21 (number)

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

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

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. As it is a semiprime with both its prime factors being Gaussian primes, 21 is then a Blum integer.

21 has an aliquot sum of 11 though it is the second composite number found in the 11-aliquot tree with the abundant 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 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
• In 'Unified String Theory', there are 21 dimensions/aspects of spacetime.

In music

• Thrash metal band Sepultura released an album titled Dante XXI in 2006.
• British indie rock band Mystery Jets released an album entitled Twenty One in 2008.
• Sam Sparro released a single called 21st Century Life in 2008.
• The number also appears in the song Spitting Feathers by The Feeling on Join With Us (song) - EP

Age 21

• In the United States (excluding insular areas), 21 is the minimum legal age at which one can purchase and drink alcoholic beverages and gamble.
• It is also the age at which it is legal to work as a prostitute. Prostitution is illegal everywhere in the US except licensed brothels in Nevada (except in Storey County and Lyon County, where the legal age is 18).
• 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 minimum age at which one can purchase a handgun or rent/use one without supervision in most states.
• Age at which one can purchase fireworks in the U.S state of New Hampshire.
• Age at which one can generally get married in the U.S state of Mississippi.

In sports

• The Pittsburgh Pirates retired number 21 in 1973 in honor of Roberto Clemente, who was killed in a plane crash while delivering humanitarian aid to victims of an earthquake in Nicaragua.
• 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.
• Captain of the 1980 U.S. men's Olympic Hockey Team Mike Eruzione wore number 21.
• The late Washington Redskin Sean Taylor wore number 21. Fellow teammates Chris Cooley and Chris Samuels wore number 21 during the Pro Bowl in his honor.
• Standout running back LaDainian Tomlinson wears number 21.
• Tim Duncan wears number 21 for the San Antonio Spurs.
• Kevin Garnett wore number 21 while playing with the Minnesota Timberwolves.
• George Gregan, an Australian rugby player wore the number 21. He was the first player to reach 100 test matches.
• In badminton, 21 points are required to win a game.

In other fields

• The letter U is the 21st letter of the English alphabet.
• 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 US-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.
• 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
• 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
• Twenty One is the title of the second album by Mystery Jets
• 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 Bill Elliott 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)
• "21" 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).
• 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

References