This article may contain indiscriminate, excessive, or irrelevant examples. (March 2016) |

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Cardinal | twenty-one | |||

Ordinal | 21st (twenty-first) | |||

Factorization | 3 × 7 | |||

Divisors | 1, 3, 7, 21 | |||

Greek numeral | ΚΑ´ | |||

Roman numeral | XXI | |||

Binary | 10101_{2} | |||

Ternary | 210_{3} | |||

Quaternary | 111_{4} | |||

Quinary | 41_{5} | |||

Senary | 33_{6} | |||

Octal | 25_{8} | |||

Duodecimal | 19_{12} | |||

Hexadecimal | 15_{16} | |||

Vigesimal | 11_{20} | |||

Base 36 | L_{36} |

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

## In mathematics

21 is:

- a Blum integer, since it is a semiprime with both its prime factors being Gaussian primes.
^{[1]} - a Fibonacci number.
^{[2]} - a Harshad number.
^{[3]} - a Motzkin number.
^{[4]} - a triangular number.
^{[5]} - an octagonal number.
^{[6]} - 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 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 (111
_{4}). - the smallest natural number that is not close to a power of 2, 2
^{n}, where the range of closeness is ±*n*. - the smallest number of differently sized squares needed to square the square.
^{[7]} - 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.

Note that a necessary condition for Let donate the quantity of the numbers smaller than We can easily see that for sufficiently large In fact, For every and so fails to hold when Just check a few numbers to see that |

21 appears in the Padovan sequence, preceded by the terms 9, 12, 16 (it is the sum of the first two of these).^{[8]}

Look up in Wiktionary, the free dictionary.twenty-one |

## In science

- The atomic number of scandium.

## Age 21

- In several countries 21 is the age of majority. See also: Coming of age.
- In all US states, 21 is the drinking age.
- However, in Puerto Rico and U.S. Virgin Islands, the drinking age is 18.

- In California, Hawaii, New York, and New Jersey, 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 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 most US states, 21 is the minimum age at which a person 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 AFL Women's, the top-level league of women's Australian rules football, each team is allowed a squad of 21 players (16 on the field and five interchanges).

## In other fields

**21** is:

- 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.
- 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.
- The number of solar rays in the flag of Kurdistan.

## References

Wikimedia Commons has media related to .21 (number) |

**^**"Sloane's A016105 : Blum integers".*The On-Line Encyclopedia of Integer Sequences*. OEIS Foundation. Retrieved 2016-05-31.**^**"Sloane's A000045 : Fibonacci numbers".*The On-Line Encyclopedia of Integer Sequences*. OEIS Foundation. Retrieved 2016-05-31.**^**"Sloane's A005349 : Niven (or Harshad) numbers".*The On-Line Encyclopedia of Integer Sequences*. OEIS Foundation. Retrieved 2016-05-31.**^**"Sloane's A001006 : Motzkin numbers".*The On-Line Encyclopedia of Integer Sequences*. OEIS Foundation. Retrieved 2016-05-31.**^**"Sloane's A000217 : Triangular numbers".*The On-Line Encyclopedia of Integer Sequences*. OEIS Foundation. Retrieved 2016-05-31.**^**"Sloane's A000567 : Octagonal numbers".*The On-Line Encyclopedia of Integer Sequences*. OEIS Foundation. Retrieved 2016-05-31.**^**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.**^**"Sloane's A000931 : Padovan sequence".*The On-Line Encyclopedia of Integer Sequences*. OEIS Foundation. Retrieved 2016-05-31.