21 (number)
From Wikipedia, the free encyclopedia
| 21 | |
|---|---|
| Cardinal | twenty-one |
| Ordinal | 21st (twenty-first) |
| Factorization |  |
| Divisor(s) | 1, 3, 7, 21 |
| Roman numeral | XXI |
| Binary | 10101 |
| Hexadecimal | 15 |
21 (twenty-one) is the natural number following 20 and preceding 22.
Contents |
[edit] 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. 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. 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.[1]
[edit] In science
- The atomic number of scandium
[edit] 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.
[edit] 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 in several states and 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).
- 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 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.
[edit] 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.
[edit] 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.
- 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
- 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 (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).
- 21 is one of the clues given to NYPD Officer John McClane (Bruce Willis) in the 1995 movie Die Hard With A Vengeance. The "21" clue referred to the 21st President, Chester A. Arthur, where a bomb was hidden in Chester A. Arthur Elementary School.
- In Silent Hill 4: The Room, Walter Sullivan needed to kill 21 people to perform "21 Sacraments".
- 21 has recently been made into a movie about gambling.
- 21 has recently been made into a new 21 Trivia game.
[edit] Historical years
21 A.D., 21 B.C., 1921, 2021, etc.
[edit] 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.
