81 is the natural number following 80 and preceding 82. In mathematics, a natural number is either a positive integer (1, 2, 3, 4, ...) or a non-negative integer (0, 1, 2, 3, 4, ...). The former definition is generally used in number theory, while the latter is preferred in set theory. ... 80 (eighty) is the natural number following 79 and preceding 81. ... 82 is the natural number following 81 and preceding 83. ...

In linguistics, cardinal numbers is the name given to number words that are used for quantity (one, two, three), as opposed to ordinal numbers, words that are used for order (first, second, third). ... Commonly, ordinal numbers, or ordinals for short, are numbers used to denote the position in an ordered sequence: first, second, third, fourth, etc. ... In mathematics, factorization or factoring is the decomposition of an object (for example, a number, a polynomial, or a matrix) into a product of other objects, or factors, which when multiplied together give the original. ... In mathematics, a divisor of an integer n, also called a factor of n, is an integer which evenly divides n without leaving a remainder. ... The system of Roman numerals is a numeral system originating in ancient Rome, and was adapted from Etruscan numerals. ... The binary numeral system (base-2) represents numeric values using two symbols, typically 0 and 1. ... In mathematics and computer science, hexadecimal, or simply hex, is a numeral system with a radix or base of 16 usually written using the symbols 0–9 and A–F or a–f. ...

In mathematics

Eighty-one is the square of 9. It is a heptagonal number and a centered octagonal number. It is also a tribonacci number, an open meandric number and a Harshad number. In square numbers, a square number, sometimes also called a perfect square, is an integer that can be written as the square of some other integer. ... 9 (nine) is the natural number following 8 and preceding 10. ... A heptagonal number is a figurate number that represents a heptagon. ... 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. ... In mathematics, the Fibonacci numbers form a sequence defined recursively by: In words: you start with 0 and 1, and then produce the next Fibonacci number by adding the two previous Fibonacci numbers. ... In mathematics, a meander or closed meander is a self-avoiding closed curve which intersects a line a number of times. ... A Harshad number, or Niven number, is an integer that is divisible by the sum of its digits in a given number base. ...

81 is also 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: 1458 Cardinal One thousand four hundred [and] fifty-eight Ordinal 1458th Factorization Divisors 3,6,9,18,21,42,63,126,189,378,729 Roman numeral MCDLVIII Binary 10110110010 Hexadecimal 5B2 1458 is one of three numbers (the other two are 81 and 1729) which, when its digits are added... 1729 is known as the Hardy-Ramanujan number, after a famous anecdote of the British mathematician G. H. Hardy regarding a hospital visit to the Indian mathematician Srinivasa Ramanujan. ...

1 + 8 = 9

9 · 9 = 81

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

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.

Table of all 110 Messier objects. ... Bodes Galaxy, also known as M81 and NGC 3031, is a spiral galaxy located 12 million light years from Earth in the Ursa Major constellation. ... The apparent magnitude (m) of a star, planet or other heavenly body is a measure of its apparent brightness; that is, the amount of light received from the object. ... A spiral galaxy is a type of galaxy in the Hubble sequence which is characterized by the following physical properties: Spiral Galaxy M74 presents a face-on view of its spiral arms. ... Orion is a remarkable constellation, visible from most places on the globe (but not always the whole year long). ... Ursa Major (Ursa Maior in Latin) is a constellation visible throughout the year in the northern hemisphere. ... Bodes Galaxy, also known as M81 and NGC 3031, is a spiral galaxy located 12 million light years from Earth in the Ursa Major constellation. ... The M81 group of galaxies (or simply the M81 group) is a physical group and a galaxy cluster. ... The New General Catalogue (NGC) is the most well-known catalogue of deep sky objects in amateur astronomy. ... A spiral galaxy is a type of galaxy in the Hubble sequence which is characterized by the following physical properties: Spiral Galaxy M74 presents a face-on view of its spiral arms. ... Andromeda is a constellation named for the princess Andromeda (which is Greek for Ruler over men), a character in Greek mythology. ... A Saros cycle is a period of 6585 + 1/3 days (approximately 18 years 10 days and 8 hours) which can be used to predict eclipses of the sun and the moon. ... Photo taken during the French 1999 eclipse A solar eclipse occurs when the Moon passes in front of the Sun and obscures it totally or partially. ... May 12 is the 132nd day of the year in the Gregorian Calendar (133rd in leap years). ... Events Kshemgupta, King of Kashmir dies and is succeeded by his young son Abhimanyu. ... June 19 is the 170th day of the year (171st in leap years) in the Gregorian Calendar, with 195 days remaining. ... An eclipse occurs whenever the Sun, Earth and Moon line up exactly. ... February 19 is the 50th day of the year in the Gregorian Calendar. ... Events March 30 - Edward I stormed Berwick-upon-Tweed, sacking the then Scottish border town with much bloodshed. ... April 19 is the 109th day of the year in the Gregorian calendar (110th in leap years). ...

In other fields

Eighty-one is also:

• The atomic number of thallium
• 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.