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# 18 (number)

18 (eighteen) is the natural number following 17 and preceding 19. It equals ten and eight, twice nine, nine times two or three times six.

## 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. Eighteen is the first inverted square-prime of the form p.q2 . .

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.

Eighteen is the second abundant number with the aliquot sum of 21 (117% in abundance) in the aliquot sequence (18,21,11,1,0) thus being the first composite number in the 11-aliquot tree. Eighteen is the aliquot sum of only one number the square 289.

In base 10 it is a Harshad number.

18, aside from 0, is the only number that equals twice the sum of its decimal digits.

18 = eighteen

## 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.
• There are 18 chapters in the Bhagavad Gita, which is contained in the Mahabharata, which has 18 books. The Kurukshetra War which the epic depicts, is between 18 armies (11 on the Kuru side, 7 on the Pandava).

## 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

## Age 18

In many parts of the world, 18 is the age minimum for purchasing tobacco and alcoholic beverages. 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.

In addition, eighteen-year-olds in the United States have the right to bear arms, own property, marry without parental consent(in almost all states), get an abortion, donate their bodies to science, and serve on a jury.

In the United Kingdom, eighteen-year-olds can purchase tobacco, alcohol and pornography, go to war, vote, and model for pornography; (but the age of consent is 16).

In many countries 18 is the age of majority.

In many states the age at which one can obtain a learner's permit without parental consent.

Minimum age at which one can be admitted to rent or buy an MPAA-rated NC-17 movie. This is also the age at which one can see an NC-17 rated movie.

Minimum age at which one can play, rent, or buy an ESRB-rated AO game.

Minimum age at which one can buy tickets to an 18A rated film in Canada without being accompanied by an adult. Ratings in Canada are provincial, so they may vary. A film can be 18A in some provinces and 14A in other provinces. A film can also be 18A in some provinces and R in other provinces. 18 is also the minimum age at which one can view, rent, or buy an R or an A rated movie.

Minimum age at which one can buy, rent, view, or buy tickets to an 18+ rated movie in the Canadian province of Quebec.

Minimum legal age to drink or buy alcohol in many countries in the world.

Minimum age at which one can get a full driver's license in many U.S states and Canadian provinces.

Minimum age to drive a car in many countries in the world. See also: 18 certificate, Coming of age

Minimum age at which one can legally buy marijuana in the Netherlands.

## In other fields

Eighteen is also:

• a 2005 movie directed by Richard Bell and starring Brendan Fletcher and Alan Cumming.
• Number worn by Red Sox japenesse pitching sensation Daisuke Matsuzaka.
• Jersey number of NHL star Marian Hossa.
• 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.
• A 2002 album by electronic artist Moby.
• The number of holes in a stipulated round of golf
• The number of chapters into which James Joyce's epic novel Ulysses is divided
• Russ Wheeler's car number in the film Days of Thunder
• A slang term used in association football for the Penalty Area
• The number of wheels on the most common type of North American 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
• 18 (Dragon Ball), a fictional character in the metaseries Dragon Ball.
• 18th Street gang, a Hispanic American gang
• Jersey number of football player Peyton Manning.
• Jersey number of football player Sage Rosenfels.
• Jersey number of former basketball player Phil Jackson
• Jersey number of baseball player Héctor Luna.
• Jersey number of basketball player Anthony Parker.
• 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 number of league titles Liverpool FC has won.
• 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)
• In the Chinese mythos, Hell has 18 levels
• In Chinese folklore, the Shaolin temple had a group of 18 Bronze Monks. Initiates could only be considered full graduates of Shaolin martial arts if they could defeat them in combat.
• In 1990 Megadeth released Hanger 18 as a single for the Rust in Piece Album.
• Eighteen Visions (often abbreviated to 18V), an alternative metal/metalcore band from Orange County, California.
• Eighteen Eighteen, a South African Rap artist.

## Historical years

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

## Notes

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