81 (number)

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

← 80 81 82 83 84 85 86 87 88 89 → List of numbers — Integers 0 10 20 30 40 50 60 70 80 90 →
Cardinal	81 eighty-one
Ordinal	81st eighty-first
Factorization	$34$
Divisors	1, 3, 9, 27, 81
Roman numeral	LXXXI
Binary	01010001
Hexadecimal	51

Look up eighty-one in
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1 In mathematics
2 In astronomy
3 In other fields
4 In Culture

[edit] In mathematics

Eighty-one is the square of 9 and the fourth power of 3. Like all powers of three, 81 is a perfect totient number It is a heptagonal number and a centered octagonal number. It is also a tribonacci number, an open meandric number. 81 is the ninth member of the Mian-Chowla sequence.

In base 10, it is a Harshad number, and one of three numbers (the other two are 1458 and 1729) which, when its digits are added together, produces a sum which, when multiplied by its reversed self, yields the original number:

1 + 8 = 9

9 · 9 = 81

(although this case is somewhat degenerate, as the sum has only a single digit).

[edit] 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
The Saros number of the solar eclipse series which began on -322 May 12 and ended on 958 June 19. The duration of Saros series 81 was 1280.1 years, and it contained 72 solar eclipses. Further, the number of lunar eclipse series which began on -20 February 19 and ended on 1296 April 19. The duration of Saros series 81 was 1316.2 years, and it contained 74 lunar eclipses.

[edit] In other fields

Eighty-one is also:

The atomic number of thallium
The symbolic number of the Hells Angels Motorcycle Club
the title of a short film by Stephen Burke: 81
the model number of: Unterseeboot 81, Sinclair ZX81
the number of the French department Tarn

Department 81 of France (Tarn)
the designation of Interstate 81, a freeway that runs from Tennessee to New York.
the code for international direct dial phone calls to Japan
the number of times a popular rhyme (erroneously) claims Lizzie Borden struck her parents with an axe
One of two ISBN Group Identifiers for books published in India
Number of stanzas in the Tao te Ching (in the most common arrangements).
Baby 81, Abhilasha Jeyarajah, a baby survivor of the tsunami, claimed by many different parents.
In milligrams, the only dosage made of St. Joseph's aspirin.
The year AD 81, 81 BC, or 1981.
Jersey number of NFL Star Terrell Owens
The number of points that Kobe Bryant scored on Jan. 22, 2006 in the NBA game as the Los Angeles Lakers posted a 122-104 victory over the Toronto Raptors. 81 is the second highest single-game point total in NBA history, behind only Wilt Chamberlain's 100 over 40 years ago.
The number of chapters in the Tao Te Ching.
Number of provinces in Turkey. 81st one is Düzce.
Turkish pronounciation of 81 is exactly sex-N-beer, may lead to wordplay for people who speak both Turkish and English.