20 (number)

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]

← 19 20 21 →
← 20 21 22 23 24 25 26 27 28 29 → List of numbers Integers ← 0 10 20 30 40 50 60 70 80 90 →
Cardinal	twenty
Ordinal	20th (twentieth)
Numeral system	vigesimal
Factorization	2^2 × 5
Divisors	1, 2, 4, 5, 10, 20
Greek numeral	Κ´
Roman numeral	XX
Binary	101002
Ternary	2023
Senary	326
Octal	248
Duodecimal	1812
Hexadecimal	1416

In mathematics

 

An icosahedron has twenty triangular faces.

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]}

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.^[11][12]

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.^[13][14]

The largest number of faces a Platonic solid can have is twenty faces, which make up a regular icosahedron.^[15] A dodecahedron, on the other hand, has twenty vertices, likewise the most a regular polyhedron can have.^[16] 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. 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.^[17]

The Happy Family of sporadic groups is made up of twenty finite simple groups that are all subquotients of the friendly giant, the largest sporadic group. The largest supersingular prime factor that divides the order of the friendly giant is 71, which is the 20th indexed prime number. Both 71 and 20 represent self-convolved Fibonacci numbers, respectively the seventh and fifth members ${\displaystyle j}$  in this sequence ${\displaystyle F_{j}^{2}}$ .^[18][19]

In science

• The atomic number of calcium
• The third magic number in physics
• The International Astronomical Union shower number for Coma Berenicids

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". (Note that this applies only to 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"^[20]

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),^[21] but also often used as an indefinite number^[22] (e.g. the newspaper headline "Scores of Typhoon Survivors Flown to Manila").^[23]

In sports

For retired sports figures who wore this number, see List of retired numbers#20

 

A standard dartboard is laid out as 20 sectors.

• Twenty20 is a form of limited overs cricket where each team plays only 20 overs.^[24]
• A standard dartboard is laid out as 20 sectors.
• The Kentucky Derby currently has a maximum field of 20 horses.^[25]
• In rugby union, 20 national teams currently qualify for each edition of the men's Rugby World Cup.
• In chess, 20 is the number of legal moves for each player in the starting position.^[26]
• In Australian rules football, most games consist of four 20-minute quarters.^[27]
• Under NCAA men's basketball rules, games consist of 20-minute halves.^[28]
• Ice hockey games are played in three 20-minute periods.^[29]
• The current Formula 1 field size is 20.
• 20 was used by Tony Stewart and Joe Gibbs Racing to win the 2002 and 2005 NASCAR Cup Series championships. It is currently used by Christopher Bell.

Age 20

• In many disciplines of developmental psychology, adulthood starts at age 20.^[30]
• Formerly the age of majority in Japan and in Japanese tradition.^[31]
• Age 20 is the age at which Levites in the time of King David were allowed "to do the work for the service of the house of the Lord", the Temple in Jerusalem (see First Chronicles Chapter 23, verses 24 and 27). In the time of Ezra and Nehemiah, following the Babylonian captivity, it was Levites from the age of 20 upwards who were assigned "to oversee the work of the house of the LORD" (Ezra Chapter 3, verse 8).

Music

Albums
20	Related	Twenty	Related
Harry Connick, Jr.^[32]	20+ Baron Rojo^[33]	Bob Baldwin^[34]	Twenty+ All-4-One^[35]
Orbital^[36]	20% Maria Bamford^[37]	Boyz II Men^[38]	20 [Twenty] FT Island^[39]
TLC^[40]		Chicane^[41]
Jars of Clay^[42]		Robert Cray^[43]
Africa Unite^[44]		Jebediah^[45]
Kate Rusby^[46]		Lynyrd Skynyrd^[47]
Hyukoh^[48]		Taking Back Sunday^[49]
Pastora Soler^[50]
The Gift^[51]
Sofa Surfers^[52]
Capo Plaza^[53]
John Dahlbäck^[54]

Songs
20	Related	Twenty
Ebbo Kraan^[55]	#20 - Fat Tuesday^[56]	The Rippingtons^[57]
Seventeen^[58]	#20 - Aphex Twin^[59]
Janet Jackson^[60]	#20 - Tony Wilson Sextet^[61]
Gorky's Zygotic Mynci^[62]	#20 - David Paul Mesler^[63]
Covenant^[64]	$20 - Low (band)^[65]
Car 44^[66]	$20 - Qwiksand^[67]
Karma to Burn^[68]	$20 - Norman Palm^[69]
Ike Yard^[70]	20% - Furyus^[71]
Noah Stone^[72]	20% - Banfi^[73]
Alec Empire^[74]	-20 - Brendon Moeller^[75]
Beans^[76]	-20 - Mr. 76ix^[77]
Damián Schwartz^[78]	20 (intro) - Janet Jackson^[79]
Anarchestra^[80]	20 (Palbarrio) - El Barrio^[81]

