# 21 (number)

 ← 20 21 22 →
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
Quaternary 1114
Quinary 415
Senary 336
Octal 258
Duodecimal 1912
Vigesimal 1120
Base 36 L36

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

## In mathematics

21 is:

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

## 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 other fields

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

21 is:

## References

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.