||This article may contain excessive, poor, or irrelevant examples. (March 2010)|
|Factorization||3 × 7|
|Divisors||1, 3, 7, 21|
Twenty-one is the fifth discrete Semiprime and the second in the (3.q) family. With 22 it forms the second discrete Semiprime pair. As it is a semiprime with both its prime factors being Gaussian primes, 21 is then a Blum integer.
21 is the sum of the first six natural numbers (1+2+3+4+5+6=21), making it a triangular number.
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.
21 appears in the Padovan sequence, preceded by the terms 9, 12, 16 (it is the sum of the first two of these).
The sum of divisors for the integers 1 through 6 is 21.
21 is the smallest non-trivial example of a Fibonacci number whose digits are Fibonacci numbers and whose digit sum is also a Fibonacci number.
21 is a repdigit in base 4 (1114).
21 is the smallest natural number that is not close to a power of 2, 2n, where the range of closeness is ±n.
|Look up twenty-one in Wiktionary, the free dictionary.|
In most US states 21 is the drinking age.
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.
- 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 the number 21 has long been the car number 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.
- Illinois is the 21st state to join the United States.
- 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.
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