81 (number)

In mathematics

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

In astronomy

• Messier object M81, a magnitude 8.5 spiral galaxy in the constellation Ursa Major, also known as Bode's 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

  In other fields

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

  In culture