | AllMusic, retrieved 2020-08-17
74. ^ 20 - Noah Stone | Song Info | AllMusic, retrieved 2020-08-17
75. ^ 20% - Banfi | Song Info | AllMusic, retrieved 2020-08-17
76. ^ 20 - Alec Empire | Song Info | AllMusic, retrieved 2020-08-17
77. ^ -20 - Brendon Moeller | Song Info | AllMusic, retrieved 2020-08-17
78. ^ 20 - Beans | Song Info | AllMusic, retrieved 2020-08-17
79. ^ -20 - Mr. 76ix | Song Info | AllMusic, retrieved 2020-08-17
80. ^ 20 - Damián Schwartz | Song Info | AllMusic, retrieved 2020-08-17
81. ^ 20 (Intro) - Janet Jackson | Song Info | AllMusic, retrieved 2020-08-17
82. ^ 20 - Anarchestra | Song Info | AllMusic, retrieved 2020-08-17
83. ^ 20 (Palbarrio) - El Barrio | Song Info | AllMusic, retrieved 2020-08-17
84. ^ "international phone country calling codes". www.globaltel.com. Retrieved 2020-08-16.
85. ^ "CB Slang: Ten Codes". www.cbslang.com. Retrieved 2020-08-16.
86. ^ "Welcome to the Online Browsing Platform (OBP)". www.iso.org. Retrieved 2020-08-16.
87. ^ "Topical Bible: Kaph". biblehub.com. Retrieved 2020-08-17.
88. ^ Toone, Matthew (2009). Great Games!: 175 Games and Activities for Families, Groups and Children. Great Games Book. p. 145. ISBN 978-0-9798345-5-4. Much like the classic game of 20 Questions,...