In other fields

• +20 is the code for international direct dial phone calls to Egypt.^[82]
• CB slang for a place, being short for the ten-code "10–20" meaning "What is your location?"^[83]
• 20/20 is a primetime newsmagazine program on ABC, which in turn is taken from the expression for normal eyesight (20/20 vision).
• 020 is the ISO 3166-1 numeric 3 digit country code for Andorra.^[84]
• The UIC Country Code for Russia identifying member countries of the International Union of Railways (UIC).
• 20 is the value of the letter J when computing the check digit in the serial number of an intermodal (shipping) container, as defined by ISO 6346.
• In Hebrew numerals, the letter Kaph (כ) represents twenty;^[85] see also gematria, the Hebrew system of numerology. Similarly the Arabic letter kāf (ك) represents twenty in the Abjad numerals.
• The number of questions in the game Twenty Questions.^[86]

See also

• List of highways numbered 20

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, and 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. Sloane, N. J. A. (ed.). "Sequence A006958 (Number of parallelogram polyominoes with n cells (also called staircase polyominoes, although that term is overused))". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation.
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. Sloane, N. J. A. (ed.). "Sequence A103606 (Primitive Pythagorean triples in nondecreasing order of perimeter, with each triple in increasing order, and if perimeters coincide then increasing order of the even members.)". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2023-07-06.
11. Maze, Gérard; Minder, Lorenz (2007). "A new family of almost identities". Elemente der Mathematik. Hensinki: European Mathematical Society. 62 (3): 90. arXiv:math/0409014. doi:10.4171/EM/61. MR 2350250. S2CID 56024534. Zbl 1213.40002.
12. 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.
13. 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.
14. 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.
15. Weisstein, Eric W. "Icosahedron". mathworld.wolfram.com. Retrieved 2020-08-16.
16. Weisstein, Eric W. "Dodecahedron". mathworld.wolfram.com. Retrieved 2020-08-16.
17. 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.
18. 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.
19. Moree, Pieter (2004). "Convoluted Convolved Fibonacci Numbers" (PDF). Journal of Integer Sequences. Waterloo, Ont., CA: University of Waterloo David R. Cheriton School of Computer Science. 7 (2): 13 (Article 04.2.2). arXiv:math.CO/0311205. Bibcode:2004JIntS...7...22M. MR 2084694. S2CID 14126332. Zbl 1069.11004.
20. "Definition of 20/20". www.merriam-webster.com. Retrieved 2020-08-16.
21. "Definition of SCORE". www.merriam-webster.com. Retrieved 2020-08-16.
22. "Biblical Criticism", The Classical Journal 36:71:83ff (March 1827) full text
23. "CBS News", Scores of Typhoon Survivors Flown to Manila (November 2013)
24. Cleaver, Dylan (2010-11-03). Brendon McCullum: Inside Twenty20. Hachette New Zealand. ISBN 978-1-86971-238-9.
25. 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 ...
26. 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
27. 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
28. 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
29. "International Ice Hockey Federation - Olympic Sport". International Olympic Committee. 9 November 2020. Retrieved 22 January 2021.
30. "Adulthood | Introduction to Psychology". lumenlearning.com.
31. "Japan's Age of Majority Changed to 18 - Living the Japon.com". www.japan-experience.com. Retrieved 19 March 2018.
32. 20 - Harry Connick, Jr. | Songs, Reviews, Credits | AllMusic, retrieved 2020-08-16
33. 20+ - Baron Rojo | Songs, Reviews, Credits | AllMusic, retrieved 2020-08-16
34. Twenty - Bob Baldwin | Songs, Reviews, Credits | AllMusic, retrieved 2020-08-16
