18 (NUMBER)

18	10	8

'18' ('eighteen') is the natural number following 17 and preceding 19.

Cardinal	18 eighteen
Ordinal	18th eighteenth
Numeral system	octodecimal
Factorization	$2\; cdot\; 3^2$
Divisors	1, 2, 3, 6, 9, 18
Roman numeral	XVIII
Roman numeral (Unicode)	XVIII, xviii
Binary	10010
Octal	22
Duodecimal	16
Hexadecimal	12

Contents

In mathematics

In science

In religion and literature

As lucky or unlucky number

Age 18

In music

In other fields

Historical years

See also

Notes

In mathematics

Eighteen is a composite number, its proper divisors being 1, 2, 3, 6 and 9. Three of these divisors (3, 6 and 9) add up to 18, hence 18 is a semiperfect number.
It is a heptagonal number, and as the sum of the first three pentagonal numbers, it is a pentagonal pyramidal number. It is also the sum of the totient function for the first seven integers.
18 is the smallest positive integer of the form $p\; q^2$ where ''p'' and ''q'' are primes with ''p'' < ''q''.
The aliquot sum of 18 is 21 and its aliquot sequence is (18,21,11,1,0).
In base 10 it is a Harshad number.
18 is the only number that equals twice the sum of its decimal digits.

In science

★ The atomic number of argon

In religion and literature

★ The Hebrew word for "life" is חי (''chai''), which has a numerical value of 18. Consequently, the custom has arisen in Jewish circles to give donations and monetary gifts in multiples of 18 as an expression of blessing for long life.

★ In Judaism, in the Talmud; Pirkei Avot (5:25), Rabbi Yehudah ben Teime gives the age of 18 as the appropriate age to get married (''"Ben shmonah esra lechupah"'', at eighteen years old to the Chupah (marriage canopy)). (See Coming of age, Age of majority).

★ Joseph Heller's novel ''Catch-22'' was originally named ''Catch-18'' because of the Hebrew meaning of the number, but was amended to the published title to avoid confusion with another war novel, Mila 18.^[1]

★ There are 18 chapters in the Bhagavad Gita.

As lucky or unlucky number

★ In Chinese tradition, the number 18, normally 十八 (shí bā), can also be read as 幺八 (yāo bā), which sounds like 要发 (yào fā), meaning that one is going to prosper. Thus, building floors numbered "18" are often very expensive in China

★ It is considered a number of the devil due to the fact that 6+6+6=18, and 666 is the number of the beast, as described in chapter 13, verse 18 of the Book of Revelation. There are also 18 letters in the words six-hundred sixty six.

Age 18

In many parts of the world, 18 is the age minimum for purchasing tobacco and alcoholic beverages (except in the United States where the drinking age remains 21 to this day). It is also the voting age in many countries. During the Vietnam War, Americans could be drafted at an age as low as 18, but were not allowed to vote in most states until they were 21. A movement spread to lower the voting age, and in 1971, the Twenty-sixth Amendment extended a guarantee of suffrage to anyone 18 years or older in all states. The voting age was lowered from 21 to 18 in the U.K. and many other nations around that time.
Eighteen is also the age in many countries at which a person may appear in a pornographic video, or express legal consent to sexual relations with another. Some countries, such as The Netherlands, have it set to 16.
An eighteen year-old is able to purchase goods from television commercials which otherwise would have been a liability issue for the seller.
In addition, eighteen year-olds in the United States have the right to bear arms, own property, marry without parental consent, get an abortion, donate their body to science, and serve on a jury. With these new privileges come responsibility: eighteen year-olds are tried as adults in court. However, there are exceptions to this tradeoff: minors can be tried as adults as early as 16 or sometimes even 7 years of age, and minors can be emancipated as early as 16 years of age, putting older minors at somewhat of a disparity of lacking the protections of children and the advantages of an adult, yet being disadvantaged by being held to the same level of responsibility or accountability as an adult.
In many countries 18 is the age of majority.
See also: 18 certificate, Coming of age

In music

★ String Quartets, Opus 18 by Ludwig van Beethoven is his first set of string quartets, containing six.

★ '''18''', the title of an album by Moby

★ Alice Cooper recorded the song ''"I'm Eighteen"''

★ Red Hot Chili Peppers recorded the song ''"She's Only 18"''

★ Skid Row recorded the song ''"18 and Life"''

★ Bow Wow recorded the song ''"Eighteen"''

★ Chris Brown turned 18 on May 5th, 2007.

In other fields

'Eighteen' is also:

★ Bobby Labonte's car number in the NASCAR Winston Cup Series when he won the 2000 Championship. This is J. J. Yeley's car number until the end of the 2007 Nextel Cup Season. Kyle Busch will take the car number in 2008.

★ The number of holes in a stipulated round of golf

★ The number of chapters into which James Joyce's epic novel Ulysses is divided

★ A slang term used in Soccer for the Penalty Area

★ The number of wheels on the most common type of tractor-trailer truck, which are hence often called ''18-wheelers''

★ The customary percentage to tip a waitress in a restaurant by groups of four or more

★ The number of the French department Cher and the Turkish province Bitlis

★ DVD-18 is double-sided, double-layered DVD format

★ A Canon error message, see E18 error

★ In neo-Nazi circles, a code word for Adolf Hitler. The number comes from the position of the letters in the alphabet: A = 1, H = 8. See also 88

★ Dragon Ball Z Character, Android 18.

★ Jersey number of football players Peyton Manning.

★ Jersey number of former basketball player Phil Jackson

★ Uniform number of former baseball player Don Larsen. The number shows prominently in a photograph taken just before the last pitch he made in his perfect game in the 1956 World Series. (When this picture was printed in ''Baseball's Unforgettable Games'' by Joseph Reichler and Ben Olan in 1960, they gave it the caption "The Writing on the Wall.")

★ The Hindu epic Mahabharata has eighteen sections, involves eighteen armies and is about a war fought over eighteen days

★ The sacred literature of the Hindus known as Bhagavad Gita has eighteen chapters.

★ In some countries the number 18 means blood (relative)

Historical years

'A.D. 18', 18 B.C., 1918, 2018, etc.

See also

★ Numeral system

Notes

1. N James. ''The Early Composition History of Catch-22''. In Biographies of Books: The Compositional Histories of Notable American Writings, J Barbour, T Quirk (edi.) pp. 262-90. Columbia: University of Missouri Press, '1996'