21 (number)

← 20 22 → 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-one
Ordinal	21st (twenty-first)
Factorization	$3 \cdot 7$
Divisors	1, 3, 7, 21
Roman numeral	XXI
Binary	10101₂
Octal	25₈
Duodecimal	19₁₂
Hexadecimal	15₁₆

21 (twenty-one) is the natural number following 20 and preceding 22.

1 In mathematics
2 In science
3 In music
4 Age 21
5 In sports
6 In other fields
7 Historical years
8 References

In mathematics

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.

Twenty-one is a Fibonacci number, a Harshad number, a Motzkin number, a triangular number and an octagonal number, as well as a composite number, its proper divisors being 1, 3 and 7.

21 is the sum of the first six natural numbers (1+2+3+4+5+6=21).

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 a repdigit in base 4 (111).

21 is the smallest natural number that is not close to a power of 2, 2^n, where the range of closeness is +- n.

21 is the smallest number of differently sized squares needed to square the square.^[1]

In science

The atomic number of scandium

In music

Main article: 21 (disambiguation)

British indie rock band Mystery Jets released an album entitled Twenty One in 2008.
"Twenty-one" is also a song from Eagles
"Twenty one" is the name of the song from No need to argue by The Cranberries
2NE1 is the name of a girl group originating South Korea meaning "New Evolution of the 21st Century"
Green Day has a song called 21 Guns.
British soul singer Adele's second album was called 21

Age 21

In several countries 21 is the age of majority. See also: Coming of age, Voting age, Legal drinking age.

In most USA states 21 is the drinking age.

In some countries it is the voting age.

In sports

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 delivering 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:
  - The Chicago Blackhawks, for Hall of Famer Stan Mikita.
  - The Colorado Avalanche, for likely future Hall of Famer Peter Forsberg.
  - The Pittsburgh Penguins, for Michel Brière.
  - The Toronto Maple Leafs have a policy of not retiring numbers unless the player honoured either died or suffered a career-ending incident while a member of the team. Other players whose numbers would otherwise be retired instead have their numbers enshrined by the team as "Honoured Numbers", which remain in circulation for future players. The number 21 is currently honoured for Hall of Famer Börje Salming.
- No NFL team has retired the number.
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 badminton, and table tennis (before 2001), 21 points are required to win a game.
Eyeshield 21(アイシールド21 Aishīrudo nijūichi) is a Japanese animation about football
In the NASCAR Sprint Cup Series the number 21 has long been the car number for Wood Brothers Racing

In other fields

21 is:

The current century—spanning the years from 2001 to 2100—referred to as the 21st century. The approach of this century inspired many forward-looking 20th century events and company and product names, such as the Century 21 Exposition (another name for the 1962 Seattle World's Fair), Century 21 Real Estate, and Century 21 Television (producers of Sylvia and Gerry Anderson Supermarionation shows like Thunderbirds and Captain Scarlet and the Mysterons).
The Twenty-first Amendment repealed the Eighteenth Amendment, thereby ending Prohibition.
Illinois - the 'Land of Lincoln' - is the 21st state to join the United States. Abraham Lincoln moved to Illinois when he was 21 and he met his future wife - Mary Todd - in Springfield when she was 21. Illinois currently has 21 electoral votes in the US Presidential Election.
Pier 21 was, from 1928 to 1971, the place where immigrants entered Canada. It was called the "Gateway to Canada."
Forever 21 (also operating as XXI Forever) is a US-based chain of clothing stores that was founded in 1984.
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
A card game, also called vingt-et-un (French for "twenty-one"), pontoon, or blackjack
There are 21 trump cards of the tarot deck if one does not consider The Fool to be a proper trump card.
Twenty One, a TV quiz show that ran from 1956 to 1958, most remembered for the scandal that the wins were fixed (see Quiz show scandals). It was remade in 2000.
Twenty-One, a 1991 independent film, starring Patsy Kensit.
The title of at least four different movies:

A 1918 comedy starring Bryant Washburn and Gertrude Selby

A 1923 drama starring Richard Barthelmess and Dorothy Mackaill

A 1991 drama starring Patsy Kensit

A 2008 drama starring Kevin Spacey, Laurence Fishburne, and Kate Bosworth, based on the book Bringing Down the House.

In the title of the TV show 21 Jump Street, starring Johnny Depp
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.
21 Demands are an Irish rock band.
Number 21 is the name of the plane alleged flown by Gustave Whitehead two years before the Wright brothers' flight
The title of a song by The Cranberries
21 is the title of Omarion's second album.
21 is the title of a novel by Patrick O'Brian
The 21 Club is a top restaurant in New York City
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. In reference to MacDougall's theory, 21 Grams is the title of a movie (although MacDougall's theory plays no direct role in the film)
"21" is the title of the debut single by British rock band The Paddingtons
21 Records, a record label
21st Circuitry, a record label
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

Historical years

21 A.D., 21 B.C., 1921, 2021, etc.

References

^ 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.