81 (number)

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80 81 82
Cardinal eighty-one
Ordinal 81st
Factorization 34
Divisors 1, 3, 9, 27, 81
Roman numeral LXXXI
Binary 10100012
Ternary 100003
Quaternary 11014
Quinary 3115
Senary 2136
Octal 1218
Duodecimal 6912
Hexadecimal 5116
Vigesimal 4120
Base 36 2936

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

In mathematics[edit]

Eighty-one is the square of 9 and the fourth power of 3. Like all powers of three, 81 is a perfect totient number. It is a heptagonal number and a centered octagonal number. It is also a tribonacci number, and an open meandric number. 81 is the ninth member of the Mian-Chowla sequence.

In base 10, it is a Harshad number, and one of three non-trivial numbers (the other two are 1458 and 1729) which, when its digits 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).

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,

\frac{1}{\left(b-1\right)^2} = 0.\overline{012\cdots(b-4)(b-3)(b-1)},

omitting only the digit b−2.

In astronomy[edit]

In other fields[edit]

Eighty-one is also:

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.


  1. ^ Kovalevski, Serge F. (November 28, 2013), "Despite Outlaw Image, Hells Angels Sue Often", New York Times .
  2. ^ http://www.sacred-texts.com/shi/jft2/jft207.htm The Eighty-One Brothers