81 (number)

Last updated
80 81 82
Cardinal eighty-one
Ordinal 81st
(eighty-first)
Factorization 34
Divisors 1, 3, 9, 27, 81
Greek numeral ΠΑ´
Roman numeral LXXXI, lxxxi
Binary 10100012
Ternary 100003
Senary 2136
Octal 1218
Duodecimal 6912
Hexadecimal 5116

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

Contents

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,

omitting only the digit b2.

In other fields

Eighty-one is also:

References

  1. Sloane, N. J. A. (ed.). "SequenceA082897(Perfect totient numbers)". The On-Line Encyclopedia of Integer Sequences . OEIS Foundation.
  2. Sloane, N. J. A. (ed.). "SequenceA000566(Heptagonal numbers)". The On-Line Encyclopedia of Integer Sequences . OEIS Foundation.
  3. Sloane, N. J. A. (ed.). "SequenceA161935(28-gonal numbers.)". The On-Line Encyclopedia of Integer Sequences . OEIS Foundation.
  4. Sloane, N. J. A. (ed.). "SequenceA016754(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.). "SequenceA000073(Tribonacci numbers)". The On-Line Encyclopedia of Integer Sequences . OEIS Foundation.
  6. Sloane, N. J. A. (ed.). "SequenceA005316(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.). "SequenceA005282(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 .