81 (number)

 ← 80 81 82 →
Cardinal eighty-one
Ordinal 81st
(eighty-first)
Factorization 34
Divisors 1, 3, 9, 27, 81
Greek numeral ΠΑ´
Roman numeral LXXXI
Binary 10100012
Ternary 100003
Quaternary 11014
Quinary 3115
Senary 2136
Octal 1218
Duodecimal 6912
Vigesimal 4120
Base 36 2936

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

In mathematics

81 is:

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,

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

omitting only the digit b−2.

In other fields

Eighty-one is also:

In culture

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.