21 (number)

For other uses, see 21 (disambiguation).

← 20 21 22 →
← 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 × 7
Divisors	1, 3, 7, 21
Greek numeral	ΚΑ´
Roman numeral	XXI
Binary	101012
Ternary	2103
Senary	336
Octal	258
Duodecimal	1912
Hexadecimal	1516

21 (twenty-one) is the natural number following 20 and preceding 22. In a 24-hour clock, the twenty-first hour is in conventional language called nine or nine o'clock.

In mathematics

21 is:

• 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.
• an octagonal number.
• a composite number, its proper divisors being 1, 3 and 7.
• the sum of the first six natural numbers (1 + 2 + 3 + 4 + 5 + 6 = 21), making it a triangular number.
• the sum of 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 (111₄).
• 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.
• 21 times 2 is 42 which is the meaning of life, this tells us that 21 is ½ of the meaning of life.

21 appears in the Padovan sequence, preceded by the terms 9, 12, 16 (it is the sum of the first two of these).

In science

• The atomic number of scandium.

Age 21

• In several countries 21 is the age of majority. See also: Coming of age.
• In most US states, 21 is the drinking age.
  • However, in Puerto Rico and U.S. Virgin Island, the drinking age is 18.
• In Hawaii and New York, 21 is the minimum age that one person may purchase cigarettes and other tobacco products.
• In some countries it is the voting age.
• In the United States, 21 is the age at which one can purchase multiple tickets to an R-rated film without providing Identifications. 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 states, 21 is the minimum age, persons may gamble or enter casinos.
• 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 (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.

In other fields

Building called "21" in Zlín, Czech Republic.

Detail of the building entrance

21 is:

• The current century—spanning the years from 2001 to 2100—is referred to as the 21st century. See also: Century 21 (disambiguation).
• The Twenty-first Amendment repealed the Eighteenth Amendment, thereby ending Prohibition.
• 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
• 21 Guns, a 2009 song by the punk-rock band Green Day
• 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 (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.
• 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
• The key value and highest-winning point total of the popular casino game Blackjack
• The number of shillings in a guinea.