21 (NUMBER)

21	20	1

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

Cardinal	21 twenty-one
Ordinal	21st twenty-first
Factorization	$3\; cdot\; 7$
Divisors	1, 3, 7, 21
Roman numeral	XXI
Binary	10101
Hexadecimal	15

Contents

In mathematics

In science

Age 21

In sports

In other fields

Historical years

In mathematics

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 fifth discrete bi-prime and the second in the (3.q) family. With 22 it forms the second discrete bi-prime pair.
21 has an aliquot sum of 11 though it is the second composite number found in the 11-aliquot tree with. 18 is the first such member as well having as 21 as its aliquot sum 21 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 5 is 21.
21 is a repdigit in base 4 (111).
21 is the smallest number of differently sized squares needed to square the square.

In science

★ The atomic number of scandium

Age 21

★ In the United States (excluding insular areas), 21 is the minimum legal age at which one can purchase and drink alcoholic beverages.

★ It is also the age at which it is legal to gamble and work as a prostitute in Nevada (except in Storey County and Lyon County, where the legal age is 18).

★ In most states, a person who is 21 or over may also rent a car (although there are usually extra fees until the person is 25).

★ Twenty-one was the minimum age of suffrage in the majority of states until the introduction of the Twenty-sixth Amendment.

★ It is also the age where one can legally buy tickets for minors who want to see an R-rated movie in 47 states. That individual must also see the movie with them as well.

★ It is also the minimum age at which one can purchase a handgun or rent/use one without supervision in most states.

★ Twenty-one was the age of majority in England and Wales before that age was lowered to 18 with the enactment of the Family Law Reform Act on 1 January 1970.

★ In the UK 21 is the age when a person can become a member of Scottish/UK Parliament or National Assembly for Wales, hire a car and adopt a child.

In sports

Players with jersey number twenty-one include:
# Roberto Clemente, Hall of Fame outfielder of the Pittsburgh Pirates
# Warren Spahn, Hall of Fame pitcher of the Boston/Milwaukee Braves.
# Sammy Sosa, MLB slugger, currently playing with the Texas Rangers
# Jason Marquis, pitcher for the Chicago Cubs
# Mark Sweeney, player for the Los Angeles Dodgers
# Carlos Delgado, first baseman for the New York Mets
# Dmitri Young, first baseman for the Washington Nationals
# Deion Sanders, NFL player, number worn with the 49ers, Falcons and the Cowboys.
# LaDainian Tomlinson, current NFL Running Back for the San Diego Chargers
# Julius Jones, running back for the Dallas Cowboys
# Frank Gore, running back for the San Francisco 49ers
# Sean Taylor, safety for the Washington Redskins
# Bob Sanders, safety for the Indianapolis Colts
# Chris McAlister, cornerback for the Baltimore Ravens
# Jason Webster, cornerback for the Buffalo Bills
# Charles Woodson, cornerback for the Green Bay Packers
# Mike Doss, safety for the Minnesota Vikings
# DeAngelo Hall, cornerback for the Atlanta Falcons
# Ken Lucas, cornerback for the Carolina Panthers
# Antrel Rolle, cornerback for the Arizona Cardinals
# Nnamdi Asomugha, cornerback for the Oakland Raiders
# Tiki Barber, retired NFL Running Back for the New York Giants.
# Dominique Wilkins, Hall of Fame basketball player for the Atlanta Hawks
# Tim Duncan, bronze medalist and current NBA basketball star for the San Antonio Spurs
# Kevin Garnett, forward for the Boston Celtics
# Josh Powell, current NBA forward for the Golden State Warriors.
# Stan Mikita, Chicago BlackHawks 1959-1980 NHL
# Mike Eruzione of the 1980 Men's Gold Olympic Hockey Team
# Peter Forsberg, hockey player for the Quebec Nordiques, Colorado Avalanche, Philadelphia Flyers, and now the Nashville Predators
# Mike Murphy, Pentucket, 2007- Lacrosse
# Leon Osman, Everton F.C. Midfielder
# Andrea Pirlo, Italian footballer and World Cup Champion playing for AC Milan.
# Nery Castillo, Mexican footballer when he sports the Mexican National Team jersey.
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 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).

★ The Twenty-first Amendment repealed the Eighteenth Amendment, thereby ending Prohibition.

★ 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 U.S.-based chain of clothing stores that was founded in 1984.

★ The number of spots on a standard cubical 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

★ A drinking game in which the goal is to get the group to 21 without saying it oneself. It's also 3 quarters (3/4) of an ounce, which among other things is the largest common "bag weight" of marijuana that the sale of which only get a misdemeanor charge in the state of New York, as 25 grams gets the perpetrator a felony.

★ The title of 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

★ 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

★ 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 car number of NASCAR race cars owned by the Wood Brothers, for whom Ken Schrader drives.

★ 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)

★ The British version of the novel ''A Clockwork Orange'' ended on chapter 21

★ 21 (single) is the title of the debut single by British rock band The Paddingtons

★ #21 is a recurring character on ''The Venture Bros.''

★ 21 Records, a record label

★ 21st Circuitry, a record label

★ The name of an anthology of short stories in French ''(Vingt et un contes).''

Historical years

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