21 (number)

In mathematics

• a Blum integer, since it is a semiprime with both its prime factors being Gaussian primes.
• a Fibonacci number.
• a Harshad number.
• a Motzkin number.
• a triangular number, because it is the sum of the first six natural numbers.
• an octagonal number.
• a composite number, its proper divisors being 1, 3 and 7.
• the sum of the divisors of the first 5 positive integers.
• the smallest non-trivial example of a Fibonacci number whose digits are Fibonacci numbers and whose digit sum is also a Fibonacci number.
• a repdigit in base 4.
• the smallest natural number that is not close to a power of 2, 2ⁿ, where the range of closeness is ±n.
• the smallest number of differently sized squares needed to square the square.
• the largest n with this property: for any positive integers a,b such that a + b = n, at least one of and is a terminating decimal. See a brief proof below.

In science

• The atomic number of scandium.

  Age 21

• In thirteen countries, 21 is the age of majority. See also: Coming of age.
• In eight countries, 21 is the minimum age to purchase tobacco products.
• In seventeen countries, 21 is the drinking age.
• In nine countries, it is the voting age.
• In most US states, 21 is the minimum age at which a person may gamble or enter casinos.
• In the United States, 21 is the minimum age to purchase a handgun or handgun ammunition.
• In the United States, 21 is the age at which one can purchase multiple tickets to an R-rated film without providing identification. It is also the age to accompany one under the age of 17 as their parent or adult guardian for an R-rated movie.
• In some countries, 21 is the minimum age to accompany a learner driver, provided that the person supervising the learner has held a full driver license for a specified amount of time. See also: List of minimum driving ages.
• In 2011, Adele named her second studio album 21, because of her age at the time.

  In sports

• Twenty-one is a variation of street basketball, in which each player, of which there can be any number, plays for himself only ; the name comes from the requisite number of baskets.
• In three-on-three basketball games held under FIBA rules, branded as 3x3, the game ends by rule once either team has reached 21 points.
• In badminton, and table tennis, 21 points are required to win a game.
• In AFL Women's, the top-level league of women's Australian rules football, each team is allowed a squad of 21 players.

  In other fields

• The Twenty-first Amendment repealed the Eighteenth Amendment, thereby ending Prohibition.
• The number of spots on a standard cubical die
• The number of firings in a 21-gun salute honoring Royalty or leaders of countries
• 21 Guns, a 2009 song by the punk-rock band Green Day
• Twenty One Pilots, an American musical duo
• 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
• Duncan MacDougall reported that 21 grams is the weight of the soul, according to an experiment.
• The number of the French department Côte-d'Or
• Twenty-One, an ancient card game in which the key value and highest-winning point total is 21
• * Blackjack, a modern version of Twenty-One played in casinos
• The number of shillings in a guinea.
• The number of solar rays in the flag of Kurdistan.
• The number on the logo for the American game show Catch 21