21 (number)

For other uses, see 21 (disambiguation).
20 21 22
Cardinal twenty-one
Ordinal 21st
(twenty-first)
Factorization 3 × 7
Divisors 1, 3, 7, 21
Roman numeral XXI
Binary 101012
Ternary 2103
Quaternary 1114
Quinary 415
Senary 336
Octal 258
Duodecimal 1912
Hexadecimal 1516
Vigesimal 1120
Base 36 L36

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

In mathematics

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).

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

In science

Age 21

In sports

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

In other fields

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

21 is:

References

Wikimedia Commons has media related to 21 (number).
  1. 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.
  2. "Article 9 Scoring" (PDF). 3x3 Rules of the Game. FIBA. January 13, 2012. Retrieved November 2, 2012.
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