81 (eighty-one) is the
natural number
In mathematics, the natural numbers are those numbers used for counting (as in "there are ''six'' coins on the table") and ordering (as in "this is the ''third'' largest city in the country").
Numbers used for counting are called '' cardinal ...
following
80 and preceding
82.
In mathematics
81 is:
* the
square of
9 and the fourth power of
3.
* a
perfect totient number In number theory, a perfect totient number is an integer that is equal to the sum of its iterated totients. That is, we apply the totient function to a number ''n'', apply it again to the resulting totient, and so on, until the number 1 is reached, ...
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 (121
8) and 26 (33
26).
* a
Harshad number in bases 2, 3, 4, 7, 9, 10 and 13.
* one of three non-trivial numbers (the other two are
1458 and
1729
Events
January–March
* January 8 – Frederick, the eldest son of King George II of Great Britain is made Prince of Wales at the age of 21, a few months after he comes to Britain for the first time after growing up in Hano ...
) which, when its digits (in decimal) are added together, produces a sum which, when multiplied by its reversed self, yields the original number:
: 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.
recurring
Recurring means occurring repeatedly and can refer to several different things:
Mathematics and finance
*Recurring expense, an ongoing (continual) expenditure
*Repeating decimal, or recurring decimal, a real number in the decimal numeral syste ...
, 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 ''b''−2.
In astronomy
*
Messier object M81, a
magnitude 8.5
spiral galaxy in the
constellation
A constellation is an area on the celestial sphere in which a group of visible stars forms a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object.
The origins of the earliest constellation ...
Ursa Major
Ursa Major (; also known as the Great Bear) is a constellation in the northern sky, whose associated mythology likely dates back into prehistory. Its Latin name means "greater (or larger) bear," referring to and contrasting it with nearby Ursa ...
, also known as
Bode's Galaxy, and the first of what is known as the
M81 Group
The M81 Group is a galaxy group in the constellations Ursa Major and Camelopardalis that includes the galaxies Messier 81 and Messier 82, as well as several other galaxies with high apparent brightnesses. The approximate center of the group ...
of galaxies
*The
New General Catalogue object
NGC 81, a
spiral galaxy in the constellation
Andromeda
In other fields
Eighty-one is also:
*The number of squares on a
shogi
, also known as Japanese chess, is a strategy board game for two players. It is one of the most popular board games in Japan and is in the same family of games as Western chess, '' chaturanga, Xiangqi'', Indian chess, and ''janggi''. ''Shōgi ...
playing board
*The year AD 81,
81 BC, or
1981.
*The
atomic number
The atomic number or nuclear charge number (symbol ''Z'') of a chemical element is the charge number of an atomic nucleus. For ordinary nuclei, this is equal to the proton number (''n''p) or the number of protons found in the nucleus of ever ...
of
thallium
Thallium is a chemical element with the symbol Tl and atomic number 81. It is a gray post-transition metal that is not found free in nature. When isolated, thallium resembles tin, but discolors when exposed to air. Chemists William Crookes an ...
*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
A short film is any motion picture that is short enough in running time not to be considered a feature film. The Academy of Motion Picture Arts and Sciences defines a short film as "an original motion picture that has a running time of 40 minutes ...
by Stephen Burke: ''
81''
*The model number of
Sinclair ZX81
*The number of the
department in France
In the administrative divisions of France, the department (french: département, ) is one of the three levels of government under the national level ("territorial collectivities"), between the administrative regions and the communes. Ninety-s ...
called
Tarn
*The
code for international direct dial phone calls to
Japan
Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the n ...
*"+81" is a song by Japanese metalcore band
Crystal Lake Crystal Lake or Crystal Lakes may refer to:
Lakes
Canada
* Crystal Lake (Saskatchewan)
* Crystal Lake (Ontario), drain into the Lynn River, which drains into Lake Erie
United States
* Crystal Lake, California, a mountain lake in Nevada Co ...
.
*One of two
ISBN
The International Standard Book Number (ISBN) is a numeric commercial book identifier that is intended to be unique. Publishers purchase ISBNs from an affiliate of the International ISBN Agency.
An ISBN is assigned to each separate edition a ...
Group Identifiers for books published in
India
India, officially the Republic of India (Hindi: ), is a country in South Asia. It is the List of countries and dependencies by area, seventh-largest country by area, the List of countries and dependencies by population, second-most populous ...
*The number of stanzas or chapters in the ''
Tao Te Ching
The ''Tao Te Ching'' (, ; ) is a Chinese classic text written around 400 BC and traditionally credited to the sage Laozi, though the text's authorship, date of composition and date of compilation are debated. The oldest excavated portion ...
'' (in the most common arrangements).
*The number of provinces in
Turkey
Turkey ( tr, Türkiye ), officially the Republic of Türkiye ( tr, Türkiye Cumhuriyeti, links=no ), is a transcontinental country located mainly on the Anatolian Peninsula in Western Asia, with a small portion on the Balkan Peninsula ...
.
*The 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 Eighty-One Brothers
sacred-texts.com
*The number of possible divinations in the '' Taixuanjing''
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 People's Liberation Army
The People's Liberation Army (PLA) is the principal military force of the China, People's Republic of China and the armed wing of the Chinese Communist Party (CCP). The PLA consists of five Military branch, service branches: the People's ...
as it was founded on August 1.
81 is used to refer to the Hells Angels Motorcycle Club, since H and A are, respectively, the 8th and 1st letters of the alphabet.
See also
* List of highways numbered 81
References
{{DEFAULTSORT:81 (Number)
Integers