ⓘ 81 (number)


ⓘ 81 (number)

81 is:

  • the square of 9 and the fourth power of 3.
  • a palindromic number in bases 8 121 8 and 26 33 26.
  • the ninth member of the Mian-Chowla sequence.
  • a Harshad number in bases 2, 3, 4, 7, 9, 10 and 13.
  • a heptagonal number.
  • one of three non-trivial numbers the other two are 1458 and 1729 which, when its digits in decimal are added together, produces a sum which, when multiplied by its reversed self, yields the original number
  • an open meandric number.
  • a perfect totient number like all powers of three.
  • a centered octagonal number.
  • a tribonacci number.
8 + 1 = 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,

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

omitting only the digit b −2.


1. In astronomy

  • Messier object M81, a magnitude 8.5 spiral galaxy in the constellation Ursa Major, also known as Bodes Galaxy, and the first of what is known as the M81 Group of galaxies
  • The New General Catalogue object NGC 81, a spiral galaxy in the constellation Andromeda

2. In other fields

Eighty-one is also:

  • The number of the department in France called Tarn
  • The symbolic number of the Hells Angels Motorcycle Club. H and A are the 8th and 1st letter of the alphabet, respectively.
  • The number of squares on a shogi playing board
  • Number of stanzas or chapters in the Tao te Ching in the most common arrangements.
  • The model number of Sinclair ZX81
  • Artemis 81 is a 1981 BBC TV science fiction drama.
  • The year AD 81, 81 BC, or 1981.
  • The Eighty-One Brothers is a Japanese fable
  • The code for international direct dial phone calls to Japan
  • The 81 is a 1965 song by Candy and the Kisses.
  • The atomic number of thallium
  • The number of possible divinations in the Taixuanjing
  • The title of a short film by Stephen Burke: 81
  • Number of provinces in Turkey.
  • Number of prayers said in the Rosary in each night.
  • One of two ISBN Group Identifiers for books published in India

3. 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 Peoples Liberation Army as it was founded on August 1. 81 is used to refer to the motor-club Hells Angels, since H and A are, respectively, the 8th and 1st letters of the alphabet.