21 (number)
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| Cardinal | twenty-one | |||
| Ordinal |
21st (twenty-first) | |||
| Factorization | 3 × 7 | |||
| Divisors | 1, 3, 7, 21 | |||
| Roman numeral | XXI | |||
| Binary | 101012 | |||
| Ternary | 2103 | |||
| Quaternary | 1114 | |||
| Quinary | 415 | |||
| Senary | 336 | |||
| Octal | 258 | |||
| Duodecimal | 1912 | |||
| Hexadecimal | 1516 | |||
| Vigesimal | 1120 | |||
| Base 36 | L36 | |||
21 (twenty-one) is the natural number following 20 and preceding 22.
In mathematics
21 is:
- a Blum integer, since it is a semiprime with both its prime factors being Gaussian primes.[1]
- a Fibonacci number.[2]
- a Harshad number.[3]
- a Motzkin number.[4]
- a triangular number.[5]
- an octagonal number.[6]
- a composite number, its proper divisors being 1, 3 and 7.
- the sum of the first six natural numbers (1 + 2 + 3 + 4 + 5 + 6 = 21), making it a triangular number.
- the sum of the sum of the divisors of the first 5 positive integers.
- the smallest non-trivial example of a Fibonacci number whose digits are Fibonacci numbers and whose digit sum is also a Fibonacci number.
- a repdigit in base 4 (1114).
- the smallest natural number that is not close to a power of 2, 2n, where the range of closeness is ±n.
- the smallest number of differently sized squares needed to square the square.[7]
21 appears in the Padovan sequence, preceded by the terms 9, 12, 16 (it is the sum of the first two of these).[8]
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Look up twenty-one in Wiktionary, the free dictionary. |
In science
- The atomic number of scandium.
Age 21
- In several countries 21 is the age of majority. See also: Coming of age.
- In most US states, 21 is the drinking age.
- However, in Puerto Rico and U.S. Virgin Island, the drinking age is 18.
- In Hawaii and New York, 21 is the minimum age that one person may purchase cigarettes and other tobacco products.
- In some countries it is the voting age.
- In the United States, 21 is the age at which one can purchase multiple tickets to an R-rated film without providing Identifications. It is also the age to accompany one under the age of 17 as their parent or adult guardian for an R-rated movie.
- In some states, 21 is the minimum age, persons may gamble or enter casinos.
- In 2011, Adele named her second studio album 21, because of her age at the time.
In sports
- Twenty-one is a variation of street basketball, in which each player, of which there can be any number, plays for himself only (i.e. not part of a team); the name comes from the requisite number of baskets.
- In badminton, and table tennis (before 2001), 21 points are required to win a game.
- In the FIA World Rallycross Championship, 21 is used by Timmy Hansen, who drives for Team Peugeot-Hansen.
In other fields
Building called "21" in Zlín, Czech Republic.
Detail of the building entrance
21 is:
- The current century—spanning the years from 2001 to 2100—is referred to as the 21st century. See also: Century 21 (disambiguation).
- The Twenty-first Amendment repealed the Eighteenth Amendment, thereby ending Prohibition.
- The number of spots on a standard cubical (six-sided) die (1+2+3+4+5+6)
- The number of firings in a 21-gun salute honoring Royalty or leaders of countries
- 21 Guns, a 2009 song by the punk-rock band Green Day
- There are 21 trump cards of the tarot deck if one does not consider The Fool to be a proper trump card.
- The standard TCP/IP port number for FTP connection
- The Twenty-One Demands were a set of demands which were sent to the Chinese government by the Japanese government of Okuma Shigenobu in 1915
- 21 Demands of MKS led to the foundation of Solidarity in Poland.
- In Israel, the number is associated with the profile 21 (the military profile designation granting an exemption from the military service)
- 21 grams is the weight of the soul, according to research by Duncan MacDougall, generally regarded as meaningless.
- 21 is the designation of a US Highway connecting Wytheville, Virginia and Beaufort, South Carolina, a truncation of a route that once connected Cleveland, Ohio and Jacksonville, Florida, among other highways past and present.
- The number of the French department Côte-d'Or
- The key value and highest-winning point total of the popular casino game Blackjack
- The number of shillings in a guinea.
References
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Wikimedia Commons has media related to 21 (number). |
- ↑ "Sloane's A016105 : Blum integers". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2016-05-31.
- ↑ "Sloane's A000045 : Fibonacci numbers". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2016-05-31.
- ↑ "Sloane's A005349 : Niven (or Harshad) numbers". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2016-05-31.
- ↑ "Sloane's A001006 : Motzkin numbers". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2016-05-31.
- ↑ "Sloane's A000217 : Triangular numbers". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2016-05-31.
- ↑ "Sloane's A000567 : Octagonal numbers". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2016-05-31.
- ↑ C. J. Bouwkamp, and A. J. W. Duijvestijn, "Catalogue of Simple Perfect Squared Squares of Orders 21 Through 25." Eindhoven University of Technology, Nov. 1992.
- ↑ "Sloane's A000931 : Padovan sequence". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2016-05-31.
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