21 (number)

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For other uses, see 21 (disambiguation).
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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[edit]

21 is:

21 has an aliquot sum of 11 though it is the second composite number found in the 11-aliquot tree with the abundant square prime 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).[8]

In science[edit]

Age 21[edit]

  • In several countries 21 is the age of majority. See also: Coming of age.
  • In all US states, 21 is the drinking age.
  • 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. 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[edit]

Number 21 on the road bicycle of Ellen van Dijk at the Ronde van Drenthe.

In other fields[edit]

Building called "21" in Zlín, Czech Republic.
Detail of the building entrance

21 is:

References[edit]

  1. ^ "Sloane's A016105 : Blum integers". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2016-05-31. 
  2. ^ "Sloane's A000045 : Fibonacci numbers". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2016-05-31. 
  3. ^ "Sloane's A005349 : Niven (or Harshad) numbers". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2016-05-31. 
  4. ^ "Sloane's A001006 : Motzkin numbers". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2016-05-31. 
  5. ^ "Sloane's A000217 : Triangular numbers". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2016-05-31. 
  6. ^ "Sloane's A000567 : Octagonal numbers". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2016-05-31. 
  7. ^ 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.
  8. ^ "Sloane's A000931 : Padovan sequence". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2016-05-31. 
  9. ^ "Article 9 Scoring" (PDF). 3x3 Rules of the Game. FIBA. January 13, 2012. Retrieved November 2, 2012.