||This article may contain excessive, poor, irrelevant, or self-sourcing examples. (March 2016)|
|Factorization||3 × 7|
|Divisors||1, 3, 7, 21|
- the fifth discrete semiprime and the second in the (3 × q) family. With 22 it forms the second discrete semiprime pair.
- a Blum integer, since it is a semiprime with both its prime factors being Gaussian primes.
- a Fibonacci number.
- a Harshad number.
- a Motzkin number.
- a triangular number.
- an octagonal number.
- 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.
21 has an aliquot sum of 11 though it is the second composite number found in the 11-aliquot tree with the abundant square prime 18 being the first such member. Twenty-one is the first number to be the aliquot sum of three numbers 18, 51, 91.
|Look up twenty-one in Wiktionary, the free dictionary.|
- In several countries 21 is the age of majority. See also: Coming of age.
- In all US states, 21 is the drinking age.
- 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. 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.
- The jersey number 21 has been retired by several North American sports teams in honor of past playing greats or other key figures:
- In Major League Baseball:
- The Cleveland Indians, for Hall of Famer Bob Lemon.
- The Milwaukee Braves, for Hall of Famer Warren Spahn; the number continues to be honored by the team in its current home of Atlanta.
- The Pittsburgh Pirates, for Hall of Famer Roberto Clemente, following his death in a plane crash while attempting to deliver humanitarian aid to victims of an earthquake in Nicaragua.
- In the NBA:
- The Atlanta Hawks, for Hall of Famer Dominique Wilkins.
- The Boston Celtics, for Hall of Famer Bill Sharman.
- The Detroit Pistons, for Hall of Famer Dave Bing.
- The Sacramento Kings, for Vlade Divac.
- The Minnesota Timberwolves have not retired the number, but have not issued it since Kevin Garnett was traded from the team in 2007.
- In the NHL:
- No NFL team has retired the number.
- In Major League Baseball:
- In basketball:
- 21 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 3x3, a formalized version of three-on-three half-court basketball, the game ends by rule once either team has scored 21 points in regulation.
- In badminton, and table tennis (before 2001), 21 points are required to win a game.
- In the NASCAR Sprint Cup Series, 21 has long been the number of the car for Wood Brothers Racing.
In other fields
- 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
- 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.
|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.
- "Article 9 Scoring" (PDF). 3x3 Rules of the Game. FIBA. January 13, 2012. Retrieved November 2, 2012.