# 81 (number)

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Cardinal | eighty-one | |||

Ordinal |
81st (eighty-first) | |||

Factorization |
3^{4} | |||

Divisors | 1, 3, 9, 27, 81 | |||

Greek numeral | ΠΑ´ | |||

Roman numeral | LXXXI | |||

Binary |
1010001_{2} | |||

Ternary |
10000_{3} | |||

Quaternary |
1101_{4} | |||

Quinary |
311_{5} | |||

Senary |
213_{6} | |||

Octal |
121_{8} | |||

Duodecimal |
69_{12} | |||

Hexadecimal |
51_{16} | |||

Vigesimal |
41_{20} | |||

Base 36 |
29_{36} |

**81** (**eighty-one**) is the natural number following 80 and preceding 82.

## In mathematics[edit]

**81** is:

- the square of 9 and the fourth power of 3.
- a perfect totient number like all powers of three.
^{[1]} - a heptagonal number.
^{[2]} - a centered octagonal number.
^{[3]} - a tribonacci number.
^{[4]} - an open meandric number.
- the ninth member of the Mian-Chowla sequence.
^{[5]} - a palindromic number in bases 8 (121
_{8}) and 26 (33_{26}). - a Harshad number in bases 2, 3, 4, 7, 9, 10 and 13.
- one of three non-trivial numbers (the other two are 1458 and 1729) which, when its digits (in decimal) are added together, produces a sum which, when multiplied by its reversed self, yields the original number:

- 8 + 1 = 9
- 9 × 9 = 81 (although this case is somewhat degenerate, as the sum has only a single digit).

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

The inverse of 81 is 0.012345679 recurring, missing only the digit "8" from the complete set of digits. This is an example of the general rule that, in base *b*,

omitting only the digit *b*−2.

## In astronomy[edit]

- Messier object M81, a magnitude 8.5 spiral galaxy in the constellation Ursa Major, also known as Bode's Galaxy, and the first of what is known as the M81 Group of galaxies
- The New General Catalogue object NGC 81, a spiral galaxy in the constellation Andromeda

## In other fields[edit]

**Eighty-one** is also:

- The number of squares on a shogi playing board
- The year AD
**81**, 81 BC, or 1981. - The atomic number of thallium
- The symbolic number of the Hells Angels Motorcycle Club. 'H' and 'A' are the 8th and 1st letter of the alphabet, respectively.
^{[6]} - The title of a short film by Stephen Burke:
*81*^{[importance?]} - The model number of Sinclair ZX81
- The number of the department in France called Tarn
- The code for international direct dial phone calls to Japan
- One of two ISBN Group Identifiers for books published in India
- Number of stanzas or chapters in the Tao te Ching (in the most common arrangements).
- Number of provinces in Turkey.
- Number of prayers said in the Rosary in each night.
- The 81 is a 1965 song by Candy and the Kisses.
^{[importance?]} - Artemis 81 is a 1981 BBC TV science fiction drama.
- 'The Eighty-One Brothers' is a Japanese fable
^{[7]} - Legend days that one half of a heart with the number 81 is still to be found at seedbox.

## In culture[edit]

The Arabic characters for the numerals 8 and 1 are visible in the left palm of the human hand; in China, 81 always reminds people People's Liberation Army as it was founded on August 1. 81 is used to refer to the motor-club Hell's Angels, since H and A are, respectively, the 8th and 1st letters of the alphabet.

## See also[edit]

## References[edit]

**^**"Sloane's A082897 : Perfect totient numbers".*The On-Line Encyclopedia of Integer Sequences*. OEIS Foundation. Retrieved 2016-05-29.**^**"Sloane's A000566 : Heptagonal numbers".*The On-Line Encyclopedia of Integer Sequences*. OEIS Foundation. Retrieved 2016-05-29.**^**"Odd squares: a(n) = (2n+1)^2. Also centered octagonal numbers".*The On-Line Encyclopedia of Integer Sequences*. OEIS Foundation. Retrieved 2016-05-29.**^**"Sloane's A000073 : Tribonacci numbers".*The On-Line Encyclopedia of Integer Sequences*. OEIS Foundation. Retrieved 2016-05-29.**^**"Sloane's A005282 : Mian-Chowla sequence".*The On-Line Encyclopedia of Integer Sequences*. OEIS Foundation. Retrieved 2016-05-29.**^**Kovalevski, Serge F. (November 28, 2013), "Despite Outlaw Image, Hells Angels Sue Often",*The New York Times*.**^**http://www.sacred-texts.com/shi/jft2/jft207.htm The Eighty-One Brothers