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

← 80 81 82 →
Cardinaleighty-one
Ordinal81st
(eighty-first)
Factorization34
Divisors1, 3, 9, 27, 81
Greek numeralΠΑ´
Roman numeralLXXXI
Binary10100012
Ternary100003
Senary2136
Octal1218
Duodecimal6912
Hexadecimal5116

In mathematics

edit

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,

 

omitting only the digit b−2.

In astronomy

edit

In other fields

edit

Eighty-one is also:

See also

edit

References

edit
  1. ^ Sloane, N. J. A. (ed.). "Sequence A082897 (Perfect totient numbers)". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation.
  2. ^ Sloane, N. J. A. (ed.). "Sequence A000566 (Heptagonal numbers)". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation.
  3. ^ Sloane, N. J. A. (ed.). "Sequence A161935 (28-gonal numbers.)". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation.
  4. ^ Sloane, N. J. A. (ed.). "Sequence A016754 (Odd squares: a(n) = (2n+1)^2. Also centered octagonal numbers)". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation.
  5. ^ Sloane, N. J. A. (ed.). "Sequence A000073 (Tribonacci numbers)". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation.
  6. ^ Sloane, N. J. A. (ed.). "Sequence A005316 (Meandric numbers: number of ways a river can cross a road n times.)". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation.
  7. ^ Sloane, N. J. A. (ed.). "Sequence A005282 (Mian-Chowla sequence)". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation.
  8. ^ Kovalevski, Serge F. (November 28, 2013), "Despite Outlaw Image, Hells Angels Sue Often", The New York Times.
  9. ^ Ansorge, Peter (9 September 2011). "Aleister Reid obituary". The Guardian. London. Retrieved 24 May 2012.
  10. ^ The Eighty-One Brothers, sacred-texts.com