35. Twenty + - All-4-One | Songs, Reviews, Credits | AllMusic, retrieved 2020-08-17
36. 20 - Orbital | Songs, Reviews, Credits | AllMusic, retrieved 2020-08-16
37. 20 - Harry Connick, Jr. | Songs, Reviews, Credits | AllMusic, retrieved 2020-08-16
38. Twenty - Boyz II Men | Songs, Reviews, Credits | AllMusic, retrieved 2020-08-16
39. 20 (Twenty) - FT Island | User Reviews | AllMusic, retrieved 2020-08-17
40. 20 - TLC | Songs, Reviews, Credits | AllMusic, retrieved 2020-08-16
41. Twenty - Chicane | Songs, Reviews, Credits | AllMusic, retrieved 2020-08-17
42. 20 - Jars of Clay | Songs, Reviews, Credits | AllMusic, retrieved 2020-08-16
43. Twenty - Robert Cray | Songs, Reviews, Credits | AllMusic, retrieved 2020-08-16
44. 20 - Africa Unite | Songs, Reviews, Credits | AllMusic, retrieved 2020-08-16
45. Twenty - Jebediah | Songs, Reviews, Credits | AllMusic, retrieved 2020-08-17
46. 20 - Kate Rusby | Songs, Reviews, Credits | AllMusic, retrieved 2020-08-16
47. Twenty - Lynyrd Skynyrd | Songs, Reviews, Credits | AllMusic, retrieved 2020-08-16
48. 20 - Hyukoh | User Reviews | AllMusic, retrieved 2020-08-16
49. Twenty - Taking Back Sunday | Songs, Reviews, Credits | AllMusic, retrieved 2020-08-16
50. 20 - Pastora Soler | Songs, Reviews, Credits | AllMusic, retrieved 2020-08-16
51. 20 - The Gift | Songs, Reviews, Credits | AllMusic, retrieved 2020-08-16
52. 20 - Sofa Surfers | Songs, Reviews, Credits | AllMusic, retrieved 2020-08-17
53. 20 - Capo Plaza | Songs, Reviews, Credits | AllMusic, retrieved 2020-08-17
54. 20 - John Dahlbäck | Songs, Reviews, Credits | AllMusic, retrieved 2020-08-17
55. 20 - Ebbo Kraan | Song Info | AllMusic, retrieved 2020-08-17
56. #20 - Fat Tuesday | Song Info | AllMusic, retrieved 2020-08-17
57. Twenty - The Rippingtons | Song Info | AllMusic, retrieved 2020-08-17
58. 20 - Seventeen | Song Info | AllMusic, retrieved 2020-08-17
59. #20 - Aphex Twin | Song Info | AllMusic, retrieved 2020-08-17
60. 20 - Janet Jackson | Song Info | AllMusic, retrieved 2020-08-17
61. #20 - Tony Wilson Sextet | Song Info | AllMusic, retrieved 2020-08-17
62. 20 - Gorky's Zygotic Mynci | Song Info | AllMusic, retrieved 2020-08-17
63. # 20 - David Paul Mesler | Song Info | AllMusic, retrieved 2020-08-17
64. 20 - Covenant | Song Info | AllMusic, retrieved 2020-08-17
65. $20 - Low | Song Info | AllMusic, retrieved 2020-08-17
66. 20 - Car 44 | Song Info | AllMusic, retrieved 2020-08-17
67. $20 - Qwiksand | Song Info | AllMusic, retrieved 2020-08-17
68. 20 - Karma to Burn | Song Info | AllMusic, retrieved 2020-08-17
69. $20 - Norman Palm | Song Info | AllMusic, retrieved 2020-08-17
70. 20 - Ike Yard | Song Info | AllMusic, retrieved 2020-08-17
71. 20% - Furyus | Song Info | AllMusic, retrieved 2020-08-17
72. 20 - Noah Stone | Song Info | AllMusic, retrieved 2020-08-17
73. 20% - Banfi | Song Info | AllMusic, retrieved 2020-08-17
74. 20 - Alec Empire | Song Info | AllMusic, retrieved 2020-08-17
75. -20 - Brendon Moeller | Song Info | AllMusic, retrieved 2020-08-17
76. 20 - Beans | Song Info | AllMusic, retrieved 2020-08-17
77. -20 - Mr. 76ix | Song Info | AllMusic, retrieved 2020-08-17
78. 20 - Damián Schwartz | Song Info | AllMusic, retrieved 2020-08-17
79. 20 (Intro) - Janet Jackson | Song Info | AllMusic, retrieved 2020-08-17
80. 20 - Anarchestra | Song Info | AllMusic, retrieved 2020-08-17
81. 20 (Palbarrio) - El Barrio | Song Info | AllMusic, retrieved 2020-08-17
82. "international phone country calling codes". www.globaltel.com. Retrieved 2020-08-16.
83. "CB Slang: Ten Codes". www.cbslang.com. Retrieved 2020-08-16.
84. "Welcome to the Online Browsing Platform (OBP)". www.iso.org. Retrieved 2020-08-16.
85. "Topical Bible: Kaph". biblehub.com. Retrieved 2020-08-17.
86. 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,...

External links

 

Look up twenty in Wiktionary, the free dictionary.

•   Media related to 20 (number) at Wikimedia Commons