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 n ...
following
80 and preceding
82.
In mathematics
81 is:
* the
square
In Euclidean geometry, a square is a regular quadrilateral, which means that it has four equal sides and four equal angles (90- degree angles, π/2 radian angles, or right angles). It can also be defined as a rectangle with two equal-length a ...
of
9 and the fourth power of
3.
* a
perfect totient number like all
powers of three.
* a
heptagonal number A heptagonal number is a figurate number that is constructed by combining heptagons with ascending size. The ''n''-th heptagonal number is given by the formula
:H_n=\frac.
The first few heptagonal numbers are:
: 0, 1, 7, 18, 34, 55, 81, 112 ...
.
* a
centered octagonal number
A centered octagonal number is a centered figurate number that represents an octagon with a dot in the center and all other dots surrounding the center dot in successive octagonal layers.. The centered octagonal numbers are the same as the od ...
.
* a
tribonacci number In mathematics, the Fibonacci numbers form a sequence defined recursion, recursively by:
:F_n =
\begin
0 & n = 0 \\
1 & n = 1 \\
F_ + F_ & n > 1
\end
That is, after two starting values, each number is the sum of the two preceding numbers.
The Fibo ...
.
* an
open meandric number In mathematics, a meander or closed meander is a self-avoiding closed curve which intersects a line a number of times. Intuitively, a meander can be viewed as a road crossing a river through a number of bridges.
Meander
Given a fixed oriented li ...
.
* the ninth member of the
Mian-Chowla sequence.
* a
palindromic number
A palindromic number (also known as a numeral palindrome or a numeric palindrome) is a number (such as 16461) that remains the same when its digits are reversed. In other words, it has reflectional symmetry across a vertical axis. The term ''palin ...
in bases 8 (121
8) and 26 (33
26).
* a
Harshad number
In mathematics, a harshad number (or Niven number) in a given number base is an integer that is divisible by the sum of its digits when written in that base.
Harshad numbers in base are also known as -harshad (or -Niven) numbers.
Harshad numbers ...
in bases 2, 3, 4, 7, 9, 10 and 13.
* 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:
: 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
The Messier objects are a set of 110 astronomical objects catalogued by the French astronomer Charles Messier in his ''Catalogue des Nébuleuses et des Amas d'Étoiles'' (''Catalogue of Nebulae and Star Clusters'').
Because Messier was only int ...
M81, a
magnitude
Magnitude may refer to:
Mathematics
*Euclidean vector, a quantity defined by both its magnitude and its direction
*Magnitude (mathematics), the relative size of an object
*Norm (mathematics), a term for the size or length of a vector
*Order of ...
8.5
spiral galaxy in the
constellation
A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object.
The origins of the e ...
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
Messier 81 (also known as NGC 3031 or Bode's Galaxy) is a grand design spiral galaxy about 12 million light-years away in the constellation Ursa Major. It has a D25 isophotal diameter of . Because of its relative proximity to the Milky Way ...
, 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 i ...
of galaxies
*The
New General Catalogue
The ''New General Catalogue of Nebulae and Clusters of Stars'' (abbreviated NGC) is an astronomical catalogue of deep-sky objects compiled by John Louis Emil Dreyer in 1888. The NGC contains 7,840 objects, including galaxies, star clusters and ...
object
NGC 81
NGC 81 is a lenticular galaxy estimated to be about 270 million light-years away 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 repre ...
, a
spiral galaxy in the constellation
Andromeda
In other fields
Eighty-one is also:
*The number of squares on a
shogi playing board
*The year AD 81,
81 BC
__NOTOC__
Year 81 BC was a year of the pre-Julian Roman calendar. At the time it was known as the Year of the Consulship of Decula and Dolabella (or, less frequently, year 673 ''Ab urbe condita''). The denomination 81 BC for this year has been u ...
, 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 every ...
of
thallium
Thallium is a chemical element with the Symbol (chemistry), 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 W ...
*The symbolic number of the
Hells Angels
The Hells Angels Motorcycle Club (HAMC) is a worldwide outlaw motorcycle club whose members typically ride Harley-Davidson motorcycles. In the United States and Canada, the Hells Angels are incorporated as the Hells Angels Motorcycle Corporati ...
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 ZX81 is a home computer that was produced by Sinclair Research and manufactured in Dundee, Scotland, by Timex Corporation. It was launched in the United Kingdom in March 1981 as the successor to Sinclair's ZX80 and designed to be a low-co ...
*The number of the
department in France called
Tarn
*The
code for international direct dial
Country calling codes or country dial-in codes are telephone number prefixes for reaching telephone subscribers in the networks of the member countries or regions of the International Telecommunication Union (ITU). The codes are defined by the ...
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 north ...
*"+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 Nevad ...
.
*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 and ...
Group Identifiers for books published in
India
India, officially the Republic of India (Hindi: ), is a country in South Asia. It is the seventh-largest country by area, the second-most populous country, and the most populous democracy in the world. Bounded by the Indian Ocean on the so ...
*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 d ...
'' (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 list of transcontinental countries, transcontinental country located mainly on the Anatolia, Anatolian Peninsula in Western Asia, with ...
.
*The number of prayers said in the
Rosary
The Rosary (; la, , in the sense of "crown of roses" or "garland of roses"), also known as the Dominican Rosary, or simply the Rosary, refers to a set of prayers used primarily in the Catholic Church, and to the physical string of knots or b ...
in each night.
*"The 81" is a 1965 song by Candy and the Kisses.
*''
Artemis 81
''Artemis 81'' is a British television play which was written by David Rudkin and directed by Alastair Reid. Commissioned by BBC producer David Rose, it was broadcast by the BBC on 29 December 1981. It was one of the last TV performances from ...
'' is a 1981
BBC #REDIRECT BBC #REDIRECT BBC
Here i going to introduce about the best teacher of my life b BALAJI sir. He is the precious gift that I got befor 2yrs . How has helped and thought all the concept and made my success in the 10th board exam. ...
...