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81 (number)

From Wikipedia, the free encyclopedia
Natural number
← 8081 82 →
Cardinaleighty-one
Ordinal81st
(eighty-first)
Factorization34
Divisors1, 3, 9, 27, 81
Greek numeralΠΑ´
Roman numeralLXXXI,lxxxi
Binary10100012
Ternary100003
Senary2136
Octal1218
Duodecimal6912
Hexadecimal5116

81 (eighty-one) is thenatural number following80 and preceding82.

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).
Look upeighty-one in Wiktionary, the free dictionary.

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

1(b−1)2=0.012⋯(b−4)(b−3)(b−1)¯,{\displaystyle {\frac {1}{\left(b-1\right)^{2}}}=0.{\overline {012\cdots (b-4)(b-3)(b-1)}},}

omitting only the digitb−2.

In other fields

[edit]

Eighty-one is also:

  • The number of chapters in theTao Te Ching, the foundational text ofTaoism.
  • The symbolic number of theHells Angels Motorcycle Club. 'H' and 'A' are the 8th and 1st letter of the alphabet, respectively.[8]
  • 81 (八一) is referenced in flags and symbols of thePeople's Liberation Army as it was founded on August 1 (8/1).
  • The career high of points scored in a single game byKobe Bryant on January 22, 2006 against theToronto Raptors. 81 is the second most points scored in an American professional basketball game, behindWilt Chamberlain's100.
  • The international calling code forJapan

References

[edit]
  1. ^Sloane, N. J. A. (ed.)."Sequence A082897 (Perfect totient numbers)".TheOn-Line Encyclopedia of Integer Sequences. OEIS Foundation.
  2. ^Sloane, N. J. A. (ed.)."Sequence A000566 (Heptagonal numbers)".TheOn-Line Encyclopedia of Integer Sequences. OEIS Foundation.
  3. ^Sloane, N. J. A. (ed.)."Sequence A161935 (28-gonal numbers.)".TheOn-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)".TheOn-Line Encyclopedia of Integer Sequences. OEIS Foundation.
  5. ^Sloane, N. J. A. (ed.)."Sequence A000073 (Tribonacci numbers)".TheOn-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.)".TheOn-Line Encyclopedia of Integer Sequences. OEIS Foundation.
  7. ^Sloane, N. J. A. (ed.)."Sequence A005282 (Mian-Chowla sequence)".TheOn-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.
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